The Williamson Amplifier

Transcription

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SECOND
Three
EDITION
ShUnnp • Sixpence Net
•
�-
'.'-
- ' --.... -
""
Tronsfor ers ond Chokes
for
WILLIAMSON
NATIONAL
ON
Tested for:
,
,I
TESTS
OF
REPORT
LABORATORY
TRANSFORMER (Marked: Lab: Output, Series Il)
Vortexion Ltd., 257/263 The Broadway, London, S.W.19.
TABLE
I
Primary Resistance and Inductance
at 50 c/s
5
2.
AMPLIFIER
PHYSICAL
The series resistance and inductance of the primary
winding with the secondary winding opcn-circuitcd,
was measured at 50 cycles per second with 5 and 20
volts applied. The room temperature was l<'°C.
The results are given in Table 1.
Voltage
the
95
142
4700
7900
Inductance
Resistance (0.)
Date ..
(H)
2nd JUlIt, 1951.
The series resistance and inductance ofthe primary
winding with the secondary winding short-circuited,
and of half the primary winding with the oilier half
short-circuited were measured at 1,000 cycles per
second. About 10 volts were applied. The results
are given in Table IL
TABLE
11
Resistance and Inductance at 1,000 c/s
Connections Resistance (0.) Inductance (lnH)
1'Ii':�;r�h,,!:�'<In'I'
K�1I !'rin...",_ K�U
J'ri,,,,,,y
.homed.
Reference ..
495
222
5.3
5.5
E.475.100.
This Laboratory Model Transformer Series 11 easily permits
6 db more feedback with complete stability than the maximum
of our Series 1 which Mr. D. T. N. Williamson described.
PRICES
Series Il Output Transformers as above ...
Series I Output Transformers
Mains Transformers ...
CHOKES 1O.12H, 150 mA ...
CHOKES 30/H, 2e mA
£7. 7.0
£6. 6.0
£4.
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£1. ll. 6
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WILLlAMSON
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H ONE:
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41959
J.T.l.
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The Williamson
Amplifier
A Collection of Articles,
reprinted frolll "Wireless World,"
011
"Design for a High-quality Amplifier"
By
D_ T_ N. WILLIAMSON
(formerly of the M.O. Val"c: Company, now
with
Ferranti
Research
Published
Laboratories)
for
•
LONDON: ILIFFE & SONS, LTD.
The W iIIiamson Amplifier
CONTENTS
Page
5
Introduction
7
Basic Requirements:
Alternative Specifications
(April
1947)
Details of Chosen Circuit and Its Performance
11
(May 1947)
NEW VERSION
Design Data:
ifications:
Mod
14
Further Notes
(August 1949)
Design of Tone Controls and Auxiliary Gramophone Circuits
20
Design for a Radio Feeder Unit
30
(October and November 1949)
(December 1949)
Replies to Queries Raised by Constructors
34
(January 1950)
Modifications for High-impedance Pickups and Long-playing Records
(May 1952)
3
35
Introduction
Introduced by
Wireless World in '947 as merely onc of a series of
amplifier designs, the .. Williamson .. has for several years been widely
accepted as the standard of design and performance wherever amplifier s
and sound reproduction arc discussed.
Descriptions of it have been
published in all the principal countries of the world, and so there are
reasonable grounds for assuming that its widespread reputation is based
solely on its qualities.
This hooklet includes all the articles written by D. T.
on the amplifier.
N. Williamson
Both the 1947 and 1949 versions are reprinted. as the
alternative output transformer ratios cover a wide range of require
ments.
an
Modifications and additions include pee-amplifier circuits and
d. unit. with recently published information 011 adaptation
impedance pickups and correction for 331 r.p.m. records.
to
bigb
We would stress the importance, if the full potentialities of Hle
amplifier are to be realized, of following the author's recommendations
in detail.
Even in the U .S.A., where several modified versions have
been descri bed, many users adhere to the designer's exact specification
with the original valve types.
It is not the circuit alone, but the
properties of the valves and such components as the output tnlnsfonner,
together with the welding of theory and p ractice into a rational layout,
which produce the results.
Editor,
,
.'
Wireless World.
♦
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Basic Design Requirements:
Alternative Specifications
ECENT
R
improvements in the
field of commercial sound
recording havc made prac
ticable the reproduction of a
wider range of frequencies than
hitherto.
The useful range of
shellac pressings has been ex
tended from the limited 50-8,000
c/s which, with certain notable
exceptions,
has been standard
from 1930 until the present, to a
range of some 20-15,000 c/s. This
increase in the frequency range
has been accompanied by an
overall reduction in distortion and
the absence of peaks, and by the
recording of a larger volume range,
which combine to make possible a
standard of reproduction not pre
viously attainable from disc re
cordings. Further improvements,
notably the subst itution of low
noise plastic material for the

present shellac composition, are
Likely to provide still f urther
enhanced performance.
The resumptio n of the tclcvision
service with its first-class sound
quality. and the JX>S.<;ible extension
of
u.h.£.
high-quality
trans
missions, increase the available
sources of high-quality soun d
Full utilization of these record·
inss and transmissions demands
reproducing equipment with a
standard of performance higher
than that whieh has served in the
past. Extension of the fr equenc y
range, involving the prcs<:nce of
large-amplitude low-frequency sig
nals, gives greater likelihood of
intermodulation distortion i n the
reproducing system, whilst the
enhanced treble response makes
this type o( di�1.ortion more
readily detectable and undesirable.
Reproduction of sound by elec.
trical means involves the ampli
fication of an electric al waveform
whi ch should be an exact counter
part of the air pressure waveform
which constitutes the sound. The
purpose of the amplifler is to
produce an exact replica of the
electrical input voltage waveform
at a power level suitable for the
.
operation
of
the loudspeaker.
This in turn reconverts the elec·
trical waveform into a corres
ponding sound pressure waveform,
which in an ideal system would
be a replica of the original.
The perfor m ance of an amplifier
int ended to reproduce a given
waveform is usually stated in
terms of its ability to reproduce
accurately the frequency com
ponents of a mythical Foutier
analysis of the waveform. While
this method is convenient and
indeed corresponds to the manner
in which the mechanism of the
car analyses sound pressure wave
forms into component frequencies
and thcreby transmits intelligence
to the brain, the (act that the
function of the s),stem is to repro
duce a waveform and not a band
of frequencies should not be
neglected. Sounds of a transient
nature having identical f requency
contents may yet he very different
in char acter, the discrepancy being
in the phase relationship of the
component frequencies.
The requirements of such an
amplifier may be listed as :(1) Negligible non-linear dig·
tortion up to the maximum rated
output.
(The term
non- linear
distortion" includes the produc
tion of undesired harmonic fre·
quencies and the intermodulation
of component frcquencies of the
sound wavc. ) This requir� that
the dynamic output/input char
acte ristic be line.."l.r within close
limits up to maximum output at all
freque ncies within the au dible
"
range.
(2) (a) Linear frequency re·
sponse within the audible fre·
quency spectrum of 10-20,000 C/S.
(b) Constant power handling
capacity for negligible non-linear
distorti on at any frequency within
the audi ble frequency spectrum.
This requirement is less strin
gent at the high-frequcncy end of
the spectrum, but should the
maximum power output/frequency
response at cither end of the
7
spectrum (but especially, at the
low frequency end) be substan
tially less than that at medium
freq uencies,
filters
must
be
arrange d to reduce the level of
these frequcncies be/ore they reach
the amplifier as otherwise severe
intermo dul ati on will occ ur
This
is especially noticeable during the
reproduction of an organ on
incorrectly designed equipment
where pelial notes of the order of
16-20 c / s cause bad distortion,
even though they may be in
audible in the sound output.
-
.
(3) Negligible phase shift with
in the audible range.
Although
the phase relationship bctweell
the componcnt frcquencies of a
complex steady-state sound ooes
not appear to affect the audible
quality of the sound, the same is
not true of sounds of a tra nsien t
nature, the quality of which may
be profoundly altered by disturb
ance oC the phase rel at ionsh ip
between c omponent frequenc ies
(4)
.
Good transient response. In
addition to low phase and fre·
quency distortion, other factors
which arc essential for the accu
rate reproduction of transient
wave-forms arc the elimination of
c hanges in effective gain due to
current and voltage cut-off in any
stages, the utmost care in the
de sign of iron-cored components.
and the re duct ion of the n umber
of such components to a minimum.
Changes in effective gain during
" low· fr equency " transients occur
in amplifiers with output stages
of the self·biased CIa.'iS AB type,
causing serious distortion which
is not revealed by steady-state
measurements.
The
transient
causes the current in the output
stage to ris e. and this is followed,
at a rate detennined by the time
constant of the biasing network,
by a rise in bias voltage which
alters the effective gain of the
amplifier .
(5)
This
with
Low
output
resistance.
requirem ent is concerned
the attainmcnt of good
The salient fea
of
these
tures
"
,
methods are of
interest.
Push-pulltriodc
without
valves
IMPUT
INPUT
the refinement of
negative
feed
back
form the
mainstay of prehighsent-day
fidelity
equip(a)
(b)
ment. A stage of
Fig. I. Output/input characteristics (a) without
this type has a
f�dback (b) with negative feedback.
number of
dis
advantages.
With
reasonable
frequency and transient response
.
efficiency III the power stage
from the loudspeaker system by
such an arrangement cannot be
ensuring that it has adequate
made to introduce non-linearity to
electrical damping.
The cone
an extent less than that represen
movement of a moving-coil loud
ted by
about
2-3
per
cent
speaker is restricted by air loading,
harmonic distortion. The output/
suspension stiffness and resistance,
input characteristic of such a stage
and electro-magnetic damping. In
is a gradual curve as in Fig.
the case of a baffle-loaded loud
1 (a). With this type of characteris
tiipeaker, the efficiency is rarely
tic distortion will be introduced at
higher than 5-10 per cent, and the
all signal levels and intermodula
air loading, which determines the
tion of the component signal
radiation, is not high.
In order
frequencies will occur at all levels.
to avoid a high bass-resonance
The intennodulation with such a
frequency, the suspension stiffness
characteristic is very considerable
in a high-grade loudspeaker is
and is responsible for the harsh
kept low, and obviously the power
ness and "mushiness" which
10tifI in such a suspension cannot
characterizes amplifiers of this
be large. Electra-magnetic damp
type.
In addition, further non
ing is therefore important in
linearity and considerable inter
controlling the motion of the cone.
mcxiulation will be introduced by
This effect is proportional to the
the output transformer core.
current which can be generated
If the load impedance is chosen
in the coil circuit, and is therefore
to give maximum output the
proportional to the total resistance
load impedance/output resistance
of the circuit. Maximum damp
ratio of the amplifier will be about
ing will be achieved when the coil
2, which is insufficient for good
is effectively short-circuited, hence
loudspeaker damping.
the output
resistance
of
the
It is difficult to produce an
amplifier should be much lower
adequate frequency response char
than the coil impedance.
(6) Adequate JX1wer reserve.
acteristic in a multi-stage ampli
The
realistic
reproduction
of
fier of this type as the effect of
multiple valve capacitances and
orchestral music in an average
room requires peak power capa
the output transformer primary
and leakage inductances becomes
bilities of the order of 15-20
watts when the electro-acoustic
serious at the ends of the a.f.
transducer
is
a
baffle-loaded
spectrum.
moving-coil loudspeaker system
The application of negative feed
The use
of normal efficiency.
back to push-pull triodes results
of horn-loaded loudspeakers may
in the more or less complete sol
reduce the power requirement to
ution of the disadvantages out
the region of 10 watts.
lined above. Feedback should
be applied over the whole am
The Output Stage
plifier, from the output transform
An output of the order of 15-20
er secondary to the initial stage as
watts may be obtained in onc of
this method corrects distortion
three ways, namely, push-pull
introduced by the output tran!'>
triodes, push-pull triodes with
fonner and makes no additional
negative feedback, or push-pull
demands upon the output capabili
tetrodC5 with negative feedback.
ties of any stage of the amplifier.
�V
/
S
The functions of negative feed
back are:(a) To improve the linearity
of
the amplifier, and output
transformer.
(b) To improve the frequency
response of the amplifier and
output transformer.
(cl To reduce the phase shift
in the amplifier and output trans
former within the audible fre
quency range.
(d) To improve the low-fre
quency characteristics of the out
put
transformer,
particularly
defects due to the non-linear
relation between flux and magne
tizing force.
(e) To reduce the output
resistance of the amplifier.
(/) To reduce the effect of
random changes of the para
meters of the amplifier and supply
voltage changes, and of
any
spurious defects.
A stage of this type is capable
of fulfilling the highest fidelity
requirements in a sound repro
ducing system. The output/input
characteristic is of the type shown
in Fig. 1 (b), and is virtually
straight up to maximum output,
when it curves sharply with the
onset of grid current in the out
put stage.
Non-linear distortion
can be reduced to a degree repre
sented by less than o. I per cent
harmonic
distortion,
with
no
The
audible
intermodulation.
frequency respon� of the whole
amplifier from input to output
transformer secondary can be
made
linear,
and
the
power
handling capacity constant over
a range considerably wider than
that required for sound reproduc
tion.
The output resistance, upon
which the loudspeaker usually
depends for most of the damping
required, can be reduced to a
small fraction of the speech coil
impedance. A ratio of load im
pedance/output resistance (some
times known as "damping fac
tor") of 20-30 is easily obtained.
" Kinkless" or "beam" out
put tetrodes used with negative
feedback can, with care, be made
to give a perfonnance midway
between that of triodes with and
without feedback. The advantages
to be gained from the use of
tetrodes are increased power effi
ciency and lower dri ve voltage
requirements.
It must be emphasized that the
characteristics of the stage are
dependent solely upon the char
acter and amount of the negative
The feedback
feedback used.
must
remain
effective
at
all
frequencies
within
the
a.f.
spectrum under all operating con
ditions, if the quality is not to
degenerate to the level usually
associated with tetrodes without
feedback.
Great care must be
taken with the design and opera
tion of the amplifier to achieve
this, and troubles such as parru,itic
oscillation
and instability
arc
liable to be encountered.
When equipment has to be
operated from low-voltage power
supplies a tetrode stage with
negative feedback is the only
choice, hut where power supplies
are not restricted, triooes arc
preferable because of case of
operation and certainty of results.
It appears then that the design
of an amplifier for sound reprO
duction to give the highest possible
fidelity should centre round a
push-pull triode output stage and
should incorporate negative feed
back.
The most suitable types of valve
for this service are the PX25
and the KT66. Of these the KT66
is to be preferred since it is a
more
modern
indirectly+heated
type with a 6.3-volt heater, and
will simplify the heater supply
problem. Triode-connected it has
characteristics
almost
identical
with those of the PX25.
Using a supply voltage of some
440 volts a power output of 15
watts per pair may be expected.
The Output Transformer
The output transformer is prob
ably the most critical component
in a high-fidelity amplifier.
An
incorrectly designed component
is capable of producing distortion
which is oftcn mistakenly attribu
ted to the electronic part of the
amplifier.
Distortion prodllciblc
directly or indirectly by the
output transformer may be listed
as follows :(a) Frequency distortion due
to low winding inductance, high
leakage reactance and resonance
phenomena.
(b) Distortion due to the phase
shift
produced
when negative
feedback is applied across the
transformer.
This usually takes
the form of parasitic oscillation
due to phase shift produced in the
high frcquency region by a high
leakage reactance.
(cl Intermodulation and har
monic distortion i n the output
stage caused by overloading at low
frequencies when the primary
inductance is insufficient. This is
primarily due to a reduction in
the effective load impedance below
the safe limit, resulting in a very
reactive load at low frequencies.
This may cause the valves to be
driven beyond cut-off since the
load ellipse will tend to become
circular.
(d) Harmonic and intermodula
tion distortion produced by the
non-linear relation between flux
and magnetizing force in the core
material. TillS distortion is always
present but will be greatly aggra
vated if the flux density in the
core exceeds the safe limit.
(el Harmonic distortion intro
duced by excessive resistance in
the primary winding.
The design of a practical trans·
former has to be a compromise
betwccn these conflicting require
ments.
At a low frequency Ib' such that
the reactance of the output trans
former primary is equal to the
resistance formed by the load
resistance and valve a.c. resist
ances in parallcl,
the output
voltage will be 3db below that at
medium frequencies. At a fre
quency 3/b the response will be
well maintained, the transformer
reactance producing only 200phase
angle.
Similarly at the high
freq llency end of the spectrum the
response will be 3db down at a
frequency It such that the leakage
reactance is equal to the sum of
the load and valve a.c. resistances.
Again at a frequency It/3 the
B
c
A
ALTERNATlNC
EXCITATION VOLTAGE
Fig. 2. Variation of iron-cored
inductan«; with a,c. excitation.
9
response will be well maintained.
If then the required frequency
range in the amplifier is from
10-20,000 eIs, fb may be taken as
3.3 cls and It as 60 ke/s. A tram;·
former which is only 3db down at
frequencies as widely spaced as
these would be difficult to design
for some conditions of operation.
and where this is so the upper
limit may be reduced, Uti the
energy content of sound at thes:!
frequencies is not usually high
The limiting factor will be the
necessity of achieving stability
when feedback is applied across
the transformer, i.e., that the loop
gain should be less than unity at
frequencies where the phase shift
0
reaches 180 .
To illustrate the procedure,
consider the specification of an
output transformcr coupling two
push-pull KT66 type valves to a
15-ohm loudspeaker load_
Primary load impcdance= IO,OOOD:
"
Turns rat10=
J
IO,OOO
'5
= 25.
___
8
:1
Effective a.c. resistance o f valves
= 25000
Low-frequency Response
Parallel load and valve resistance
=
2500 X 10,000
12,500
=
2oo0D:
Ib
3·3 c/S(wb�21) response
should be 3db down.
Primary incremetllal inductance
=
2000
L � -- =95 H,
"
fligh-frequency Response
Sum of load and a.c. resistance:;
=
10,000 + 2500
= 125°0 n
At I, = 60 kc/s (Wj = 376,000.)
response should be 3db down.
Leakage reactance
=
=
12,500
�
33
mHo
A 20-watt transformer having 10
primary and 8 secondary sections
and using onc of the bettec grades
of core material can be made to
comply with these requirements.
Winding data will be given in an
appendix (see page 11).
Some confusion may arise when
specifying an output transformer
as the appar
. ent inductance of
the windings will vary greatly
with the method of measurement.
The inductance of an iroll-cored
(,)
OtJTPUT
STAi;E
Fig. 3.
Block diagrams
of
ci rcuit arrangements discussed
component is a. function of the
excitation, the varia.tion being
of the form shown in Fig. 2. The
exact shape of the curve is
dependent on the magnetization
characteristic for the core material .
The
maximum
in(lucbnce,
corresponding to point C o:.;curs
when the core material is nearing
saturation and is commonly 4-6
times the "low excitation" or
"incremental' value at A, which
corresponds to operation near the
origin of the magnetization curve.
]n a correc1.ly designed oUlput
transformer the primary induct
ance corresponding to the voltage
swing at maximum output at
50 cls will lie in the region of B
in Fig. 2.
III spccifying the component,
the important value is the incre
mental illducta!ICe corresponding
to point A, since this value deter·
mines the frequency response at
low outputs.
phase Shift
The reduction of phase shift in
amplifiers which arc to operate
with negative ffcdback is of
prime importance. as instahility
(c)
in
the text.
will reauIt, should a phase shift
of 1800 occur at a frequency where
t.he vector gain of the amplifier
and feedback network is greater
than unity. The introduction of
more than one t.ransfonner into
the feedback path is likely to
give risc to troublc from insta
bility. As it i� desirable to apply
feedbaek over the output tran!l
former the rest of the amplifier
should be R-C coupled.
Alternative Circuits
Although the amplifier may
contain push-pull stages it is
desirable that t.he input and output
should be "single ended" and
have a common earth terminal.
Three circuit arrangements suggest
themselves.
The block diagram of Fig. 3 (a)
shows the simplest circuit arrange
ment. The output valves are
preceded by a phase splitter
which is driven by the first stage.
The feedback is taken from the
output transformer secondary to
the cathode of the first stage.
This arrangement is advantageous
in that the phase shift in the
amplifier can easily be reduced
>0
to a low value as it contains the
minimum numhcr of stages. The
arrangement, however, has a
number of disadvantages which
render it unsuitable. The input
voltage required by the phase
splitter is rather more than can
be obtained frolll the first stage
for a reasonable distortion with
the available h.t. voltage, and in
addition the phase splitter is
operating at an unduly high level.
The gain of the circuit is low even
if a pentode is used in t.he first
stage, and where it low�impedance
loudspeaker system is used. in·
sufficient feedback voltage will
be available.
The addition of a push-pull
driver stage to the previous
arrangement, as in Fig 3 (b).
provides a solution to most of thc
difficulties. Eaeh stage then works
well within its capabilities. The
increa&.d phase shift due: to the
extra stage has not been found
unduly troublesome provided that
suitable precautions are taken.
The functions of phase splitter
and push-pull driver stage may
be combined in a self-balancing
" paraplJase" circuit giving tl:c
arrangement of Fig. 3 (c).
The
grid of one drive valve is fed
directly from the first stage, the
other being fed from a resistance
network between the anodes of
the driver valves as shown in
Fig. 4. This arrangement forms
a good alternative to the preceding
onc where it is desirable to use the
minimum number of valves,
I---�HIOUTPUT
�WV--r'ww+--l'1c
R
Fig. 4.
" Paraphase"
circuit
combining
the
functions
of
and push-pull
phase splitter
driver stages.
Details of Chosen Circuit
and Its Performance
THE
consider ation s
under
lying the de sign of a h igh
quality amplifier were dis
cussed in the first part of th i s
article. A circu i t of the complete
amplifier is shown in Fig. 5. This
foll ows the basic a rrangemen t of
Fig. 3(b). The des ig n of the indi
vidual sta ges will not be treated
i n detail, but a review of the
s<tlient features may be of value.
As a measure of standardization
an valves except those of the out
pu t stage ar e type L63, triodes of
about 8,ooo ohms a.c. resistance.
Initial Stages.-In order to
Fis.5.
keep the ph ase shift in the ampli
fier at low frequencies as small as
possible the first stage has been
directly coupled to the phase
splitter,
el i min atin g
one R·C
coup lin g . The first two stages are
thus des igned as a single entity.
The phase-splitfer section, which
consists of a triorle with equal
loads in anode and cathode cir
cuits, operates partly as a cathode
follower, its grid being some 100 V
pos i tiv e with respect to chassis.
The anode of the first triode is also
arran g ed to be about 100 V pos i
tive and is coupled to t he pha se
spli t ter g rid . Due to the cathode
follower action of V, t he operating
conditions arc not critical and no
trouble is likely to be encountered
from normal changes in valve
parameters.
resistor of V" t o
The
calhode
bias
which feedback
is applied from the outpllt tra ns
forIller secondary, is .kept a.s small
us p racticab l e to av oid gain reduc
tiOll ill the first l>tage, due to series
feedback.
Driver Stage_-The ontpu t from
the phase-splitter is taken to the
push-pull driver stage. Provision
is ma.de for var yin g the load rc
Circuit diagram of complete amplifier. Voltages umrerlin!d are peak signal vcltages at IS watts output.
I MO t watt ± 20 per
R)
R2
aa,ooo n I watt ± 20
U"
47,000 n I watt ± 20
H,,,,
470nlwatt:l:10
n5• RG• R7 22,000 n 1 watt ± 10
Ra, Rg
0.47 MO: i Wi\tt ± 20
ItJ{)
390 n i watt ± 10
39,000 0: 2 watt ± 10
Rn. R13
25,000 0 1 watt wireR12
wound variabltl.
0.1 MO i watt ± 20
Ra, RIll
cent
"
..
..
"
..
..
CIRCUIT VALUES
n iwatt ± 20 por ('cnt
Joo n I watt ± 20
100 n 2 watt wirc1.000
wound v-ariable.
1[;0 n 3 WlItt ± 20
100 n i watt ± 20
"
..
1,200v'spcecil coil impedance,
! wntt.
S "V 450 \', wkg.
0.05 p.l' 350 V, wkg.
9.2;) pJo' ;w> V. wkg_
"
c,
C,
CH,
CH,
"
Sill' 600 V, wkg.
:lO H at 20 lOA (min.)
IQ II at 100 mA (min.)
Power tran�formcr.
Seeondarv <!25·0-<!25 V.
l;)() mA ( in.) 5 V. 3A,G.a
V. 4A, c.t.
Lo3
KTOO.
1152
8 !.F 550 V, wkg.
m
The Witliamson Amplifier
,.
-
0
•
,
-
"
,
•
•
:;
-
•
,
,
WITHOUT fE(DUC�
WITH FEEDSAC�
�
>
•
•
,.
>
.,
"
,.
IMPUT VOlTS
(� .. s.)
.
"
...
•
•
�
•
Fig. 6.
Input-output characteristic and harmonic
distortion curves, with and without feedback.
(Right)Oscillograms of input-output characteristic ; left-hand
column, without feedback; right-hand column, with
feedback. (I) At 300 c/s with slight overload (2) At 300
eIs, output voltage 15% below maximum, (3) and (4)
Conditions as in (1) and (2) respectively, but at 30 e/s.
sistors of this stage which, in COll
junction with a common unby
passed cathode bias resistor.
allows a considerable range of
adjustment to be made in the
drive voltagcs to the output valves
to compensate for any inequality
in gain.
Output S tage. The balance of
quiescent anode current in the
output stage is a matter of some
importance, as it alTects the per
formance of the output trans
former to a marked degree . In
this amplifier, provision is made,
by means of a network in the
cathode circuits of the KT66
valves, for altering the grid bias
of each valve, giving complete
control of the static conditions of
the stage.
A feature of this
arrangement is that the valves
operate with a common unby
passed cathode bias resistor,
which assists in preserving the
balance of the stage under
dynamic conditions.
Output
Transfonner.
The
turns ratio of the output trans
former will be determined by the
impedance of the loudspeaker
load. It is convenient to make
each secondary section of such an
-
-
impedance that by series-parallel
arrangement a number of suitable
load impedances may be provided
utilizing all the sections of the
transformer. A suitable value of
impedance is 1.7 ohms per sec
tion, giving alternatives of 1.7,
6.8, 15.3, 27 ohms, etc.
Winding data for a suitable
transformer are gi ven in the
Appendix.
Negative Feedback Network._
The design of this amplifier is such
that no difficulty should be experi
enced in U1e application of nega
tive feedback up to a maximum
of some 30 db. Provided that the
threshold of instability is not
reached, the benefits of negative
feedback increase as the amount
of feedback is increased, at the
sole expense of loss of gain, but
there will be little if any audible
improvement to be gained with
this amplifier by increasing the
amount of feedback beyond 20db.
The feedback network is a
purely resistive potential divider,
the bottom limb of which is the
cathode bias resistor f)f the first
stage.
With component values as
<2
specified no trouble should be ex
perienced from instability due to
the effects of unintentional posi
tive feedback. Should instability
arise it will probably appear as
oscillation at a supersonic fre
quency. This may be transient,
occurring only at some part of the
cycle when the amplifier is oper
ated near maximum output. Its
c.."tuSC may bc bad layout or an
output transformer with a higher
leakage reactance than specified,
or it may be due to resonance in
the output transformer.
A remedy, which should only
be used as a temporary measure,
is to reduce the high-frequency
response of one of the amplifier
stages, so reducing the loop gain
at the frequency of oscillation to
a value below unity. This may
conveniently be done by connect
ing a small capacitor (s.."\y 200 pF)
in series with a 5,000 n resistor
from the anode of VI to chassis.
Performance
Linearity.-Thc
linearity of the
ampli.fier is well illustrated by the
series of oscillograms. These show
that, up to maximum output, the
linearity is of a high order, and
that the overload characteristic is
of the desirable type shown in
Fig . I(b) in the previous Issue.
The improvement due to the
application of negative feedback,
especially at low frequencies. is
clearly
demonstrated
by
the
oscillograms.
Equipment for measuring inter
modulation
products was
not
available, but measurement of the
total
harmonic distortion
was
made with an input frequency of
400 c I s. The result is shown in
Fig. 6, from which it will be seen
that the harmonic distortion at
maximum rated output (15 watts)
is less than 0 . 1 per cent. Inter
modulation. with this degree of
linearity. is not present to an
audible degree.
Frequency Response.-Thc fre
quency response of the amplifier
is greatly dependent upon the
characteristics of the output trans
fonner. In the amplifier tested,
the output transformer had a
resonance at about 60 kc / s which
caused a sharp dip of 2.6db
around this frequency. The char
acteristic within the audible range
from 10-20,ooo c / s is linear with
n
i
o . 2 db.
Phase Shift_-The excellence of
the frequency response charaeter-
db
8
i tic indicates that little phase
shift is present. Phase shift is only
apparent at the extremes of the
a.£. spectrum and never exceeds a
few degrees.
Output Resistance.-The out
put resistance of the amplifier is
0.5 ohms measured at the Is-ohm
output terminals.
Noise Level.-In the amplifier
tested, the measured noise level
was 85 db below maximum output.
The noise i n this amplifier was,
however, almost entirely s o c / s
hum, caused by coupling between
the mains and output trans
formers.
By more careful ar
rangement of these components it
appeared that the noise level
could be reduced to better than
lOodb below maximum output.
If desired, the power output of
the amplifier may be increased
beyond IS watts by the use of
several pairs of output valves in
parallel push-pull.
The output
transformer, power supply and
bias arrangements, and the feed
back resistor R.. will require to
he modified. Amplifiers of this
design with power outputs up to
70 watts have been produced.
Listening tests carried out in
conjunction with a wide-range
loudspeaker system havc fully
+I
"
I
0
8
0
0
fREQUENCY IN CYClES
HR
SECOND
Frequency response (without feedback) of 20 watt output trans
described in appendix.
Generator resistance 2,5000 load
resistance 1.7n.
Measured with SV r.m.S. on primary. At higher
excitations the bass response improves progressively up to saturation.
Fig. 7.
former
supported th� ml::'<1sured pt'rform
ance.
No distortion can be de
tected. even when the amplifier is
reproducing organ music includ
ing pedal notes of the 20 (,, / S order.
which reach the threshold of
maximum output. Transient8 are
reproduced with extreme fidelity;
tests using a dircct microphone
circuit with noises such as j ingling
keys reveal extraordinary realism.
The amplifier can be de!;Cribed
as virtually perfect for sound
reproducing channels of the high
est fidelity. It provides an ideal
amplifier for sound-recording pur
poses,
where
" distortionless ..
amplification and low lIoise level
art: of prime importance.
APPENDIX.
Output Transrormer.
Specification .
Primary load impedance
=
10,000 ohms c.t.
Secondary load impedance
1.7 ohms per section.
=
= 76 : I .
Turns ratio
Primary inductance= 100 H
(min.)
Leakage inductance= 30 mH (max.)
Winding. Data.
COre: �lin 5tack of Pattern No.
�8A .. Super Silcor " laminations
(.\'lagnetic and EIt:ctrical Alloys,
Burnbank, J lamilton, Lanarks).
The
winding consists of two
identical interlcaved coih. each Ilin
wide. wound Oil liin x I l i n pax01in
formcf';. On each former i!; wound:
5 primary sections each c()n!;i�tin�
of 5 layer.; (8B turns per layer)
of 30 s.w.g. enamelled copper wire
interleaved with 2 Illil . paper, alter
nating w ith 4 secondary sections.
each cOllsi�ting of 2 layers (29 turns
per layer) of 19 s.w.g. enum. copper
wire, interleaved with :l mi l. p;ll�r.
Each section is insulated Jrom it�
llI�ighbour5 by ;.I layers of 5 mil.
Empire tape.
All connections are
brought out on one side of the wind
i ng. but the primary section!; may lJe
connected in series when winding.
only two prilllury connec;tioll!; per
coil being brought out.'"
Measured Performance.
Primary inductance = 100 H.
(measured at 50 ers wit h .'i-V r.m.s.
on primary, equivalent to �.5 mW)
Leakage inductance = 2� mHo
(a) Input waveform, 300 c/s. (b) Output wavefonn with feedback and
slight overload. (c) Output waveform with feedback but output voltage
15% below maximum.
'3
(measured at 1,000 cts)
Primary resistance
=
�50 ohms.
.. Secondary cQ1mectio"sfor different
ratios tue given in the Table OH p.17.
The WiIliamson Amplifier
The New Version
Design Data : Modifications: Further Notes
INCE the publication in the
April and May, 1947. issues
S of Wireless World of an am
plifier design suitable for high
quality reproduction of sound.
correspondence bas revealed that
a morc complete explanation of
some of the features of the design,
with the addition of some informa
tion about construction, would be
of interest.
The correspondence
also shows that considerable de
mand cxlsts for a pre-amplifier
unit to enable the amplifier to be
used in conjunction with gramo
phone pickups and microphones 01
low output. In the present article
it is proposed to deal witb the
amplifier,
and
in
subsequent
articles to present the design of
auxiliary equipment to form a
domestic sound-reproducing in
stallation.
The list of
Circuit Diagram.
component values are printed
again. These differ in minor detail
from the originals . In tile circ uit
previollsly printed a potentio
meter, RIl• was provided in the
penultimate stage to enable the
signal to be balanced . Due to the
use
of
common
unbypassed
cathode resistors for the push-pull
stages, the amplifier is largely
self-balancing to signal, and it is
permissible to dispense with this
adjustment. Accordingly, revised
values and tolerances are shown
for resistors Rs' RI' RIL and RIJ "
A transiti onal phase-Shift net
work consisting of R•• and Cu
which
was previ ollsly
recom
mended as a temporary measure,
has been added as a permanent
feature to increase the margin of
stability at high f requencies . This
will be discu ssed later when the
stability of the ampli fier is con
sidered.
Finally, an indirectly - heated
rectifier has been substituted as
this p!"cvenls a damaging voltag£'
surge
when
the
amplifier is
switched all. No suitable type was
'.0
Fig. 1_
Circuit diacram of complete amplifier.
J watt ± 20%
I watt ± 20%
1 watt ± 20%
".
4-iOn
R,
t watt ± 10%
R" R7 22,0000 1 watt ± 5%
R,
",
IMO
33,OOOn
47,OOOn
ma.teI H
.. od)
R.
22,0000
I
R"
3!lOO
i watt
(or
R•• R. 0.47Mn
wlllt ± 20%
;I watt ± 20,%
±
Ru,Ru 47,OOOn 2 watt ±
(ur matchro)
10%
5%
Voltages underlin� are peak s:gnal voltages at IS watts output.
H.u, RII,O.BHl
Rn, R2(J 1.0000
I{.I&, R l8 1000
RI?' R2IlOOO
J watt ± 10%
watt ± 20,%
1 watt. ± 20%
2 watt. wirewound
variable
3 watt ± 20%
t watt ± 20%
Cs, C1
C,
c"
-i wa.tt (see tabla)
V3, V.
i
CH,
CIll!
0.2SI'F
350V wkg.
200pr'
a.
=;OV wkg.
81'F
30H
lOll
600V wkg.
at 20mA
at 150mA
R"
1500
R�3. Ru lOOn
1,200 V lpee<:b coil impedanOt
R26
Power transformer
T
Secondary 4.25_0_42.'lY 150 Ill."-. aY. 3A,
6.3V 4.-\., oollt.re-tappod
4,700n
t watt ± 20%
500V wkg.
Cl' O2, C6• Cs S/,l<'
C3• C.
0.05/,F 350V wkg.
'-a, Ve KT66
R"
'4
VI> V2 2 x L63 or 6J5, GSN7 or BM
"7 Coasor 53K U,
do.
do.
5V4
>0
"
0
"
"
0
1-- -
I
/
/
Wll
.=
-
!��t:�:H �:��(ri��(K
I
-
I
-
I
-
Fig.
2.
t-
tI
/
00
00p-
-
III
§
H
-
-
,
,
,
,
§
Loop gain and phase-shift characteristics of the amplifier.
available when the circuit was
originally published.
A list of
alternative valve types is also
shown.
Amplitude
and
Phase I frequency Response. A curve show
ing the transmission and loop
gain of the amplifier at frequencies
between I c l s and I Mc I s is shown
in Fig. 2 . Although only the sec·
tion between lO e / s and 20,ooo c / 5
is useful for sound reproduction,
the curves outside this range are
included as they may be of in
terest to those who may wish to
use the amplifier for other pur
poses.
They may also serve to
emphasize that. in a feedback
a.mpl:ifier, the response must be
carefully controlled at frequencies
very remote from the uselul ra.nge
if stability is to be achieved.
General
Constructional
Data.
The layout of the amplifier is not
critical, provided that a few
simple precautions are observed.
Many different arrangements have
been used S<1.tisfactorily to suit
differing circumstances.
An ex
cellent plan is to construct the
power supply and the amplifier on
separate chassis, as this gives
greater flexibility in accommo
dating the equipment in a cabinet.
The f 0 1 1 0 w i n g precautions
should be observed : 1 . The output transformer core
should be positioned at right
angles to the cores of the mains
transformer and the main smooth
ing choke.
2. The output transformer and
loudspeaker leads should be kept
at a reasonable distance from the
input leads, which should be
screened. As the response curve
Fig. 3.
Suggested
layout of principal
components of com
bined amplifier and
power pack.
shows, the amplifier has consider
able gain at low radio frequencies,
and care is necessary to avoid
oscillation.
3. Signal wires, especially grid
leads. should be kept as short as
possible, and the stopper resistors
associated with the output stage
must be mounted on the valve
holder tags, and not on grOl;p
panels.
-4. A bus-bar earth return
formed by a piece of 12 or 14
s.w.g. tinned copper wire, con
nected to the chassis at the input
end. is greatly to be preferred to
the use of the chassis as an earth
return.
5. Electrolytic
and
p a. p e r
capacitors should be kept away
from sources of heat, such as the
output and rectifier valves.
Figs. 3 and 4 show the positions
of the major components in two
alternative layouts which have
been used successfully.
Initial
Adjustments.
'5
Before
the amplifier is pUl into service
there arc a few adjustments which
require to be made.
These COIl
cern the balancing of the standing
currents in the output stage, and
(with the original circuit.) balanc
ing of the signal currents in the
push·pull stages.
Accurate balance of the stand
ing currents in the output stage
is essential, as the low-frequency
cha.racteristics of the output trans
former deteriorate rapid.ly with
d_e. magnetizatioll .
The proc�
dure to be adopted for static and
signal balancing is as follows : -
Slatic BalaJ1( illg.
(a) Connect a
suitable milli
ammeter in the lead to the
centre tap of the output tram,
former primary.
(b) Set the total current to
1 2 5 mA by means of H. , .
(c) Connect a moving-coil
voltmeter
(O-IO V
approx . )
across the whole of the output
v,
v,
OUTrtJl
lIlANSfORM£1I
"'�IMS
' '" '""' '
:: ''=--,
- �=======::'..J
I �='='::==
®0
(.ONN[tTlN� lI�K
Fig. 4.
Layout when using separate power pack.
transformer primary and adjust
R" until the reading is zero, in
dicating balance.
Random
fluctuations of this instrument
may be noticed. These are due
to mains and valve fluctuations
a.nd should be disregarded.
Signal Baltmc;'lg.
(a) Connect
the
low-impedance winding of a small out
put transformer in the lead to
the centre tap of the output
transformer. Connect a detector
(beadphonl::s or a cathode-ray
oscillograph if available) to the
other winding,
earthing one
side for safety.
(b) Connect a resistive load
in place of the loudspeaker.
(c) Apply a signal at a fre
quency of about 4oo e / s to the
amplifier input to give an out
put voltage about hall maxi
mum.
(d) Adjust RI. for minimum
output in the detector.
The Output Transfonner. As
stated
previously,
the output
transformer is tht: most critical
component in the a.mplifu::r and
satisfactory performance will not
be obtained with a. component
differing substantially from the
specification.
The effect of de
creasing the primary inductance
will be to produce instability a t
low frequencies. which can be
cured only by altering the time
constants or the other coupling
circuits. or by dt:Creasing the
amount of feedback. At high fre
quencies the situation is more
complex,
as
there
are
more
variables.
The leakage induc
tance. the self-capacita.nce of the
windings, the ca.pacitance between
windings and the distribution of
these parameters determine the
transmission of the c.omponent a.t
high frequencies, and great varia
tions arc possible.
I n the output transformer �peci
fied, the only parameter which is
likely to vary appreciably is the
inductance of the primary at low
signal levels. due to the use of
core material with a low initial
careless
permeability.
or
to
assembly of the core. The high
frequency characteristics are not
dependent on the core material to
They arc
a substantial degree.
dependent only on the geometry of
construction. and to some extent
upon the dielectric properties of
the insulants used, and are there
fore reproducible with a high
degree of accuracy.
Comments are frequently ex
pressed about the size of the out
put transformer.
It is true tha.t
it is considerably larger than the
transformers which are usually
fitted to 15-watt amplifiers.
The
fact that the peak flux density of
7,250 gauss for maximum output
at 20C/S lies on the upper safe
limit for low distortion is sufficient
comment on current practice.
Some confusion arose regarding
the method of connection of the
transformer secondary windings to
match
loads
of
various
im
pedances. whilst utilizing all the
secondary sectioDs.
The correct
is
p r i m a r y load impedance
10,0000 and as the turns ratio in
the original design is 7 6 : I the im
pedance of each secondary section
is lo,ooo nl 76� or 1 . 7 0 .
When
secondary sections arc connected
in parallel, the turns ratio, and
hence the impedance ratio, re
mains unchanged.
If now two
secondary sections, or sets oC
paralleled sections, are connected
in series the turns ratio is halved.
,6
and the secondary impedance.
being proportional to the square
of
the
turns
ratio,
becomes
I . 7 l!( 2'=6.80. Similarly if three
sections are connected in series the
impedance
becomes
I -7 x 3� "'"
15.30.
Thus the a v a i l a b l e
secondary impedances. keeping a
10,000 !l primary load impedance,
are 1.7, 6.8, 15·3, 27, 42.5, 61,
83 and I()()!l. The connections to
obtain these values are shown in
the table.
Should it be nectlssary, in an
emergency, to match loads of
other impedances to the ampli
fier, it is permissible to reduce the
primary load impedance to 6,000 n
giving another series of secondary
impedances, namely I, 4. g, 16,
25, 36, 49 and 64 n. Under these
conditions the power output wj]]
be increased slightly and the dis
tortion will be doubled.
The
value of the feedback resistor Ru
must remain unaltered, as the
turns ratio is unchanged.
The
values of R.� are given in the
table.
Winding data for an output
transformer to match loads in the
region of 3.5!1 are givcn in the
Appendix and the connections
and other data are included in the
lower section of the table.
The two outer layers of the
output transformer primary should
normally be connected together to
form the centre tap, the inner sec
tions of the winding being taken
to the valve anodes.
This gives
tile mmunum external electric
field.
Stability with Negative Feed
back.-Much has been written
about the stability of amplifiers
under conditions of negative feed
back, and the criteria for stability
arc now widely appreciated. The
article by
Cathode Ray " in the
May,
1949,
issue,
states
the
matter simply and with character
istic clarity.
Continuous oscillation will occur
in a feedback amplifier if the loop
gain-that is the transmission of
the amplifier and the feedback
network-is greater than unity at
any point where the phase shift
of the amplifier has reached 180°.
It is also possible for an amplifier
to be unstable in the absence of
continuous oscillation if these con_
ditions should occur in a transient
manner at a critical signal level.
This latter condition is particu
Jarly likely to occur in badly de..
signed amplifiers with iron-cored
components, where the inductance
and, therefore, the time constant
controlling the phase and ampli
tude chamcteristics of one OT more
stages may increase by as much as
a factor of five between zero and
maximum signal levels. If this
variable time constant is shorter
than those of the fixed coupling
circuits, an increase in its value
due to a high signal level may be
sufficient to render the system un
stable. In order to avoid this
condition the fixed time constants
must be made much longer than
that of the variable stage. This
condition would lead to undesir
ably large interstage couplings if
good low-frequency response were
required. Alternatively, the var
iable time constant must be
chosen in relation to the fixed
time constants, such that its mini
mum value is sufficiently longer
than the fixed values to produce
stability. An increase in its value
then serves only to increase the
stability margin. This method is
used in the amplifier under dis
.
CUSSlOn.
To ensure a wide margin of
stability, whilst at the same time
preserving the high loop gain
necessary to reduce the effect of
transformer distortion at fre
quencies of the order of IG-20 C/S,
would require a transformer with
a very large initial primary induc
tance. This would necessarily be
expensive, and a compromise
must be drawn between the three
factors.
Because of this, the
margin of stability must be kept
to the lowest practicable value.
When the amplifier is repro
duced, the " spread " in tolerance
of components will normally be
such that changes in character
istics due to departure from the
nominal value of one component
will be balanced by opposite
changes produced by departure in
another component, and the
amplifier as a whole is likely to
have characteristics close to the
average.
Individual amplifiers
may. however, have charac
teristics wbich differ substan
tially from the average, due
to
an upward or downward
trend in the changes produced by
component deviations.
If the
trend is in a direction such that
the loop gain is reduced, no in
stability will result, the only efiect
being a slight degrading of the
performance. If, on the other
hand, the loop gain is increased
by an amount greater than the
OUTPUT TRA.NSFORMERS.
--.
No. of seeondary
groups of sections
in series
I
Connections
--.
Original
Output
Transformer
10,000/1.70
Altcroative
Output
Trnmformer
(800 Appendix)
10,000/3.60
Correct seoondary
impeda.m.'e (ohms)
I
impedance
"Y
permissible (ohms
Minimum second.
Foodback resistor
R2r. (ohms)
2
J1 r J�
•
.JU1
L7
6.S
15.3
I
,
9
1,500
3,300
4,700
I.
6,800
3.6
l4A
32.5
57.5
2,200
4,700
6,800
9,000
52.5
26.25
17.5
13
"
"
27
19
:--:-
�
�
:::::
-
42.5
61
83
109
25
3.
49
64
8,200
10,000
11,000
12,000
15.2
12.6
10.8
'.5
17.
230
18,000
90
I
8
7
::-,,-
J�
Correct secondary
impedance (ohms)
25.4
•
�
�
� �
�
�
-11 -1� �
38
I
Assuming that the amplifier is,
or has been rendered, stable with
the feedba.ck disconnected, the
next step should be to check the
phase and amplitude character
istics at low frequencies. It is not
practicable to make direct mea
surements of these characteristics
without very special equipment,
as inspection of Fig. 2 will show
that the interesting region lies
•
•
76
Turns ratio
Primary Inductance
TA.BLE OF CONNECTIONS.
Turns ratio
Foodba.ck TCsistor
Ru (ohms)
margin of stability, oscillation will
occur. It should be emphasized
that this will happen only very
rarely, and when it does the
remedy is obviously to reduce the
loop gain to its corrcct value.
To assist the unfortunate few
who experience instability, the
following procedure is recom
mended.
If oscillation should
occur at a low frequency (about
2 C/S) the first step should be to
disconnect the feedback resistor
R.J• If the oscillation continues
the decoupling circuits should be
checked and any faulty compon
ents replaced.
The amplifier
should also he examined to ensure
that it is operating correctly
balanced in push-pull, and not in
an unbalanced manner due to the
failure of some component.
130
11,I'iOO
13,500
16,000
10.5
8.15
7.5
�,
below
10 c / s .
Tt is therefore
necessary to arrive at the desired
re::>ult by indirect means, namely
by measurement of the component
parameters which determine the
characteristics.
The parameter
which is most likely to show a.
large deviation from specification
is the initial primary inductance
of the output transformer, since
the quality of the COre material is
not easy to control accurately, and
careless assembly of the core may
cause considerable varia.tions in its
permeability.
The initial primary inductance
should be checked by connecting
the primary winding across the
5-V, 50-C / S rectifier heater wind
ing of the mains transformer and
measuring the current in it. The
secondary willdings should be on
open circuit. The current, which
can just be read on the 10 mA
8..c. range of a Model 7 Avometer,
should be Ijo p.A or lower.
The
component should be rejected if
the current exceeds 200 p.A.
If the output transformer is
satisfactory the values of the other
components should be checked,
particular attention being paid to
the coupling components. Should
the time constants of the coup
lings, that is their RC product, be
higher than the nominal values by
more than 20 per cent, the resis
tors should be adjusted to give
the correct value.
The trouble will probably have
revealed itself by this time, but,
if upon reconnectillg R'5 the oscil
lation is still present, it is very
likely to be due to the use of
valves with mutual conductances
higher than average, and it is
legitimate to increase the value of
If
R,s to reduce the loop gain.
instruments are available, the
loop gain may be measured by
disconnecting
RH
from
the
cathode of VI and reconnecting it
via a 470!l± 10 per cent resistor to
chassis.
The voltage gain, mea.
sured from the input grid to the
junction of RH and the 4700 re
sistor, should be 10 at frequencies
between
30c/s
and
IOkc/s.
Care must be taken not to over
load the amplifier when this mea
surement is being made.
The adjustment of the loop gain
to its correct value at medium
frequencies should render the
amplifier stable at high fre
quencies. It is unlikely that the
phase characteristic at high fre-
quencies of individual amplifiers
will deviate appreciably from nor
mal unless the layout is very poor
or the transformer is not to speci
fication.
Capacitive Loads
The
amplifier
is
absolutely
stable at high frequencies with a
resistive or inductive load, but it
is possible for oscillation to occur
when the load impedance is capa
citive at very high frequencies,
for example, when a long cable is
used to connect the amplifier and
loudspeaker. To avoid this pos
sibility, and to give an increased
margin of stability, a transitional
phase-shift network consisting of
R•• and C,o in conjunction with
the output resistance of V" has
been included in the circuit. This
has the effect of reducing the loop
gain at frequencies from 20kc/s
upwards without affecting the
phase shift in the critical region .
The use of a phase advance net
work consisting of a capacitor
shunting R.� has been advocated
as a means of stabilizing this
amplifier.
The effect of such a
network is to increase the loop
gain at high frequencies, at the
same time reducing the amount of
It is sometimes pos
phase lag.
sible by this means to steer the
phase curve away from the 180Q
point as the loop gain is passing
through unity, thus increasing the
margin of stability.
The connection of a capacitor
across Ra' however, will not
stabilize this amplifier if it bas
been consfructed to specification,
although it may produce improvc
menl if oscillation is due la some
brge departure from specifIcation,
such as the u�e of an output trans
former with completely different
high - frequency
characteristics.
The writer has no information
about this.
The use of separate RC bias
impedances for the output valves
has also been suggested .
This
procedure is not endorsed by the
writer, as there are numerous <1is
advant..-g
-t es in its use and no re
deeming features whatsoever. If
the time constant of the bias net
work is made sufficiently long to
ensure that the low-frequency per
formance of the amplifier is un
impaired, the phase shift of the
bias network will have its maxi
mum at or near the lower critical
frequcncy
and
may
provoke
,8
oscillation. If, on the other hand,
it is made sufficiently short to
avoid this, the ability of the
amplifier to handle low
fre
quencies will be impaired.
The
use of separate bias impedances
destroys the self-balancing pro
perties of the amplifier, and if two
dissimilar valves are used in the
output stage " motor boati n g " is
likely, due to the presence of sig
nal in the h.t. line. The perlorm
ance of the output transfonncr
may be seriously affected by the
out-of-balance current caused by
valves whose anode currents lie
within the manufacturer's toler
ance limits. Finally, there can be
little justification of this modifica
tion on economic grounds, as the
costs are roughly similar. Indeed,
if the question of replacement due
to failure is considered, the com
mon bias arrangement shows a
definite saving.
It is to be hoped that these re
marks on stability will not have
the effect of frightening those who
already possess amplifiers of this
type or are contemplating acquir
ing them.
Their purpose is to
help the occasional " outer limit "
case where instability is experi
enced, but if they serve to impress
upon the reader that negative feed
back amplifiCrs are designed as
an integral unit, and that any
modifications, however insignifi
cant they may appear, may seri
ously affect the performa.nce or
stability, a useful purpose will
have heen accomplished.
Such
modifications should be attempted
only by those who are confident
that they know what they are do
ing, and who have access to mea·
suring equipment to verify results.
APPENDIX
Output
Trun.;Jonner
with
3.6-ohm
Secondaries
Winding Dala
l1in. stack of 28A Super
Si1cor laminations.
(Magnetic and
Electrical Alloys, Burnbauk, Ham
ilton, Lanarks.). The winding con
sists of two identical interleaved
coils eaeh I �in. wide on paxolin
formers liin. X Ijin. inside dimen
sions.
On each former is wound
5 primary ficctions, each con_
sisting of 440 turns (5 layers, 88
turns per layer) of 30 s.w.g.
enamelled copper wire interleaved
with 2 mil. paper, alternating with
4 secondary sections, each con·
Core :
The WilIiamson Amplifier
sisting of 84 turns (2 layers. 42
turns per layer) of 22 s.w.g.
enamelled copper wire interleaved
with :1 roil. paper.
Each section is insulated from its
neighbours by 3 layers of .5 roil.
All connections are
Empire tape.
brought out on onc side of the wind·
ing. but the primary sections may be
connected in series when winding.
two primary connections ollly prr
holobin being brought out. \Villdings
to he assembled on core with the
bobhin reversed. and with iusulating
cheeks a nd centre spacer.
'Vhy the
lVILLlA1USON
A1UPLlFIER
should eDlploy
PARTRIDGE
TraosforDlers
THE widest possible audio range-the lowest possible distor
tion and an output of 20 watts . . . these critical demands of the designer of
this now famous Amplifier implied the finest that technical skill and craftsmanship
could provide for every component.
Little wonder that from the inception of the
Williamson Amplifier in 1947 Partridge have specialised in the transformers and
chokes.
The all important output transformer was the special care of Partridge and
this H Williamson specification " component is now available for a varied range of
impedance.
(A model is also available for American 807 tubes, see the modified
circuit in " Audio Engineering," November 1949.) All secondary windings are brought
out as eight separate sections of equal impedance.
Stock types comprise 0.95 ohm.
1.7 ohm, 3.6 ohm and 7.5 ohm sections ; this latter giving a 500 ohm secondary
for American requirements.
The Partridge
H
Williamson " Output Transformer
is acknowledged the most efficient 0/ its type.
Available
UN'DTTED
or
'DTTED
(Style
(Style
Technical data sheets wuh fullest details
(complete with alternatIVe mounrmgs) are
available on rt4uesl. Also available, complete
catalogue of the Partridge range including the
mains components for this and orher amplifiers.
VDN/436B)
VDN/436B)
PARfiPOGE
*
IMMEDIATE DELIVERY can be made.
and these components are ready for shipment
to all parts of the world.
ROEBUCK ROAD
:
KINGSTON BY·PASS
TRANSFORMERS LTD
TOLWORTH
19
:
SURREY
:
Elmbridge
6737f8
Design of Tone Controls and
M
OST
A uxiliary Gramophone Circuits
power amplifiers in
tended for sound repro
duction are designed to
have a uniform response to fre
quencies within the audible range,
and it is the aim of designers of
pickups, microphones and loud
speakers to give similar character
istics to their products. This re
presents an attempt to fulfil one
of the conditions for the creation
of a perfect replica of the original
sound and provides a common
basis for the design of individual
units, which, when connected to
gether. will provide a complete
channel with a uniform gain /
frequency characteristic.
Considerations of an engineer
ing nature sometimes make it de
sirable, and even essential, to
depart from this ideal of a uni
form response in certain soctions
of equipment, and quite fre
quently the use of inferior equip
ment or long and unsuitable trans
mission lines leads te an undesir
able departure from uniformity.
(n cases like this, other " equal
izer " units have to be inserted in
the channel to provide character
istics which are the inverse of
those of the offending section , so
remedying the defect.
When listening conditions de
part from the ideal-and this, un
fortunately, happens frequently
since most rooms are unsuitable
auditoria for the reproduction of
orchestral music at realistic in
tensities-it is sometimes bene
ficial to modify the frequency re
sponse characteristic of the equip
ment in an attempt to compensate
for the more obvious defects in the
word
The
acoustics.
room
" attempt " is used advisedly.
since only very complex equaliza
tion could ever hope to provide
accurate compensation for room
acoustics.
This question of the
frequency compensation which is
desirable when conditions depart
from the ideal is a very thorny
and subjective one. It provokes
heated,
dogmatic,
and
much
usually very unscientific discus
sion, and is beyond the scope of
source, and are capable of being
ameliorated.
In addition , fixed
compensation must be provided
(or deviations from a uniform re
sponse which are deliberately in
troduced in gramophone records.
The degree of complication
which is worthwhile in such a
unit must be considered.
In
theory, it is possible to compen
sate precisely for deficiencies in
the
amplitude/ frequency
and
phase / frequency response cbanc-
the present article. It must suffice
to say that the matter is one i n
whic h the individual must enr·
cise his own judgment and act
accordingly.
In order that he may have
scope to do this, a pre-amplifier
designed to be used in conjunction
with gramophone recordings and
radio transmissions should there
fore be capable of providing vari
able compensation for such de
fects as are likely to occur in the
,
..
IHPUT
fAU
C"
0..
I
III SE SJ,...-
TRE6l[
fAll
�I�E I
C"
I
e�ss
0·1
""
'"
BASS
:51
I
AIS(
' ..
MIN .
00>
'"
TREBLE
Cn
I
FlIt
::--
VALVE
G�ID
Cn
' ..
Fig. 5. Basic frequency compensation circuit. Typical
values (for use after an EF37. triode-connected) are !
R.cn. 3sokO, log ; �h lookO ; Ru. 6.8kO ; R.u,
lokO ; Rc•• IookO hnear. C20• ISopF mu. ; Cn.
o.oIl'F. Cu O'OSl'F ; CUt looopF.
I- "� (!i "
I
1'-,1
.
11
I' ;
f-- ,�
:.1,,[
I
�
'0
'''[ ( n
'�. ",'
FR£QUlIICY
(c/.)
""
I ,
V
r-
I
,
II
�
,
I
,i.
Fig. 6.
1 1111 -
�
Response curves of circuit of Fig. S.
teristics, but the equipment to do
this is complicated and expensive.
When a considerable portion of
the channel is outside the control
of the listener, as is the ca..<;e
when
reproducing
records
or
broadcast transmissions, he has
no means, apart from the sensi
tivity and training of his cars, of
determining the defects which
have occurred in that portion.
Since it is impossible to determine
the nature and amount of phase
distortion by listening to a trans
mission, and since it is not usual
for much attention to be paid to
this form of distortion at the re
cording or transmitting end, there
would seem to be little justifica
tion for the inclusion of phase
correcting networks in domestic
equipment.
In the case of a
sound reproducing system which
is completely under the control of
the user, particularly if ::.iereo
phonic, phase distortion should
not be allowed to occur if the
finest possible quality is to be ob
tained. This is especially true at
low frequencies, where consider
able time delays are involved.
Low phase distortion is best
achieved by designing a system
with a bandwidth considerably
greater than the audible range,
but where this is not possible com_
pensation may be provided.
Consideration of the causes of
frequency distortion leads to the
conclusion that it is normal for the
levels at the ends of the spt."Ctrum
to be accentuated or attenuated
progressively with respect to the
level at middle frequencies and a
form of compensation to correct
this fulfils most requirements. It
is not possible to lay down hard
and fast rules about the amount
of compensation necessary, hut
rates of attenuation or accentua
tion greater than 6db/ octave are
not usually required.
As it is often desirable to change
the amount of compensation dur
ing a programme without calling
attention to the fact, methods
which give continuous control
over the response are to be pre
ferred to switched systems, unless
the latter are graded in very fine
steps.
The use of inductors to provide
gain/frequency compensation is
to be deprecated as, apart from
possible troubles due to resonance
eflects and non-linearity, they are
very liable to pick up hum from
CURVE A lOOP r.AIN .. 0
B
.. 20
(
. .. 100
90°
.... �
"
-
w
....
/
-�
-
l
c'!:Vl
I�
Fig. 7.
'1--
I
1
Basic filter circuit.
stray alternating magnetic fields,
especially if they are air-cored ,
Metal- or dust-cored toroids are
less troublesome in this respoct,
but are expensive and not readily
obtainable.
Frequency Compensation.-Fig.
5 shows a simple compensation
circuit which will accomplish bass
and
treble
accentuation
and
attenuation without the use of in
The controls consist of
ductors.
two potentiometers, each asso
ciated with a changeover switch.
Consider the low frequency con
trols R •• and 52'
When RH is
fully anticlockwise (minimum re
sistance) the response to fre
quencies below I,OOOC / S is uni
form.
If the switch S. is set to
" rise," as R•• is rotated clock
wise,
the
amplitude/frequency
characteristic will rise at low fre
quencies to the maximum shown
at A in Fig. 6.
If 53 is set to
" fall .. and Rn rotated clockwise
from the minimum position, pro
gressive low-frequency attenua
tion will be introduced, up to the
maximum shown at B. In a simi
lar manner, by the use of R .. and
S. the high-frequency response is
continuously variable from a level
response to the extremes shown at
C and D with the values given.
The curves may be shifted bodily
along the horizontal axis by
modifying the capacitance values
as shDwn by the arrows in Fig. 6,
..
HEOUENCY
Fig. 8.
Characteristics
of Fig. 7.
of circuit
The attenuation introduced by
the Iletwork when controls are at
the level position is 24 db, and the
network must,
of COllrse, be
introduced into the system at a
signal level sllch that the valve
feeding is not overloaded.
Low-Pass Filter,-The majority
of medium-wave broadcast trans
missions, when reproduced with
wide-range equipment. exhibit a
most objectionable fonn of non
linear distortion.
This takes the
form of a mttle or buzz often
accompanying transient sounds
such as pianoforte music .
This
type of distortion is commonly
caused by minor discontinuities in
the transfer characteristic and is
frequently associated with Class
" B " amplifiers.
Recording and processing de
fects, record wear and imperfect
tracing by the pickup produce a
similar type of distortion from
gramophone records.
The most offensive frequency
components
of the
rattJe
or
buzz are generally present at
the extreme upper end of the
audible spectrum,
and spread
downwards as the severity of the
effect increases. Fortunately, the
concentration of this type of dis
tortion into the extreme upper end
of the spcctrum makes it possible
to effect considerable improve
IDent by removing or reducing the
energy in the signal at these fre
quencies. A low-pass filter with
a cut-oil frequency variable be
tween the limits of 5 and r3kc/s
and a fairly high rate of attenua
tion above the cut-off frequency
is a great asset in securing the best
possible aural result from indiffer_
ent transmissions or recordings.
Although it is practicable to
provide a. filter with a continu-
the response rises to a fraction of
its value below resonance and then
falls off due to the attenua
tion produced by the capacitor C.
The addition of a further R-C
attenuatin g network external to
the circuit will produce a Ire
quency response characteristic as
FREQUENCY
filter characteristic produced
Fig.
9.
Modification
of
additional phase shift.
basic
by
ously variable cut-off frequency,
the expense and complication are
not normally justified and a
switched selection of frequencies is
satisfactory.
To attain the high
attenu ation rates necessary to se
cure satisfactory results a norma}
resonant-section
type
of filter
could be used, but this carries
with it the disadvantages asso
ciated with the use of inductors.
An alternative type of filter
using only resistive a nd capac iti ve
elements based on the parall el -T
network' is capable of giving very
satisfactory results. Briefly. the
princip le of this filter is as fol
lows. In Fig. 7 is shown an ampli
fier feedi ng a parall el -T null net
work. the output from the net
work being fed back to the input
of the amplifier. Such a system
has amplitude and phase charac
teristics of the general shape
shown in Fig. 8 . By altering the
loop gain of the amplifier, it is
possible to produce a resonance
characteristic of any desired de
gree of sharpness.
If now a lagging phase shift is
introduced into the amplifier. for
example . by connecting the capa
citor C from grid to earth. it will
be seen that the total phase shift
due to network and amplifier j ust
below resonance will be greater
than 90" and the feedback volt
age will have a positive compon
ent, whilst above resonance a
greater negative component will
exist. The effect of this is to un
balance the amplitude character
istic as shown in Fig. 9.
A rise
in response occurs just before the
resonance frequency due to the
positive component of feedback,
and above the resonant frequency
. R·e filter Circuits."'
' Thie.sen. G. J.
Jo.m.�l 0/ the Aoou.lioal Sotiety of
Am",;o".
Vo1. 16. No. 4. pp. 275-279
April. 19-15
FREQUENCY
Fig. 1 0. Final low-pass charac
teristic resulting from addition of
external R-C attenuator.
shown in Fig. 10. The similarity
of this curve to the response of a
resonant element L-C filter will
readily be appreciated. There is
a practical limit to th e rate of
atlenuation which can be achieved
with a single stage, since the
attenuation rate and the level to
which the response rises above the
frequency of maximum attenua
tion are interrelated . Thus a high
rate of attenuation is achieved
with simplicity only at the expense
of a low ratio of response below
cut-off to peak response above
cut-off.
However. a rate of
attenuation of 40 db / octave can
be obtained from one stage with
a minimu m attenuation above
cut-off of nearly 30 db, which is
quite satisfactory .
By cascading
a number of these filter stages any
desired attenuation characteristics
may be achieved. and high-pass
filters may be similarly formed by
the addition of leading phase shift
to the amplifier.
A filter desi gned on these lines,
with five switched positions giv
ing nominal cut-off frequencies of
5· 7, 10 and 13 kc/s and a
" lin ear " position is inc orporated
in the final circuit. The perform
ance is shown in Fig. 1 1 .
Gramophone
Pee.amplifier.
The arrangements just described
are generally all that is necessary
to compensate for defects in radio
transmissions. For record repro
duction. howe ver. additional fixed
compensation is required .
The
nature of this compensation will
depend on the rec ording charae22
teristic and the type of pickup
used.
to require repetition. lateral disc
recordings are usually cut with a
groove amplitude which i!l propor+
tional to signal below some arbi
trarily selected frequency in the
300-4oo C / s region and with a
lateral groove velocity which is
proportional to signal above this
For reasons now too well known
frequency.
To improve signal /
noise ratio it is now common prac_
ttee to increase the level recorded
at high frequencies. This is par
tic ularly effective, since the noise
energy per cycle increases with
frequency due to tbe structure of
the record material . In Fig. 12 is
shown the recording characteristic
used by Decca. The E.M.I. char
acteristic does not differ substan
tially at low frequencies but the
rise above 3,OOO C / S is absent. It
is proposed to use the Decca cbar+
acteristic as a basis for design.
When playing KM.1. recordings,
onc fixed capacitor in the pre
amp lifiers to be described later may
be switc hed out of circuit, giving a
level response. Alternatively the
gramophone prc-amplifier may be
left unchanged and correction pro
vided by means of the variable
treble control in the tone c ompen
sation unit. This. when C�. is set
to 100 pF and R .. (Fig. 5) ad
vanced by one quarter of maxi
mum rotation, gives almost per
fect correction.
The majority of pickups . with
the exception of piezoelectric
types, give an electrical output
which is proportional to the lateral
velocity of the stylus . The out
put of such a pickup when play
ing a Decca recording will be of
the form shown in Fig. 12, with
ordinates of voltage instead of
velocity. A pre-amplifier suitable
for such a pickup should have a
frequency characteristic which is
the inverse of this.
Some desirable properties of a
pickup pre-amplifier are : I.
Low noise level.
Low distortion at signal
2.
levels likely to be encountered
with pickups in common use.
3. Sharp attenuation below
20 C / S
to
suppress
turntable
rumble. etc.
4.
Provision for varyi ng the
gain electrically.
Noise Level.-The attainment
of a low noise level in high-quality
sound systems is of such vital im-
sponse fiat to 20,OOO C / S operat
ing at a realistic volume level pro·
duces, in the absence oC a signal,
noise which is just audible as a
very gentle rustle and is com
pletely inoffensive.
Most modern microphones and
pickups
are
electromagnetic,
although there is a tcndency for
microphone design to gravitate
towards carrier-operated capacitor
types.
These have problems of
their own and will not be treated
here.
Electromagnetic
micro
phones and pickups are manufac
tured with impedances ranging
from a few milliohms to several
thousand ohms, but are normally
used in conjunction with a trans-
former which raises the impedance
to a suitably high value to match
the input impedance of a valve,
For obvious reasons it is desir
able to make this secondary im
pedance as large as possible-say
partance that a few remarks of
a general nature will not be out
of place at this juncture.
It is an unfortunate fact that
improvements in microphones and
pickups in the direction of wider
frequency range a.nd absence of
other fonns of distortion are
almost invariably achieved at the
expense of the electrical output.
This does not necessarily mean
that the efficiency of the trans·
ducer is reduced by the other im
provements, but merely that it re
moves
less
energy
from
the
acoustical field or from the record
groove which actuates it, causing
less disturbance of this field, or
less wear of the record groove.
There is, however, a. limit to
this tendency set by the noise
generated by thermal agitation in
the transducer and its auxiliaries
and by the noise produced in the
first valve of the amplifier.
It is
desirable in a wide-range, high
quality sound system to attempt
to maintain a peak signalJ noise
This
ratio of at least 70 db.
figure represents the best that can
be achieved with Cl direct cellulose
disc recording when everything is
" just right," and it is to be ex
pected that the standards of com
mercial disc recordings will ap
proach this level whel; improved
techniques are combined with new
disc materials.
A well-designed
magnetic tape recorder will give
a signal /noise ratio of 70-80 db,
and the increasing use of this type
of equipment will doubtless give
impetus to the research necessary
for the achievement of similar
standards in other forms of re
cording.
With a signal /noise
ratio of 70 db, a sound reproduc
ing system with a frequency re-
I
o
.
,.
----
Fig. I I.
+
/'
1
FREOUENCY (els)
Measured overall response of low_pass filter, in conjunction
with pre_amplifier circuit (Fig, IS, page 26).
I
V
1/
Fig.
12.
several megohms-since the volt·
age output from the transducer
will increase simultaneously. re
ducing the gain required from the
electronic equipment and
the
amount of noise contributed by it.
lt is not practicable, however,
to increase the secondary imped
ance much beyond 0 . 1 MO if a
flat frequency response is required
from the transformer over the
audible range.
The noise generated by thermal
agitation in a 0.1 MO resistor at
room temperature is about 6/AV
for a bandwidth of 20.000 C / S . To
this must be added the noise pro
duced in the first valve of the
amplifier. By careful design and
construction, and by the use of a
suitable valve, the noise from all
causes, including mains hum, can
be reduced to a value equivalent
to about 3 P.V at the grid, but
under normal conditions a figure
fREQUENcY (c�)
Decca recording characteristic.
'3
of 5 /AV is fairly representative.
The total noise may be taken as
the square root of the sum of the
squares of these values. or about
To obtain a signal/noise
B I'V.
ratio of 70 db, then, the peak sig
nal must be 70db above this level.
say 25 mV Lm.s. The pre-ampli
fier should have sufficient gain to
enable the main amplifier to be
fully loaded by a signal at this
leveL
The choice of a valve type for
the first stage must be made care
fully.
In theory, for equal gain
the noise level in a triade stage is
lower than that produced by a
pentode, since the pentode has an
additional noise component due to
electron parlItLOu between screen
and anode.
In fact, however,
there are no h1gh-gai.n triodes
commercially available with the
requisite characteristics and elec
trode
structures for
Jaw-noise
operation. A valve designed for
such conditions should have a
rigidly braced electrode structure
to reduce microphony and a
balanced " double helical " heater
construction to minimize the alter_
nating
field
surrounding
the
cathode. The Mullard EF37 has
this construction and, connected
as a pentode, the noise levels men
tioned earlier are obtainable. Be
fore commencing work, the reader
who is not familiar with the te<:h
nique of high-gain amplifier con
struction should consult an article
on this subje<:t.2. I
Considerable
reduction of residual hum may
usually be obtained by demagnet
izing the valve.'
In order to
obtain the best signal /noise ratio,
the principle which should be fol
lowed, when valve noise is the
limiting factor in high-gain ampli
fiers, is to put the whole of the
available signal into the valve
grid, and to provide any fre
quency compensation which may
be ne<:essary after the signal has
been amplified. By this method
valve noise is included in any
attenuating operations which may
be performed and the overall sig
nal noise ratio is improved.
Low Distortion.
Numerous
methods of providing a. response
which varies with frequency are
possible and, of course, each
method has advantages and dis
advantages.
Where the response
has to be continuously variable
the method which gives greatest
simplicity
of
control
usually
triumphs.
Other things being
equal. however. methods which
employ selective negative feed
back are to be preferred. as cir
cuits of this nature generally have
a high signal-handling capacity
and non-linear distortion is kept
to a mlnLmum. In a pickup pre
amplifier this may be of import
ance where pickups with widely
varying output levels are to be
used.
.
Buandall. P. J.
Hum in lIi.h Gain Am·
-
•
p lilien."
2.
lI'iu/eJJ II'Drld. VDI. 53. No.
.
.
pp. 57·61, Februa� . 1947.
.
.
• Dickenon
A.
F.,
Hum
Reduction.,
•
EI'clro�iu. Vol. 21, No. 12, p. 112. De
cember. lM8.
Correspondence_
Eledro�ic Efttiut";ftl.
Vo1. 20, No. 245, p. 235. July. 1948: No.
248, I'. 339. October, 1948; No. 250. p.
.06. Dccembtr, 1948.
High
_
Pass
Characteristic.
Gramophone motors tend to pro
duce vibrations which can cause
unpleasant rumbling noises in a
wide-range system. Although the
energy contained in the " rumble "
components may be relatively
low, the frequency is also very
low,
and
consequently
loud
speaker cone movements of high
amplitude may be caused. If the
driving coil should move out of
the region of uniform flux-density,
the whole spectrum being repro
duced will be distorted in a par
ticularly unpleasant manner. Dis
tortion in the output transformer
is also possible.
This situation can be improved
materially by the insertion of a
high-pass filter with a cut·off fre
quency of about 'lo c / s and a
fairly rapid attenuation below cut
off.
At these low frequencies.
such filters are conveniently com
posed
of
resistance-capacitance
networks and may be incorpor
ated in the bass-compensation pre
amplifier.
Electrical
Fading
Control.
When the pickup is placed on, or
removed from. the disc the gain
must be reduced to avoid un
pleasant noises.
While this may
be done by a mechanical poten
tiometer the method is clumsy
and does not facilitate rapid re
cord changing. It has been found
convenient to employ an electrical
method in which the gain of one
of the stages is reduced to zero at
the flick of a switch by a bias volt·
age applied and removed by
means of a network with a suit
able time constant.
Pra-Amplifiers
Although all the refinements
outlined so far are desirable. in
dividual requirements will vary
considerably and will detennine
how much complication should be
attempted.
Two
gramophone
pre-amplifier circuits will there
fore be described. which should
COver most requirements.
Fig. 13 shows a simple circuit
which gives good compensation
for the Decca recording charac
teristic.
The circuit constants
have been adjusted to give as
high a degree of attenuation
below 'l0 C / s as is consistent with
simplicity. This involves a slight
sacrifice of the response at 20 e / s .
The method of operation is as
follows :
Negative feedback is
'4
applied to the valve by the
potential divider formed by Ru
and the impedance of CH ' en
and nU' At medium frequencies
the reactance of CL� is small, and
that of Cu large compared with
the resistance of Ru and Ru' and
the gain of the stage is determined
by the values of these resistors.
As the frequency is lowered the
impedance of the top limb in
creases, giving a progressive rc
duction of feedback.
This pro
duces a gain/ frequency charac
teristic which rises to a maximum,
determined by the circuit con
stants, and then decreases due to
the coupling components C LI ' Rn
and Ru'
With increasing fre
quency the impedance of Cu de
creases, increasing the negative
feedback and producing a falling
gain/ frequency characteristic.
The capacitance between the
input transformer s(:condary wind
ing and earth may. f
i large, affect
the response at the extreme upper
end of the audible spectrum. This
effect is negligible with a well
designed component, but long
leads should be avoided.
The
transformer should be mounted
on
the
pre-amplifler
chassis,
which in turn may conveniently
be fixed beneath the motor board.
The overall cbaracteristic with
input
from
a
p e rf e c t
an
. . velocity " pickup on a. Dccca
disc is shown in Fig. 14.
A more complex circuit, which
gives nearly perfect compensation
and a very rapid attenuation
(30db/ octave) below 'l o c / s , is
shown in Fig. 1 5 .
This pre
amplifier has a higher gain than
the previous onc. and is particu
larly suitable for use in equip
ment where the pickup is located
at some distance from the rest of
the amplifier as the circuit ter
minates in a cathode follower.
The construction oL this circuit
is not recommended for those
without access to facilities for
checking the response of the
finished unit, as the performance
may be seriously affected by an
error in component values.
The frequency characteristic of
this amplifier is produced by the
combination of two curves shown
at A and B in Fig. 16.
These,
when added, give the curve C.
Curve A is produced by the cir
cuit associated with V,.. which is
similar in principle to that of
List or Components for Fig. 13.
Rn
It"
Ru
R"
't"
Rn
n"
RM
Ru
Typo
Value to suit High·stability
transformer
carbon
0.1 ]'0[0
do.
0.68 MO
do.
0.22 MO
do.
do.
47 kO
4.7 kO
do.
0.22 MO
Compoaition
22 kO
do.
2.2 MO
do.
Rating Tolerance
!W
IW
jW
IW
I
10%
10%
All resistors may 00 iW rating tolerance 20% unless
otherwise specified.
Ra.ting
(V d.e.
Type
working) Tolerance
Paper
2f)ij
Electrolytic
12
ElectrvJylic
450
Silvered mica
350
10%
Silvered mica.
250
10%
Paper
500
Fig. 13. The attenuation at low
frequencies is due to the combined
effect of the intervalve couplings.
Curve B is produced by feedback
over V 14 through a parallel-T net
work tuned to 20 c I s.
The overall frequency response
curve. taken under the same con
ditions as that of Fig. 14. is shown
in Fig. 17.
Fading Control.�The circuits
of Figs. 13 and 15 have no pro
vision for electrical fading. Fig. 18
shows a network which. when
connected to the cathode of V. in
Fig. 1 3 or V11 in Fig. 15. enables
the gain to be reduced to zero in
about a second when the switch
S5 is closed. On opening 55 the
gain is restored to its normal value
in a similar period.
Complete Variable Compensa.
tion Unit.-It is now necessary to
connect together the circuits just
dcscribed to fonn a flexible tone
compensation unit. This must be
done in such a manner that each
works well within its signal
handling capacity and does not
influence the others adversely.
Fig. 1 9 on pages 28 and 29 shows
the final arrangement.
Power
Supplies.�The
High
Quality Amplifier has a frequency
response which is useful down to
2 e/s.
This necessitates a few
precautions when auxiliaries are
connected to the input. At these
very low frequencies. the balance
of the push-pull stages may not
be good. and there may be con
siderable signal in the supply line.
Fig_
Simple
gramophone
preamplifier designed
for the Decca recording characteristic. When playing
E.M.I. records CH
may be switched out
of circuit.
Alter_
natively, compensa"l n'
tion can be effected R
in the tone-control
circuits.
��(:R"
L-I "--r-
R ..
R" .
R.
f
Fig. 14.
.c"
�:.
c..:
�
�
r
�" C, :
;
mu:
FRf:QUEHC.Y
"'V
I c,!.- R
",
h
: R"
-
R"
:R,.
•
w
w"
z·
�:;;
"£
""
OZ
o�
'z
"w
5�
0
"
tf-f-!;----;
(t/l)
Response curve of circuit of Fig. 13 with ideal " velocity "
pickup.
A very carefully designed and
necessarily expensive decoupling
system is required if a high-gain
pre-amplifier is to operate satis
factorily from the amplifier power
supply.
The cost of such de
coupling is higher than that of a
separate power supply unit pro
ducing. say. 350 V at 20 mA. and
therefore the use of a unit of this
type is strongly recommended.
Performance.�F,.equency
Re
sponse.�Reference to Figs. 6, H,
I4 and 17 will enable the fre
quency response of any combina
tion of units and control settings
to be detennined.
The effect of
intermediate control settings may
be arrived at by interpolation.
Gain.�The figures underlined
in Fig. 19 are the peak signal
voltages necessary to give maxi
mum output at I.ooo c / s when the
pre-amplifier is used in conjunc
tion
with
the
High
Quality
Amplifier.
The simple pickup pre-amplifier
'5
(Fig. 13) has a gain of H at 1.000
c / s . Thus. when this unit is used.
full output may be obtained with
a pickup which produces 18 mV
peak.
Should it be required to
use the system with an insensitive
microphone. disconnection of C..
in Fig. 13 will raise the gain of
the stage to about 150. with a
sensibly
linear
frequency
re
sponse. Full output will then be
obtained with an input of 1.3mV
peak. The more complex pickup
pre-amplifier (Fig. IS) has a gain
of approximately 250.
Noise
Letlel.�With
careful
construction and by adjllstment
of R51 to give minimum hum. the
noise level may be reduccd to an
equivalent input signal of 3-5 IAV
at the pickup pre-amplifier grid.
excluding the noise due to the
pickup
transformer
and
auxiliaries.
Distortion.�The
har
total
monic distortion produced by the
units when used up to the signal
Fig. IS.
R"
Value to suit
trllnsformer
R" 0.1 '1\10
Reo 0.68 MO
R" 0.22 1\10
R" 4.7 k Q
Rn 0.22 1\[ 0
R. 20 kO·
R6/j 22 kO
R..
H"
R..
R..
R,o
R"
R"
Ro3
It,,
R"
HoG
Rn
R"
0.22 l\lO
0.20 MD·
4.7 i\Hl
1.0 1\1 Q
0.22 MO
2.2 kQ
2.0 :\1 fl:
'fYre
High .�tability
cnrbon
do.
do.
do.
do.
Composition
do.
High·stability
carbon
Rating Tolornnce
�W
IlY
IlY
1.0 MD:
do.
Composition
do.
do.
do.
levels indicated is considerably
less than 0.1 per cent.
Form of the Equipmcnt.-The
outward form which a complete
domestic sound equipment takes
is very much a matter of personal
taste.
The suggestions which
follow have been found in prac·
tice to provide ease of operation
combined with absence of trouble
some feedback effects,
The equipment is best con·
structed in two units, one con
taining the loudspeaker and the
R,. 0.22 MO
Hao IO kO
20%
10%
5%
20%
20%
20%
'
I ""
or matched
1%
or matched
OT
IlY
Type
Composition
do.
Hating Tolerance
IlY
20 %
20%
• lIIay require adjustment.
All resistors may be !W rating, except where other.
wise stated.
Hating
(V d.G.
Type
working) Tolerance
Puper
0.5 1'F
250
20%
060
12
E:1ectro1ytic
C" 50 1'1<'
450
Electrolytic
C" 16 ,..F
Paper
10%
:I;}()
C� 0.02 1'P
10%
Sih'ered mica.
3W
C" 4000 pF
10%
Sih-ered mica
350
Coo 100 pF
20%
250
Paper
C. 0.5 /LF
12
Electrolytic
C" f,Q /,F
1%
350
0.01 I'F
Silt'ered mica
C..
or matched
Papcr
20%
500
C.. 0.25 1'.1<'
1%
:IJO
t::iitvercd mica
Coo f>OOO pF
or matched
CO
3;:;0
Sih-ered mica.
5000 pF
1%
'
or matched
10%
Silvered mica
C"
7000 pF
3.'>0
Papcr
20%
C� 0.5 /LF
500
450
EleetrviyLic
CM 16 1'1'
20%
20%
20%
20"/0
10%
! lY
do.
do.
do.
lIV
�W
do.
do.
Jligh.stability carlx)JJ
do.
high-pass filter.
Component Values ror Circuit of Fig. 15.
Composition
2.0 MD:
10 1\J 0:
47 kO:
I kQ
4i kn
Pre-amplifier with
1%
mutch(ld
5%
10%
20 %
20%
other the turntable, This pre
vents mechanical and acoustical
feedback.
The control unit fiay be a con
sole of armchair height (overall
dimensions about 18in x l4in x
20in high) easily movable on
castors. This may contain the
pickup and turntable, the pre
amplifier uDit and, if desired, a
radio receiver, complete with its
power supply. The output from
the pre-amplifier may be con
nected via a cathode follower to
26
a
multicore-screened cable. which
connects the console with the
amplifier and loudspeaker unit,
and carries the mains and aerial
connections.
The amplifier and loudspeaker
unit may be a triangular corner
cabinet, with the amplifier built
into the lower portion, and the
loudspeaker occupying the upper
section, arranged at a. convenient
level for listening.
This arrangement gives great
ea.se of manipulation, avoiding
the necessity of rising from one's
--,
350V
\
comfortable seat to attend to the "
,
I
controls or change a record. The ' ..... -, 1
C" R
Rn
..
main amplifier may be included 1-,�.. ..
:v 1
in the console, but this tends to
R"
I R"
OFF
make it heavy and bulky, and
,I I
gives rise to problems of heat dis
ON
I l
sipatio n which afe not easily
"-I.,, "
r.
�
solved.
I ,
I
I
AcknowIedgment.-The writer
1 I
is greatly indebted to Ferranti,
Ltd., for pennission to publish
Fig. 18. Circuit of fading control.
the results of work undertaken
on their behalI, and wishc!l to
thank his colleagues for help
List ot Components for Fig. 18.
freely gIven.
.:h,I � I
r'" C'T
c.
Ru
RS2
Ra;s
RBI
.�
�
"
�
w
.......
� JB;/I'
,
IW
IW
Rating
(Vd.c.
working)
- -'-L--
fREQUENCY
_
_
Fig. 16.
�
•
'"
..
•
"
w
>
�
•
-
�
w
250
350
:150
Derivation of high_pass
(;baracteristi(;.
0
- 10
-lO
/
8
Fig. 17.
fREOUENCY
Response
(�Is)
curve of cirwit
of Fig.
15·
OTHER " WIRELESS WORLD " REPRINTS
Receiver Alignment Equipment : I. Simple Cathode-Ray o�c;nos�..,pc
1 950). 2. D e.ign for � Wobbu1atQr (October 1950). By M. G. ScrQggie}
. ..
...
•.•
...
...
9d. ""'. By POSI 10!<I.
Communications Receiving Equipment : 1. Ex-RA.F. Communi
cations Receiver (July 1946). 2. Band-Pas� Converle
(October 195;:1). 3. More
!I.se., M.l.f.f.
(March
�bout Ihnd_Pas. (Anverters (February 1950.
(July 1952).
Pee-amplifier.
Television
Jl;
4. 21 m'co Band Pa.s Cnnverrer
Is. n�•. BY Plm Is. I!d.
Vaughan (Dtt"mb"r 1951).
9<1. nU. By (>QJI 10!d.
Osdlloscope : Simple llesign Wilh Fi\'c-Ineh Calhode Ray
Radio Feeder Unit :
High Quality PrC-lUncd R�ccivcr Wilh Gramophone
By J. F. O.
9:/. nU.
Tub.,. fir W. TU'ling (June and July 1952).
By
:o;
Midget Three-Valve A.C. Mains Receiver :
T.RE set.
S. W. Amo., H.SC. (Ilo
W. Amo., H.SC. (110)<15.) and G.
G.
1951).
By pOll !O!d.
M. n�l.
Longand Medium Wave
Embodying Automatic Gain Control. By s.
Johnston., B.se. (HONS.) (October and NGvemhcr
Sensitive T.R.F. Receiver :
s.) (February (950).
Obtaillable direct
...
TRANSFORMER
To A uthor's
Specification
£4-1 3-6
CHOKES FOR WILLlAMSON'S
AMPLIFIER
JOH at 20 m/a.
10H at 1 50 m/a.
50H at 20 m/a.
·
.
·
.
·
.
18/6
35/6
22/-
TRANSFORMERS
FS4J. Input 200/250v.
Output 425/0/425v. at
200 m/a. 6.3v. 4 am ps.
6.3v. 4 amps. CT.
CT.
5v. 3 amps.
Fully S h rouded
5 1 /�
MAINS
W. I .
Inpuc
200/250v.
Output
325/0/32Sv. at
20 m/a. 6.3v. 0.6 am ps.
6.3v. 1 .5 amps. Chassis
mounting . . . . . . . . 23/-
/
-300
O . p.
fl.ating
0.22 Mn
0.22 Mn
47 k n
lOOn
All resistors may 00 iW rating.
tolerance 20,% uules8 otherwise
specified.
- ""
/
ol--�
WI LLlAMSON'S
from:
Lt. nU.
B)I [H>fl 7!d.
By pcm It. Bd.
ILIFFE & SONS LTD., DORSET HOUSE, STAM,FORD ST., LONDON., S.E.1.
27
H. ASHWORTH
676, GREAT HORTON
ROAD
B RADFO R D
Y O R K S.
'Phone : BRADFORD 7 1 9 1 6
RH
S[
�
S,.�
R"
INPIIT fROM
PICI(UP OR
PilE-AMPLIfiER
S, t
I1IN.
11.1.
BASS
"'
;; LJ
-1'r;
�
�
,�
�ll
C"
I
I
CU
RISE
R..
R..
R"
Ru
Rn
R..
R"
R..
R"
R..
R..
R.,
R..
R62
R�
Rw
R"
RM
R"
I"=�
I�
�
Cu
R"
Complete tone compensation and filter unit.
The input and output voItages underlined are peak values
for full output from the main amplifier.
Fig. 19·
1<"
"l
et:: v"
.. , .
I1IN.
List of Components for Fig. 19.
L�ati!lg
0.25 �I n log.
47 kO
3.31.0
0.25 MO log.
100 k n
6.8
Typ<>
Silvered mica
OA7 Mn
0,47 MO
33 k n
100 kO
3.3 kO
1 1'.1 0
0.1 1\10
0.1 MO
50 kn
lOO n
}
10%
10%
10%
IW
IW
High.
stability
caroon
"
matched
Silvered mica
do.
do.
do.
do.
do.
do.
do.
Electrolytic
do.
C
%
1%
1%
20%
unless
eHa SO H a.t
Choke.
Rating
Cro
Ctt
C�
Cu
0"
C"
50 /AF
0.25 �·
150 pF ma.x.
0.01 f'F
0.05 f'F
1000 pF
50 pF
0.05 f!F
Type
Electrolytic
Electrolytic
Pa.per
Proset
Paper
do.
Silvorod mica
Electrolytic
Paper
(V d.c.
working) Tolerance
do.
do.
do.
do.
do.
do.
Electrolytic
Paper
Electrolytic
IW
kO
2.2 k O
0.1 MO
8 pF
Rating
kO
10 kO
0.1 MO linear
100
Tolerance
IW
IW
47 kO
All resistors may be 1\V rating, tolerance
otherwi${: sptlcified.
C"
C..
C"
R ..
R"
Cu
c"
R"
C"
R"
R..
RCu
R
R�
R"
"
FALL
JR"��V'
11.1.010
'NPIIT
R"
c"
(V d.c.
working) Tolerance
20 mA.
Resistam:e about
Mains Transformer.
r.oo
12
'50
or
matched
''''
r.oo
1,500 O.
500
20%
Primary : 10·0-200-220·240 V, 50 cls.
Secondaries : 1. 325-0·325 V, 20 mA d.e.
2. 6.3 V, 0.6 A.
3. 6.3 V, 1.5 A.
250
250
20%
20 %
20%
Switche�.
S,.
Singlc pole double throw.
S.
U{lublo pole double throw.
20%
S,.
12
450
12
500
8,.
,8
Single pole double throw.
5
bank,
5
po�ition selector switch.
5%
5%
5%
5%
5%
5%
5%
20%
1
1%
1%
1%
1%
1%
1%
1%
Tht WiIliamson Amplifier
THE WILLlAMSON
AMPLIFIER DESERVES
JOINTS SOLDERED WITH
Radio Feeder Un it
(.!u puge 30 jor 9trlerlll detail.!)
ADDITIONAL COIL DATA
The radio feeder unit described
on succeeding pages was desigoed
originally to provide high-quality
reception from medium-wave !ita
lions and coil-winding data covered
a range of frequencies from 500 kc/s
to r.6Mc/s.
Since then there have been many
requests for guidance in the con·
struction of coils for the reception
of
:wo
the
Droitwich
transmitter
on
kc/s, and the author has !Sup
plied the following additional data
for those who get a higher signal
strength for the RRC. Light Pro
gramme from the long-wave trans
mitter.
'"I SllE 1 CARTON
COIL-WINDING DATA FOR THE LONG-WAVE RANGE
Transformer
Aerial
Coupling
Wlndlng
Primary
Secondary
No. of turns
Inductance
(.... H)
180
700
330
2,000
Primary
260
Secondary
330
Coils are wound with 40-42 s.w.g.,
d.s.c. copper wire.
To give the correct coefficient of
coupling the spacing between the
windings of the aerial transformer
should be increased to 0.25ill. The
disposition of the coupling trans·
former windings is unaltered.
When continuous tuning is to be
Coefficient
of coupl�
(apprOL)
0.3
1,500
0.6
2,000
---
used, the minimum capacitance of the
ganged capacitor should be increased
by tbe addition of a JoopF silvered_
mica capacitor across each secondary
winding of the transformers, giving a
coverage of approximately 150-300
kc/so
For fixed tuning, the capacitors
should be 300 pF.
29
Jomt may
One imperrectl y
endanger th� successful assembly of the
Willill mson Amplificr.
Solder with
Multicore and run no risks. Multicorc
contains 3 cores of extra-.lctive, non
corrosive Ersin Flux ensuring speedy
and rdiable soldering without waste or
t rouhle and guardnteeing that there are
no lengths of solder without flux. Correct
proportions of both flux and solder are
automatically applied. Fast-acting, fast
holding Muhicore is used exclusively
hy leading manufacturers of radio, T!V
and electronic equipment. Make certain
of a good job-with Mult;core.
(shoWI! abol·e) has
beell (Iesiglled specifically for easy
Simply pull out the length you
Ilse.
require.
CI6018 specijirotiQn (60/40
al/oy) is particularly recommended for
the Wi/liomSOI! Amplifier
RADIO
4 TIV SERVICE
ENGINEER'S 1 1tl. REEL
For UIoCI whio;h dcmal>d
raor q...anll!� of solder.
Contain.
approximately
167 feel of 18 S. W.G.5O/SO
Idlo),
SOlder.
El'$in
Mulliooft
US;" MlIlticore Solder can he obloilled
from radio shops el·ery.,.,·here. Size I
('orlon 5/- relail. I lb. reel 15/-.
MULTICORE SOLDERS LTD.
HEMEL HEMPSTEAD, HERTS (IOXMOQR MM)
MULTICQRE WORKS, MAYUNDS AVENUE,
The Willialllson Amplifier
Design for a
Radio Feeder Unit
HE
preceding articles in this
series have described ampli
fier, tone compensation and
gramophone pre-amplifier units
which are capable of driving a
loudspeaker Lrom the output of a
pick up or a radio receiver. The
design of a radio receiver which
would be suitable lor use under
the varied reception conditions
which exist in the populous parts
of the country, and which a t the
same time could be constructed
simply and with certainty of re
sults, would be a difficult under
In addi tion ,
such
a
taking.
receiver would be unnecessarily
cornplex for the needs of tha t
section of the community which
lives within the primary service
area of high-powered twin-wave
T
Fig.
20. Circuit diaof 1ocal station
gram
radio receiver. Posi_
tions of selector
switches for pre-set
t ning shown at X.
u
T
,�
�
;
"--r-
R"
R"
R.,
RM
R..
R"
nJl ]
R"
R"
",.
Coo
'"
�
-
R"
transmitters, and which desires
only to receive transmissions from
these by the simplest possible
means.
In order that the units described
in the series should form a com
plete domestic sound installation.
it is proposed to outline the design
of a small two-stage receiver suit
able for the reception of mediurn
wave transmissions within the
primary service area. The type of
receiver to be described givessatis
factory results where the spacing
between the carrier frequencies of
the principal transmitters is high,
say 200 k c / s . Tt is not suitable
for use in districts where dosely
spaced
powerful
transmissions
exist , or where interference is
severe.
The receiver circuit is
.
-"
I
EF:W
oc
KTW61
I
I
I
I
C ,.
I
I
I
�
,
RIl
I
(Ill
r7...,
-
I
I�
�
Coo
R..
I
I
T,
c'----
t
.
-"
offered as an indication of the
general lines all which to proceed,
and is capable of being adapted to
individual requirements and con
ditions.
The basic circuit, shown in
Fig. 20, consists of an r.f. ampli
fier.
transformer-coupled
to a
negati ve-feedback detector.
Cir
cuit values for a number of alter
native bIlling arrangements are
given.
Possibly
the
simplest
scheme. from the point of view of
construction, is to use a twin
gallged capaci tor to cover the
range, although by this method it
is not easy to secure a uniformly
good performance at each cnd of
the medium-wave band.
Alter
natively the receiver may be p re
tuned, stations being selected by a
Cn
I�
\s;
R..
1.5 kil
0.1 MO
IQ kO
47 kQ
4.7 kO
22 kfl
2.� MO
2 TV
All resistors may be ! W rating, tolerance
20 per cent unless otllcrwisc specified.
R"
R"
Cn
C"
..
R"
OUTPUT
i
Rating
jW
R"
CH
COMPONENT VALUES FOR CIRCUIT OF FIG_ 20
0.[ 'l\H1
0.1 Ma
330 a
+ 150V
'"
See text
O.lJ.lF
O.II'F
O.IJ.lF
lI.il'lo'
IOOpF
lOOpF
O.I,
IJ
J
..�
Type
Paper
Pa.per
Paper
Electrolytic
Silvered mica
Sil�-ered mica
Paper
Rating (V d.c.
working)
25O
350
35.
450
500
The
push-button or rotary switch.
Tbe use of variable indudors in
this arrangement provides a simple
method of achieving a uniform
selectivity and sensitivity over the
range, with the disadvantage that
two coils or tuned circuits must be
provided for each st..1.tion to be
received. In the unlikely event of
serious thermal drift, correction is
easily applied by the use of nega
tive temperature coefficient capa
citors.
R. F. Transformers. - Winding
data are given to enable Lf. trans
formers to be wound simply on
standard formers without the llse
of a wave-winding machine.
Tbe
correct number of turns are pile
wound in a random manner be
tween thin Paxolin or cardboard
cheeks, which serve to guide and
support the edges of the winding.
This gives an approximation to
the performance of a wave-wound
coiL
The table gives winding data for
transformers to be used with a
twin-ganged capacitor with
a
capacitance swing of 485 pF with
t.rimmers, covering a Irequency
range of approximately 550-1,550
kc/so
When separately-switched tuned
transformers are to be llsed, the
values of secondary inductance
and tuning capacitance may be
read from the curve of Fig. 2 1
against
transmitter
frequency.
This curve has been cOTJJ.puted for
an L / e ratio of unity (L in ,..H,
C in pF), which is nearly opti
The number of turns
mum.
necessary to produce the required
inductance with the formers and
dust-cores specified may then be
obtained from Fig. 22.
The
Williamson Amplifier
WINDING DATA FOR R.F. TRANSFORMERS
Transformer
Aeria.l
Coupling
WindIng
No. of turns
Inductance
Prima.ry
35
30
Secon dary
Primary
dimensions of the coil formers and
windings are shown in Fig. 23 .
\Vhen the capacitance is being
chosen, allowance should be made
Ior strays, which will probably be
about 25 pF. The values used
should therefore be less than those
indicated by this amount. In prac
tice llie nearest standard value
should be chosen and allowance
made in the value of inductance.
Movement of the core will enable
a variation of approximately ± I 8
per cent to b e made i n the induct
ance.
Construction.-In order to pre
serve stability, precautions must
be observed when constructing the
receiver.
The most likely cause
400
21.
95
100
0.35
80
.
.
.
0.65
of instability is the presence of
undue st.ray capacitance between
the anode and control grid of V,..
The valve types used have an
anode-grid capacitance of less than
0.003 pF, and a layout should be
chosen which does not materially
increase this figure.
The design ,
based on this value, has a factor
of safety of about 4.
Although
the valve is metallized, a screening
can may be necessary to reduce
leakage to the valve base .
All
components in the grid circuit
should be kept above the chassis,
and all component.<; in the anode
circuit below the chassis. Where
components in t.he anode circuit,
or in the following grid circuit
0
..
�M�:$�
<.,..I::i
-
V-
l.-'V-1/I/
U
�
,
,,'"
,
"
.,.
1/
1/
1/
I/
"�f-f;0
/
.0
,�
'
.
'
'"
10 0
,
Fig.
lOO
0
1+
100
",
95
/
40'
200
(f.lH)
60
Secondary
Coefficient
of coupling
(approx.)
t-
0
70
'.000
fREQUENCY (k�IS)
,
,
I
"
i
'
"
"'
NuMBER Of TURN'>
12'
I
'"
140
Fig. 22. Curve relating inductance and num.
bee of turns fOf windings discussed in teat.
Curve relating tuned circuit parameters
and resonance frequency.
3'
The Willi_mlon Amplifier
must be brought ablJve the chassis.
as is the case when tuning is by
means of a. ganged capacitor. they
must b e screened carefully from
the aerial circuits.
Figs. 24 and
25 show suggested layouts for con
tinuously variable and switched
1° 0'3"
tuning arrangements.
SLOTS CUT WITH
The Detector.-To give low
THIN SAW
!
i
,
distortion, the detector I'equires
,
L.. <
,,
to work at a fairly high signal
---,
level-say 5V r.m.S. output.
As
the receiver is intended to feed the
"
MATERIAL; O ' O I � BAKElIZfO fABRIC
tone compensation unit, which
OR PAXOllN
requires an input of only 200 mV
peak, the output is taken from a
tapping on the detector load resis
COUPLING TRANSfORMER
AERIAL TRANSFORMER
tance.
This greatly reduces the
a.c. loading on the detector and
enables it to handle high modula
tion levels without distortion.
SEC.
Alignment Procedure.-(l) Set
SEC.
ganged capacitor at a position
PR I.
about five degrees from the mini
PRI.
mum capacitance cnd, and adjust
trimmers for maximum output
from the high-frequency Third
Programme.
(2) Set capacitor about twenty
OATA FOR FIXED TUNING
degrees from maximum capaci
PRIMARY WINDING.
tance position and adjust dust
PRIMARY WINDIN(;. (OIR£O URI.U
CONNECTION )
cores for maximum out
lh NUMBER or SECONDARY 'UA�S
'fI NUMBER Of SECONDARY TURNS
put from the low-fre
srCONDARY WINDING. SEE TEXT
quency
Third
ProPRIMARY WINDING. (lOn fEEDER)
gramme.
'hv Nur'I8ER OF SECONOARY TURNS
(3) Repeat this pro
SECONDARY WIMOIN(;. SE[ TEXT
"'"" "" IIl0C�
cess until both stations
are accurately tuned.
P o w e r Supplies.
Formers are standard
Fig. 23.
The receiver is intended
moulded type, fitted with a-mm
threaded iron-dust cores_ All coils
are wound with Litz wire consisting
Fig. 25. Plan
of 7-9 strands of 45-48 s.w.g.
view of top
enamelled copper wire.
o f chassis.
Swi tched
model
Call
FOFlHE.RS
I, ,-:=::�-I"
I_L L
�
,__
_ _
.
.
e-l'- --- I 'I,.
.
-----
�
.
s'f;------->j
.
llUMltllUM OR COPPH 5tR£{H
EXTENOING BELOW CH,t.,SSIS
O -I::ll::ji
��::::�: �q._C__''__
�3l:ll/CUTDUT
G
.
CO
L. l��:::::::�===n�----j�J
4'
.0
>,
..
Cn
�
AND
llf: ABO�E
CHASSIS 'oIITH
fOil GANGrD CAPACIT()�
""'" ,'I; ""
�
�g. 24· This di�ram
,..
.ows a p,an V1ew 0f
top of chassis_
3'
to bc Sllpplied from the
pre-amplifier
power
supply. The decoupling
is
not
adequate
to
enable it to be fed from the main
amplifier supply.
Acknowledgment.
The writer
is indebted to Mr. A . T. Shepherd
of Fcrranti, Ltd" for his assis
tance in the compilation of data
for these notes .
-
the
3C67A
High quality
output
T RANSFORMER
we know, unique and permits of seven different
secondary impedances with virtually no change in
leakage reactane<: as against thc normal four obtained
with CUSlOmary arrangements of eight sections.
10 the 3C67A these impedances are 0.450, 1.8n,
40, 70, 110, 16n and 300. Onc additional impcd
ance-220----can be obtained if required with an
increase in leakage reactance of about 10 per cent.
It gives us great pleasure to announce OUT latest OU1 u,
transformer for the " Williamso n " amplifier. • he
result of some painstaking research, this instrument
represents quite a considerable advance on our 2836R.
Typical technical details are as follows :
F
Pritnary
D.e. Resistance 900
·f 900.
Incremental inductance taken at ') v. 50 1'.'5" 100 hys.
minimum.
Nominal impedance 10,0000 centre tapped.
Leakage reaClance tested at I v. 800 ds. :
Whole Pdmarr to the secondary connected for JOn
and short-cm�uited, 16 m 'Hys.
Insertion loss
0.5 d.h. (6i per cent.).
Half primary to the 300 secondary, 8 m 'liys.
!-Ialf primary to the other half short-circuited,
17 m,lHys.
Frequency Response
With no feedhack and assuming a generator impedance
of 2,5000 the frequency response, even at incremental
levels, is virtually fiat (0.25 d.b.) from 12� c.'s. to
25 kc s. There is a smooth high Irequency " roll
off" with no objectionable resonances.
Power handling capacity
At 15 watts at 121 e{s., the core material is being
worked at 10,000 gauss aod the distortion intmduced
by the iron at this frcquency and power level w;lho1/t
an)' feedback should not exceed 2.5 per cent.
At 25 � the transformer will handle 60 watts with
half this distortion, and in either case quite a modest
amount of feedback will reduce the distortion to
negligible. proportions.
Secondary
Considerable thought has been given to the question
of secondary impedances, and we havc decidcd that,
for Loudspeaker matching, the most useful range is
givcn by a basic im cdancc of 0.450.
The arrangcmcnt 0 secondary sections is, so far as
r
U
..
outstanding results "
of his Wireless World artieles, the output trans
former is probably the most critical component
" . . . As Mr. Williamson himself observes in one
in a high-fidelity amplifier. Your transformer
performs this exacting duty with full honours,
and I shall always recommend its use to anyone
undertaking construction of a " Williamson."
With many thanks for a first-class job."
E. L.
N U RSTEED
ROAD
Telephone
DEVIZES
Devizes
33
536
J., Swindon.
WILTS
Replies to Queries
T
Raised
HE
series of articles recently published on the
High-Quality Amplifier has aroused consider
able interest and given rise to correspondence.
It is hoped that these notes, whieh deal with
matters of general interest ansing from the corres
pondence, may he of assistance to readers who have
similar difficulties.
�
Output Transfonner.-Vvhen assembling the core
of the transformer, care should be taken to ensure
that the edges of the T and U laminations butt to
gether. The magnetic properties of the core are de
pendent upon careful assembly and tight clamping.
the pickup.
The tone compensation unit of Fig. 19 may be
constructed on orthodox lines, the only essential being
to provide sufficient frontal area to accommodate
seven controls.
Grid leads should be kept short to
avoid hum pick-up. The blank valveholder tenninals
(pin 6) should not be used as anchors for the leads
to the top-cap grids. The power supply components
can, with advantage, be assembled on a separate
Static Balancing.-The method of balancing the
standing currents in the output valves, which was
suggested in the article in the August, 1949, issue, is
dependent for its success on close matching of the
d.c, resistances of the halves of the output trans
former primary. Nominally the sections are identi
cal, and when carefully machine-wound from the
same reel of wire, the I"esistances should not differ
materially.
It is possible, howeve1, due to varia
tions in wire diameter and insulation thickness, for
the resistances to differ by up to S per cent and even,
in extreme cases, 10 per cent, Should this occur, a
compensating resistor should be added in series with
the low-resistance side in order to equalize the resist
ances, and the meter connected across the equalized
sections.
Other more direct methods may, of course, be used
Characteristics of 2,000 Receiving
Valves and C.R. Tubes
Constructors
to adjust the anode currents to equality, but unless
the transf0n:ter has a split primary winding they
.
are Inconvement, and great care should be taken to
ensure that the insertion of instruments does not
�ause oscillation which could give misleading read
mgs.
Construction.-There is little to add to the con
structional data on the main amplifier given in the
August, 1949, issue, except perhaps to explain that
the purpose of the sub-chassis screen, shown in Fig 3
(see page IS), is to prevent feedback from the anode
connections of the output valves to the input of the
amplifier, It should extend downwards to the full
depth of the chassis.
The method of construction of the preamplifier
and tone-compensation units will usuaUy be adapted
to individual circumstances. Onc suggested method
?f construction for the preamplifier circuit of Fig. I S
I S to use a shallow chassis about 9in x 3in x lino The
valves and electrolytic capacitors are mounted in a
group along the centre of this chassis, and the other
components mounted vertically above the chassis on
tag strips arranged on each side of the central group.
The connections to the valveholde!'s are taken
through slots cut in the top of the chassis.
The
input transformer should be mounted on the top of
he chassis at one end. With the sizes given, there
IS ample room for a screened component of dimen
sions up to 3in x 3in x 'Zin.
The whole unit should
be fitted with screening covers, and mounted on the
underside of the motorboard as close as possible to
Valvcs.-There is no exact equivalent for the
Osram type KT66, and its use is recommended where
possible. When the equipment is to be used over
seas, the KT66 may be difficult to obtain, and 6L6
glass and metal types may be regarded as direct
replacements, with the proviso that the total anode
and screen dissipation should be reduced from 25 W
to 21.5 \V by reducing the total current from 125 mA
to llamA by adjustment of R". The use of these
valves with reduced rating entails Cl. slight reduction
of the maximum output. The 807 may be used at
the full rating of 25 W, with modifications to the
valve connections.
Since the articles were written, a modification of
the EF37 has appeared under the nUIi1bcr EF37A.
This has improved heater construction giving greater
freedom from hum, and its use may be advantageous
for VI and V,.,
No other changes in valve types can be recom
mended, as their use would involve radical redesign.
RADIO VALVE
DATA
by
chassis,
Conc1usion.-The circuits published i n the series
have been evolved over a. considerable period of time
and are capable of giving a. very high standard of
performance . Requests have been received for data
on modifications, but as it is rarely possible to
determine the full effect of these without carrying
out tests, in general, no such data ean be supplied
by the writer,•
Or. fn' that matl�r. by Wirel.n lVaT/d.-Er>.
3rd Edition. Compiled by the staff of WIRELESS WORLD. Gives the
main characteristics and base connections of over 2,000 types of British
and American radio valves, and over ISO cathode-ray tubes. These arc
further classified into obsolete, replacement or current types as recom
mended by the makers. 80 pp. 3s. 6d. net. By post 3s. IOd.
Obtaillable from all booksellers or direct from :
ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON, S.E.t,
34
The WilliamsoQ Amplifier
Modifications for High-impedance
Pickups and Long-playing Records
T
HE
introduction of long-playing records in Great
Britain, after the public..1.tion in November,
1949. of gramophone pre-amplifier circuits for
the " High Quality Amplifier " which were suitable
only for the 78-r.p.m. standards. has made it nece s
sary to revi5e these designs .
The principle of recording with a rising frequency
characteristic at high frequencies and reproducing
with a correspondingly falling characteristic. in order
to effect a reduction in the level of surface-noise from
the material, is a well-established and useful one. In
the case of long-playing records it results, in conjunc
tion with the u s e of a homogeneous plastic for the
record material, in an almost silent background .
There arc. however. dangers attendant upon its
use. The scheme is based on the hypothesis that the
energy level of music decreases with increase of fre
quency above about 500 c J s. Thus it should be pos
sible steadily to increase the gain of the recording
channel above this frequency. This appears particu
larly attractive at first sight. since with the n ormally
used constant-velocity characteristic the recorded am
plitude for a constant recording level i s inversely
proportional to frequency and is therefore very small
at high frequencies.
Initially. a rising frequency response characteri stic
producing practically constant amplitude at constant
level was used, the energy level distribution being
relied upon to restrict the amplitude at high frequen .
cies. The effect of this was, in practice, to cancel the
improvement in tracing, which the small-groove
system offered, by producing, at high frequencies and
high orchestral levels. recorded waveforms with radii
of curvature too small to be traced accurately. The
resulting distortion manifested itself as a tearing
sollnd superimposed on the full orchestra.
There is additional evidence to suggest that the
original hypothe sis required revision, since it is
demon strable that it break s down when such per
cussion instruments as cymbals and castanets are COII
sidered, particularly when the frequency range is
wide. Indeed, the peak power level required to re
produce cymbals exceeds that normally required at
medium frequencies.
"'20
This early experi ence has led to
the adoption of a characteristic
�
+I0
which is a better compromise
between these conflicting factors
t:
and gives much more satisfactory
g 0
�
results in practice. Fig. 26 shows
the provisional recording charac:;l
teristic now in use by the Decca
0
Record
Company
for
L.P.
�
records.
The amount of treble
-20
0
boost is lower than the theoreti
cal optimum, but the use of even
this amount of compensation
� -I
means that the first stages of the pre-amplifier m ust
be capable of handling occasional hig�-frequel�cy
peaks which arc greater than those expenenced With
standard records. unless the pick-up is a constant
amplitude one, or its output at high frequencies i s
attenuated before reach ing the pre amplifier.
The original designs of pre-amplifler employed
negative-feedback methods of compensation, and
hence are particularly suitable for a wide range of
inputs. However, pickups are available wilh s�ch a
.
.
wide variety of output levels that no Single ClrcUlt
will cope adequately with them , and external attenu
atars may have to he used.
·
Modifications.-Dealing first with the single-valve
pre-amplifier (original circuit, Fig . 13. p. 25), the
revised circuit of Fig. 27 shows the modifications
necessary to provide alternative standard and long
playing characteristics.
To simplify the switching,
by using a single-pole challgeover switch, the capa
citor C 1 . i s left permanently in circuit, giving a
Decca. 78_Lp.m. character stic in the " 78 " position .
Alternatively, Cl_, may be removed to give the KM.I.
characteristic. In either case, correction for the
other 78-Lp.m. characteristic may be made by means
o[ the treble control on the tone compensation unit .
The advantage of this simplified switching i s that
it becomes practicabl e to gang the switch to the
motor spccd·change control to give automatic com
pensation. H this arrangement is not desired, a two
pole multi-position switch may be used, to give three
or more combinations. as in Fig. 28.
It should be noted that the position of Cl6 has been
altered. so that the whole of the feedback netwcrk is
at earth potential. This avoids switching transients
wh ich would otherwise occur, due to charging and
discharging of capacitors as the switch i s operated .
A small capacitor, C,;, has been connected across
the input tra.nsformer secondary. This is to prevent
any tendency to instability or peaking at high frei
Recording characteristic used for current Decca
long-playing records.
Fig. 26.
-
I
,
I
N
35
--0
I
rREQUENCY
v
-
§
( cfo )
0
0
go
N
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S.E.I
The Witliamson Amp li fier
quencies, caused by the presence in the feedback loop
of the stray secondary reactances of the transformer.
The necessity for this capacitor and its minimum
value will vary with the individual transformers. Its
value sbould be kept ali lilllall as possible, consistent
with stability.
Modifications to the three-stage high-pass prealll
plifier (original circuit Fig. 15, p . 26) are on the samc
lines, and Fig. 29 shows the revised circuit.
With these pre-amplifier circuits, the wiring to the
selector switch must be kept short. and the switch
should. if possible. be mounted on the pre-amplifier.
Should the position of the pre-amplifier render suc:h
)I'V
R"
R"
R,,:
" �i
FC�.,..
r�,
t"'-
jl[ �R"
L..-
Rn
I-
a switch inaccessibLe, consideration should be gi\'cn
to the usc of a relay in place of the selector switch.
rather than the use of extension Leads. This has the
additional advantage that it could easily be operated
from the speed-change lever by means of a mino·
switch or from the additional switched pin which is
a feature of some pickups with interchangeable heads.
Pickups without Transformers_-A number of pkk·
tipS are available which do not normally require a
"transformer.
It is possible to use the majority of
these with the prc-amplifier circuits. by interp:Hing a
suitable 1 : 1 transformer. In other cases. when thc
connecting leads an: short. it may be practicable to
connect the pickup directly in place of
the transformer seeondary. The limit·
ing factor will be the capacitance
between the leads and their screening.
which will be shunted across R
or
Ru' and which, if sufficiently l�ge.
would upset the treble compensatiotl.
The value of this stray capacitance
shoul not be all owed. to exceed 50 pF.
and If e'l or CU IS switched out.
should be compensatod by a capacit
ance of one tenth of its value in
�arallel with Rn or R", t o give a
hnear frequency-response character
istic at high frequencies.
."
.'
Resistors R" and R�. must he
-"
R"
-o·
.Left : Fie. �7. Simple two.position twitch
-'
.
o·
Ine In smtle-volve pre-omplifier (or
playin,
Decca l8-r.p.m. standord and
Ht-r.p.m. L.P. records.
Comf't;nsatian
(or !he E..M.I. 7B-r.p_m. stondard choroc
terlsric should be opplied seporotely by the
treble tone control.
�
C"
Cn
� I 'm:,':" �5,
"
,C
r
C,
;
Coo:
� R"
[C"
o.
o·
R"
o�
R ,.
List of Components for Fig. 27
R"
R"
R"
R"
R"
R32
R"
R�4
R"
Value to suit
transrormer
O.IMn
0.68MCl
0.22Mn
47kn
4.7kn
0.22Mn
22kO
2.2Mn
Type
High-stability
carbon
do.
do.
do.
do.
S,
Tolerance
C"OR Cu
-f
IW
jW
IW
IW
do.
Composition
do.
10%
10%
do.
All resistors may � j W rating, tolerance 20% unless
OIherwise specified.
C"
C"
C"
C"
C"
C"
Coo
C"
Coo
Rating
0.5f'F
5Of'F
16f'F
1000F
O.05f'F
1�5OpF
2500pF
1500pF
300pF
Type
Paper
Electrolytic
do.
Silvered mica
Paper
Silvered mica
do.
do.
do.
Single-pole changeover switch
Rating
CV d_c.
working)
250
12
450
250
500
250
250
250
250
Below : FI.e. 28. Alternoti'le circuit (applic
able to F'es. 27, 29 ona 30) with three
poSition switch eMng compensation (or
Decco 33!. Decca 7B ond E..M./. 78-r.p.m.
recording characteristi".
Tolerance
'·00IS/--I-F
l·sMn
DE':�/_
:n ' )
: Q-004.uF
78?,
Rn
"11
100PF R"
IOOpF
10%
10%
10%
10%
'7
14 [.H.J.
"
R,.
R,.
"
HANNEY OF BATH
W l lLlAMSON
7
25 Resistors lnd pOts for William .
•on main amplif,er
9 Rui.torl lor fig. I]
23 R".istors for fir. 15 _ _ _ _ _ _ _ _ _ _ _ . ..
"
•
"
"
15
22 Resi.tor. and potslor fig. 19. .
6 Condens"., for "I. 13 ..
1 5 Condensers for f" . 15........
33 Condenser. for fi,. 19 ..
10 Resistors for fi,. 20. . .
7 Conden.er< for fir. 20 (le. .
• nden)
"
..
C68
•
"
,
Match�d p.i.oH7 K lwut rcsinars
Matched pair
re.istor.
of 21K
It
Wltt
6 position ....itch. .
S.P.D.T. ,,,nuin,, OAK .witche....
5 Sank
n.p.D.T. Icnuin. OAK Iwitch• •_ _
Relian"," 100 ohm potf!ntiometers
Tile obo e i. on
v
Small quantiti". of WODEN main. tran.·
fo.mer., cholc.., and ouput tran,form ..ro
are available from time to time, as de
tailed in our current list lupplement.
15 •
1 2 1 % .ilye. mira condensers for
fir. 1'.1
7 5% ,ilver mica condensers for
fi , . 19
.. ... .. . . . . . . . . . .
ly
0
,
•
,
•
,
C O M P O N ENTS
AMPLIFIER
•
.
•
.
.
•
.
T.C.C.
.25
mfd.
metal packs
(co�plin, conden.ers for n,. I )
ndt
metalmitu
mId.
.05
T.C.C.
(couplOn, cond"Mer. for n,. I )
each
KT 66
.
valv�., motchcd
clud,n, tax
•
,
"
p�ir, in. . . p" r
....
(includins tUt)
. .. . . . . . . . . .each
Ma,ns <ranslormer <0 Jpecincnion
.
.
•
.
•
.
'"
"
"
10 "cnry 150 mA choke to .p"ci
ncuion
"
30 "enry 20 mA choke to .pecilieuion
..•.....•...........•..•......
William.on OUtput tran,former.
by ELSTONE (3.6 o"m ..,con_
dary) . ............ .
.e/ection of our
lor,e nod.,
Acos GP
LP
..
nd 6<1. sto
B.S.R. type M U I O 2 .peed moto..
,
COLLARO type ]RC521 luto_
chan,er., 3 opeed unmixed, with
2 Hi_Fi cry....l he.d,....
.
CHANCERY LP utuhment, (on
vert. 78 r.p.m. motor to LP in a
maner of ucond.
COLLARO type 3RC511 autochan,erl. as .bove but play•
10 and 11 inch record. MIXED
.
BRISTOL
Tele p hone :
38 1 1
£11
GOODMANSAXIOM 150 'peakers
end .aye you"e/f time end trouble,
LOWER
£18 1� 8
currenl ,i.1 ond
."pp/em..nt,
BATH
ROAD,
RADIO DE S IG N E R'S HAN D BOOK
A
compreh�nsivc::
rdc::rc::ncc::, the work
of
10 amhors and 23 colbborating cnglllcers
Handbook
have
achieved
exceptional
success, many
thousands of copies of this standard reference
work having been sold In all parts of the
world. The book deals with general theory,
Previous editions of Radio Designer's
components,
testing
and
design.
The
new
especially interested in the design and appli
radio receivers or audio amplifiers ;
it contains an enormous amount of data
cation of
which
means
and
has been
made readily
of a fully-detailed list
previous
,·olume,
and
contains
work
It is the work of ten
The
accessible
a comprehensive index,
edition is more than four times as large as
the
main
subjects
testing, general theory
ar'"'
valves
of
by
contents
and
valve
gmeers, under the editorship
and COmponents, audio
frequencies, power supplies,
design of complete A-M and F-M receivers,
Smith.
and reference data.
hitherto unpublished.
authors
and
The
twenty-three
work
is
collaborating
of F. Langford
intended for those
Publication May, 1953.
JLlFFE
&
Obtainable from
SONS
en
LTD.,
all
frequencies, radio
42s.
net.
By post 43s. 6d.
booksellers or direct by post from
DORSET
HOUSE,
"
J
n 6
£10 S 6
GOODMANS AUDIOM 600pnkers £8 11 •
£8 5 0
ROLA G l l .pnkerl. . . . ... . .
.
.
0
" ,
" .
£9 19 •
£1 J -4
U I� 4
B.S.R. type GU-4 ,ram_motors ..
L. F. II A N N E Y
77
117
pick"p', ...ndard or
. . . . . . . . . . . . . . . . . . . . . ..
B.S.R. type M U I 4 ] speed motors
mp now for our
••
20
Hnds only, sc�ndard Or LP....... .
' "
EF37A valve. (EF37 now ob.olete)
.
PARTRIDGE type WWFB/O/1.7
output tran.formers unpotted.
includi", packin,
the address below.
S T A J\·f F O R D
ST.,
LONDON
S.E.l
of Fig . 14, p. 25.
Thi 5 circuit is suitable for most
moving-iron va.riable-reluctance pickups. and can be
used w ith piezoelectric pi ckups which havc been
loaded to give an output proporti onal to recorded
veloc ity .
retained to provide a conducting path to the valve
grid when the pickup heads are being interchanged .
There may be cases, where one side of the input
must be earthed, in which it is impracticable to utilize
the pre-amplifiers in this way.
In tbis event the
circuit may be modified as shown in Fig. 30. This
ci«:uit applies to both prc-amplifiers. In it. the trans
formcr had been replaced by a resistive network R"",
Ra,' mixing the input and feedback voltages.
The input resistace of this circuit is approximately
0 . 1 MD, alld its voltage gain at I.OOO C / S is 9.
The
frequency-response curve is almost identical with that
be restricted to :.IoomV in the case
Danger of Overloading.-The
amplifiers 5hould
input to the prc
of the single-stage circuits and 50 mV for the three
stage circuit, and if necessary a potential divider
should be used.
Piczoelec::tric Pickups.-Lightweight piezoeleclric
pickups have recently become popular, particularly
for L.P. recordings . Since these give a rel atively h igh
output, no pre-amplifier is necessary and any correc
tion required may be achieved by means of simpl e
Below ; fig. 29. Revised three-stage pre-ampli(ier circuit with
high-pass (ilter, fO play Decca 33t- and lB-r.p.m. records.
,..
c"
' ..
'.
.
�:.
c.. '
t'-,'� ,'
"
'..
"-'"
ic.�·
R" Value to suit
Transfonner
O.IMO
R..
O.68MO
R"
O.22MO
Rn
R"
4.7kO
O.22MO
R"
R..
20kO*
R"
22kO
R..
R"
R"
R..
R"
R"
R"
O.22MO:
O.20M!l*
4.7M!l
1.0MO
O.22Mn
2.2kO
2.oMO
Rn
R,.
R"
R"
R"
R"
R"
R"
R..
2.0Mn
l.OMD
lOMQ
47kO
lkn
47kfl.
0.22Mn
lOkO
2.2MQ
•
O(ctIAI�:'
�
�'"
'"
c.. '
f 'Type
s•
rc"
: ' :'
"
..
IC.:,1 .�,; �!!r
r '
,;
,;,
'.. f
r!&
�
c"
c" ' : '
.
..
'"
c..'
'..
c"
[(." '
.
.
c"
''''''
:
,..
Coo
..
®
L�:'
' '
.
"
'.
"
"'"
'..
.
i!
Component Va)ues for Circuit of Fig. 29
do.
do.
do.
do.
High-stability
carbon
do.
do.
Composition
do.
do.
do.
do.
do,
do.
May require adjusuncnt.
�:.
'..
High-stability
""'oon
do.
do.
do.
do.
Composition
do.
High-stability
carbon
Composition
do,
'..
'"
Rating
Tolerance
!W
jW
!W
20%
20 %
20%
20%
10%
IW
20 %
10%
jW
,W
5 "
'"
20 %
20%
20%
1 %
IW
Coo
COO
O.2Sp.F
5000pF
C"
C62
C"
or matched
do.
IW
Cos
O.5p.F
50p.F
1 6p.F
O.02p.F
lOOpF
O.5" F
50pF
O.01,..F
C,"
C"
Cb2
C�3
CM,
Coo
C"
CH
C"
C"
C"
C"
do.
5%
10%
20 %
20%
20%
20 %
20 %
Coo
S,
All resistors
Rating
(V d.c.
working)
Type
250
Paper
Electrolytic
12
450
do.
Paper
350
Silvered mica 350
Paper
250
ElccU"olytic
12
Silvered mica
350
Paper
Silvered
500
350
350
do.
SOOOpF
350
do.
7000pF
Papcr
500
O.5p.F
Electrolytic
450
1 6,..F
10-SOpF
Silvered mica 250
Paper
O.lp.F
500
Silvered mica
2500pF
250
1500pF
do.
250
do.
300pF
250
Single-pole changeover SWilCh.
Tolerance
200;.,
1 O '}(,
10%
20 %
oc
1 %
matched
20 %
1 %
oc matched
do.
10 ��
20 ��
10 %
10%
10%
may be l W rating, except where otherwise stated.
39
The WiJliamson Amplifier
F",g. JO M0difi�d input circuit for use without tran,
former when °"'
side of the pick"P
must be earthed.
.
(t-.....
' ..
loo kfi
"CKUr
".uT
I
----
.�
�
R"
IOOkn
I
r
I
RC networks, details of which have alre3.dy been
published. '
Checking the Pre·amplifiers.-When a pre·a.mpli·
[Jer has been constructed, it is advisable to measure
its response curve over the audible frequency range
and beyond, in ordcr to ensure that nothing is amiss.
This is particularly so in the case of the three·stage
pre-amplifier.
To facilitate this measurement the networks of Figs.
JI and 32 have been devised.
These circuits, when
fed with constant-voltage variable·frequency input,
I
o"OO6fLf
I
O·03,uF
mn
INPUT
(CONSUNT VOLTAGE)
C·OOSfLF
INPUT
(COIo"iTANT VOLTA(",[)
1
""n
;
TAP AT RESISTANCE EOUAL TO
IMPEDANCE Of PICKUP IF
WER THAN llOn
t/
OUTPUT TO
PRE-AMPLlFIER
(DISCONNECT
I
FOA EM.l CHARACTEFIISTIC)
Fig. 32.
/0'<101
TAP AT RESISTANCE EQUAL TO
IMPEOAtlC< OF PICKUP IF
THAN 1·110.0
::;--
OUTPUT TO
PRE -AMPLIfiER
Simulator for Deaa and E.M.I. 78-r.p.m. recording
characteristics.
Fig. 3 / .
I
101t0
Simulator for Decco 33t r.p.m. L.P. characterisric.
produce outputs which are, respectively, replicas of
the standard and L.P. characteristics.
To test a. pre-a.mplifier, the appropriate network
should be connected between an oscillator and the
pre-amplifler input. The output from the pre-ampli
fier for a. constant voltage to the network should then
follow the response curve already published for the
appropriate circuit (Figs. 14 and n , pp. 25 and :;q ) .
Acknowledgment.-The writer i s indebted to Decca
for information a.bout their recording characteristic.
, Wen and Kelly, " PicJmp
November. 1950. pp. 386-391.
Input
1'",lIed in Englalld by ('"uTI""all I'TC$$ Lld., Pa,i. Ga"fell. Lum/VII,
Ciceuiu," WiT..l.... War/d,
5.f..1.
Ll00t-nR!,;l�l�
KS
GOODSELL
WILLI A M S O N
HIGH
-
FI D E LI T Y
.....
-
�
AMPLIFIERS
*
The Goodsell version of the World famous WHliamson Amplifier is to full specification,
*
*
*
All components are adequately rated to ensure long life and trouble free operation.
laboratory bullt and tested.
Fully troplcalrzed version. Type G W r 8/C is available for use overseas.
All models have separate power supply i n corporated on the same chassis for use with
multi-stage pre-ampllfiers, tape pre-ampllfiers and tuner units.
I
t
:
�
I
�
i
I
�
A laboratory report of i WHUamson
type GWr9/C, which w;as submitted
lation tesu. The results using an
Intermodulation DiUortion Meter and
Frequency Oscill...tor ...re given.
Amplifier, our
to Intermod".
Altee lanCing
Boonto" Seat
O.l�
2%
The report on the Goodsell pre·...mplifier showed it
to be ... compar.able unit With s(mUar figures.
(standard model)
Pr(ce
Ul ; 5 : 0
(as IUu.tr...ted)
Price
£J6 : 0 : 0
Type
GWIB
Type
GWrB;C
P R E -A M P L I F I E R
Power Output
I.H. Distortion
0.900
0.2%
TONE
I
1 5 watts
10 wattS
1 5 Wiltts
1 0 watts
I
:
I
�
Frequeney
Ratio I : "
!
10 & 2,000 c/•.
10 & 2,000 eis,
.00 & 2,000 els.
100 & 2,000 e's.
I
CONTROL
U N I TS
Multi.$tage, low noise tone eontrol units with radio input and gramophone pre·ampHfiBr. Equalization for micro
groove and stand...rd recording eharacterinics, 5 �sition .teep cut low.pu. filter and cathode follower out.put.
feedback over every suge ensures negti,ible distortion.
Type PFA with equalization for British and American
LP and 7B. Price m : 10 : 0
recording characteristics and 5 mv sen,ltivlty on
Type FUTC with LP ...nd 78 equalization ...nd 1 5 mv sensitivity.
MANUFACT URED
Price ll2 :
12: 0
BY_
GOODSELL LTD., 40, GARD N E R STRE ET, B R I G HTON, 1
f
i
Phone: Brighton
167J5
_ _ .- ----,� �
--- ---_.
........... - ------ ...... --------_. _ -----
:
!
I
:
j
I
•
I
-.....
VALVES
fo r t h e
L.63
W I LLIAM S O N
AMP
KT.66
IFIER
The following Osram valves are specified :
L.63.
*8.65.
KT.66.
U.52.
Use of KT.66 is essential to obtain optimum
perfonnance of this equipment.
The characteristics of KT.66 and 6L6G are
not identica1.
Circuit information on a Quality Amplifier
for DC/AC operation may be obtained from
the Osram Valves & Electronic Dept., Magnet
8.65
House, Kingsway, W.C.2.
dOl/ble triade
for any two LJjJ type.
* B.G:j
as
altemarive
U.52
THE GENERAL
ELECTRIC CO
.•
LTD., MAGNET
HOUSE,
K!NG5WAY,
W.C.2
MODERNIZE '10"" 'W� II�
HE Williamson amplifier ci rcuit was
T
hrst public ized in Enlland in
.10.1 in Ihis counny in 1949.
1947,
It has
.u:hievc...1 wiJe acceptance and popularity,
.111.1 lI.n 1",,,11 the basis for K'veral modi·
hl' ation s of the: ori,inal desian. The:
mnS( bill sic change was the: ulrra-lincar
version of operation, which I developed
anJ subsequc:ntly Jc:scribcd • . This ar
-
rangement corrected 2 of the: basic de
heic:nc ic:s in the: original desiln
it
the: powc:r caS-bility of the:
amplifier to n or 30 wattS, and it irn
proVN the: marlin of feedback stability.
Now, as alwa y s happens. Pro,I'CSS in
amplifier Jesi,n has continued: it is
pussible to make furthe:r improvements
in 'the Williamson Jcsigrf' (both triode
and ultra·linear venions). These im
povements again correa for limitations
with respect to power OUtput and
stability.
inc reased
Increasing Power Output
Present thinking 00 requirements for
.IuJio P�W" js vastly dilfercnt from that
nf a fe w yean ago. Then, most people
, .. id, · ....l"n waus is enoush for me."
Now, ho wever, modern program ma
terial has bcc:n increased in dynamic
range many times over that of former
�l:ars. This fact alone: has increased the:
performance is required. Even t h is mini·
be: inctcaScd in the
mum will probably
ycan to come unless the c:fficiency of
loudspeaker sYStems can he increascd.
Por theM: Itnd relatN reasons. c:lfuru
have bcc:n devoted to increasinl, the:
power OUtput of audio amplifien. The
advent of some new tube: t}·pes ha', made
this praCtical within the W:lliamson
configuration without the n�cd for com·
pletcly tebuilding the am pl ifier . Changes
required arc replacemc:ne of the output
tubes, substitution of an output trans·
former which will handle the: increased
power and provide suitable impedance
matchin& and addition of fixed bias.
.
·
, New Output Tubes
The: new tube: selected for modernization
of the: Williamson is che Amperex
('CA 7, which is also imponed anJ dis
tributed as che: Mullard EL· H. This is
a compact tube: with power capabilities
up to 100 wans, depending on th e sup·
ply voltagcs available. Ir can be: p l ulled
ditcCdy into che sockm fo r merly uscd
for �RRI',. KT(,(,'s, 161<'1·s. ;an.! mher
tubes of this type, with the single: aJJi·
tional requitement that the No. I pins
muse be: grounded.
The: 6CA 7 JEL. 34 is a pentode tube:
of extreme linearity. hs prefl:rrN oper·
ating conJition is as a pentO\lc: ;although
the: manufac tu re r furnishes trio..le r;acin�s
for che tube. rri,,..le UI'l'r;alion r�'sults in
higher .JislOrriulI an,1 rl",h • .:e..1 powl"r vut·
pu t. The convencional ultra· linear con
ncecion cannot provide opcimum rc:sults
wich dICse tubes either, since there is no
ty pe of o pe ratio n more linear than che
pentode conneccion for which chey were
designed. As will be: discu ssed later,
however. a com p romise form of opera·
tion fits the necJs of che Williamson
modernization very nicely.
To dc:rive the: potential be:nefits avail·
able in these: tubcs, proper impedance
macching muse be: obtainc."! in Ihe out·
put transformer, The Ornaco A-430
transfor mer has bc:c:n dc:signed specifi·
cally for chis purpose. This is a �O-watt
unit. che performance of which excI:cds
Mr. WiIliamson's sIX-cificarions wich re·
spect to frequency response. permissi b le
feedback. power handling ability. ;and so
on. At presenc this is the only commc:r·
cial transformer of corrt'Ct impl-Jance.
hut it is anticipate.1 thOlt orhl"rs may he:
available soon.
The Dynaco A·4.30 h as primary taps
which can be: used to fu rnish aboue 10%
screen loading. This does noe cause de·
po wer requirements substantially for
realistic, undistoncd teproduCtion. In
addition, source material frequency re
sponse ha s been cxtended, and this also
introduces the: need for a te-cvaluation
lIf amplifier power requirements. In
crcascJ f req uency response means that
the amplifier has to hand le power at
greater ntrcmes of frequcncy. At these:
e x t re mes, the i mpeda nce characteristics
of the loudspeaker change from the:
nominal values. This means that the
am plific:r is mismlllcheJ at frequency ex·
tremes, and a m ismatch dccrc:ascs the
maximum.powc:r cap.bilities
of any
;amplific:r.
To Jc:liver clean power into
a
loud
'Ilt·,.lel·r IlIa<I, an amplifier must be: cap
.• ble of at least ,,,,ic, the: powe r rcquir<....
for a resistor load such as is used in
Ill<::u uring and ratin" amplificn. Thus
the e x tension of both dynamic range and
frequency range in modern rccordinll.
I'M sourCl"s. and tape means that 2�
w.IttS are ;about a 1fIilfi1flllm if top-grade
I,. "Improyin& the
W,Uiem ... Ampliller". HaoUe .. T� N.....
... bn...,)' 1953.
'Hatler. D .• and Kef_ H.
di
.
..
�' .
��.
.----4--r---".".......
.. ....-.J
.,
••$0"'.
_.
:;&:'000.
,
"
,'�
"
"
'.lv.
iOONOT uou_o elN". 'A�
I' FILA"'", c.1.CulT '.
'.OuNOCD ... "( ....�I
,
NOT(
_•• " IN LA."
T"",
..... , "se ... ""\, .... ",cas. OT"'.
"'•• " MA., ...... , Ot"(IIt',,,
wA6.ulS OIH."Ou.c ON O"IGI�
....... c:;,( .. , .. , ""(D. '''''se
00 .. or ...... I TO.' C ...... CEO.
AUIlIOCltAFT I\fACAZINE
hi8hct ratil1�)
t h is
I
\V1f�-J
arc
serieS
III
i fom
junction, with the negative rl"Ctilier
'l'rminal� tow;lr,1 rhl." ""trut �i,'(·.
Th..
rl·s:.rhinH negative OC is filtl."rl-J by a
IO.OOO-ohm potcmiomercr and a 47,·
OOO-ohm fixed resistor, and the arm o f
bypassed by • 40-p.fd (or
the pot is
greate r ) , I � O· volt Cllpacitor. This is
s hown in the diagram. Fig. 1. The po
tc:ntiomerer can be placl-J conveniently
in the hole that form e r ly hcld the bias·
bala nc ing pot. A bias-balance ad j ust .
ment is no lonlte r requ i red , since the
OUtput transfo rm er is
of
desig n
•
in
which perfo rm an ce is noc deteriorat�-d
by moderate current unbalance, Ind the
used do not have much vari;!tio:l
current drain.
The new potentiome ter controls the
rubes
in plate
to the bottom
IOO,OOO·ohm ou t p U t .
bias voltage, which is fed
ends of the
tube
two
grid resistors. These,
of
course, are
no longer connected to the components
formerly used in bias bala ncing.
i
·i
The combination of chan,lt es descriheti
above has inc:rC';lSc:d the !'uwer ut the
amplifier to about tw ice iu u l t ra · l i nc ar
rating and
ra ting.
about 4
po"ant
improvement,
i
J
but
imp rovemen t
important
,
iu trioJe
times
This dunge alone makes In im·
an
equally
also
can
be
made by ex tending the stability m argin
of the amplifier.
Stabilizing the Amplifier
good amplifier design ha ve
Criteria for
changed in recent years, and the stress
:rioration of the extreme line:uity of
the tubes, and it does ha ve the advantage
of lower ing the int ernal im ped ance (for
bener damping). fur ther, it improves
regul.uion of the output
the inherent
stage to the point where no changes need
be made in the B+ supply of the basic
Williamson in order to
tubes.
use
the new
This tran s form e r can he interchan,lted
.Iir��rly w ith ullits fnrmerly u5<.·,1 in this
pli fier) . It fits the space, des pite its lar.fl:e
size, because it hu no flanges.
Insr.alla·
tion of the t rans fo rm er requires no me
chan ical alterations in the chas sis except, .
possibly, reaming the mou n ting holes to
accommod ate mounting sruds.
of Ilias than rubes generally
amplifiers.
Value
used in
of the
he reduced to
c ircuit. If the uri}o;in;11 output ruhes were
catho.le resistor
100·ohm screen
suppressors connected from pin 3 to pi n
4 shou ld be removed. and the trans
former leads connectt:d as indicated in
the schematic ,!iagram (Fig. I ). The
constructor should note that the circuit
ahout 200 ohm s from the conv entional
triad e connected, the
2�0 to
should
300 ohms. When this hu been
done, after substit u ting rubes and out
pu t transformer, the resulting amplifier
can put out 3� to 40 excep tional ly clean
warts.
However, the capabilities of the
m ust be tr ac ed from the: phase inve rter
to the output grids in order to ,I eterm ine
which of the output tu �S is the "top"
exploited unless the constructor is will·
Transformer lea ds
replace the orig i na l sclf· biasing ar range ·
one in the c irc uit .
must be
properly conn(:Clcd to the out
put rubes, or the fL"C\lb",k phasing will
be incorrect . I f t he re is a loud buzz
after connecti ng the transformer, the
plate and screcn Icad from onc output
rube should he transferred to the other,
and vice versa.
figs.
2
3
and
a
show
t he
A··1 \0
Hc-.lIhkit
W. �M. (Sce Appendix for s��ific con
vt:ninn in�tna:tiClll\ fC):"r.Jinj( this amTloun!ed
on
cnnven,",I
stability.
Many
amplifiers,
while performing well under steady-state
conditions, have exhibited
muddy and
harsh qualities when reproducing music.
One
reason
for this is the fact
th;!t
their transient performance is inferior
Biasing the Output Tubes
The 6CA 7 JEL- 34 requires a lower value
Williamson
is now being placed more and more on
am p l ifier
to
that
under
steady·state
conditions.
A.nother reason is that am pli fi e r
per·
formance on loudspeaker loads is often
not as good as it is with resistor loa.J�.
This point was touched on briefly he·
fore: its
feedback
ramifications w i th respect to
i nstabili ry are far· reach ing.
Many designers have come
to
the con
elusion that stability, has more effect on
listening qU:llity than disto"ion does'
ConK-quently. increases in the m:lr,IC i n of
new rubes and transf ormer are not fully
ing to incorporate a fixc<l-bias su p pl y to
ment.
Addition of
a
negati\"e DC s u pply for
fixed biu is quite si mp le.
A. ca pacito r
of .O� p.fcJ is taken from one side of the
high· voltage
secon da ry
of
the
power
transformer and connected in series with
a 27,OOO-ohm, I-watt resisto r to �roun.t.
Th�'sc form a di v idin� network which
cuts the AC v oltage from the power
transformer to less tha n �/I it s full value.
Two snull �Icnium r�"Ct i fiers (20 ma or
17
, �bilit�·
Br
arc
important Jcsi,!;n prublc:ms.
rhe51:
sranJar,l s,
rhen,
original
W iII, ; m�n amplifiers hav� ina.lequar�
. srabilir�' ;lr b o th l'xrreme IlIw anJ hi/-th
frnlll"1Il i.. �. Thi, UII 1><: ,1C:1ll01l5Ira"...1
raplJly IOwarJ
yanishi,,� point.
il
The:
frequency rl'Sponse of the a m pl ifi e r will
be app roxi m ately the same as t ha t of the
original version el[cept that peaks in the
rc:spUllllC (a",)(iaft.� with instllhility)
The r ran sient
fnr the.' IlIw ('11,1 hy IlIuchin" the: inllUr
a� eliminated.
grid momentarily with the: finge"ip and
- thar unmeasurable intangible - will
watchi n g the spt.-:aker con�.
be audibly better. It
lo w -f requ en cy
transient
The heavy
which is gen
crated tri.ltgcrs the ilml'lilier, and ther�
ar� usually sevcral sur�c:s befor�/ th�
dfl'Ct is Jam rc:J out. The: s peake r ' con�
can
be
back ar.d fo "h
com in,IC t... rc:st. This
ob 5l:rved to mov�
5I:veral timl's hdllre
means t hat �hnrt siltna l impulses will
also cause: sl'urious cone movcmcnrs
which tend to hlur the sound.
At hig h fn:quencies, the correspond
ing effect can he viewClI on an oscillo
scope with a square-wave signa l input.
A ripp lc...J square wav e is ind icative of
hasic
instah ility, and in,licates rhat
rhere is a transient distllrrion of high
frequency signa ls.
Insrabilitr is due to rhe facc rhac
the phase: characteristics of th� amplifier
C:1U� some IIf rhe fl'Cllt-ack ro he al'l'l i,...1
pnrilil·..J, illsrc:hl of Ht'X.,I;,·d, at the:
fn:quenc)' exrrc:mes.
The n:me.�y is
superficially s imple - to shift the: phase
in the right direction at the critical f�
quenci�s. le is not always simple to do
rhis. Forrunarcl�·. the phase characteris
tics of the A-4 30 trans forme r and the
\,(/iI li amson circuit arrangement permit
complete corn:ction of the low-frequ�ncy
-p1r.UeChar,u:tcristic', and apr>reciabl� -cor
rectio n of the high-freque ncy characc�r
iscic. T hese corrections a� made with a
few inexpensiv� componenrs.
The
lo w - f requency
correction
is
achieved by shunring the 0.2�-,.tfd
coupling capacitors which go co the out
pu t grids with I megohm resistors_
High-frequency cor rection is obtai n ed
with a 100-p-p-fd capacitor which is COD
nected fr o m the lowe r drive r p lat� to
rhe cathode of the firsr stage. Without
,ltoing in r o the theory underlying these:
<.t'rr,·cti,,"!, il is wurth IIICllliollill1( Ih:lt
they have a trem�ndou s effect on pe r
formance.
( It is assumed that t he
10,OOO-ohm resisto r in the input grid,
change of the .0�-p-fJ capacitors to 0.25,
an d the use: of. a small capacitor across
the feedbac k resisto r as indicated in the
sc hematic are already included in the
amp lifie r. If not, these should a lso be
added in accor,lanc� with previous
recommendations·. )
This completcs the modernization of
the Williamson. If the power su pp l y purs
out a full 150 volts with �nable regu
lation , the ou t put pow�r will be a bout
50 wattS at 1% IM d istort ion. If the
power supply provides lower voltage, the
output power will be rcducl-d somewhat.
Below full o utput, the distonion Jrops
°H.ner. D.• "A 60 Wen Uhre·Li""... AmpIiIia",
H",/i. .. r.I...-;.,.. N._. FebnIM,. IIISS.
� v ide nt
in
mo�
w ill
be
solid,
responle
particularly
speci fic
hints
will
be
hc:l p fu l ro those who arc intereste.-d in
mod erni zing the Heathkit W-3M Wit
liamson:
The bias voltage divider an.d rectifie r
can go in the power supp ly chassis. Out
put of th� selenium rectifier is connected
to pin 5 of the pow�r socket and carried
rh roug h the spare wi� in the connecting
cable to one side of the 10,OOO-ohm
bias·setting pot.
The 2�O · oh m and
tors are di scardc:d ,
as
twO
I OO · o h m resis
is the lOO-ohm pot.
The bias pot �p laces the lOO-ohm bal
ance :l,fjustment, and the hla n k tie
puints to which the WO - ohm resistors
. "'e�
fastened can then be usc...J ro con
nl'Ct lead. which a� pan of the ground
circuit.
In the
20'p-CJ
intercsu
capacitor
.houlJ i>c .lJ,kJ.
tWttn the two 6SN7's s hould be in·
c r l' ascd to O.2� p-f,1. Car� sholll,1 he
cxcn:isc.-..l in ord�·c to get thc:lIC
the SI'l\lCt'.
If di/liculry
10
hI
is enCllunrecl.'d. ic
is slJ.&:,I:esred that miniatu� capacitors be
used. such
The
as
Aemlitc.'S mad e by Aerovox.
l·megohm
phasc-corCl't:tins
rc:·
sistors across the co upli_ng capa.citocs pt.·c·
Appendix
followi ng
n(ll, onc:
The .05-p-fd cou plin,!; capacitors bc:
better· defined
bass and s m oothe r, cleaner rreble.
The
t:�UlppCJ; jf
of econom)"
which formerly
the
by-
m i t som� positive DC to appear on rhe
grids if the nega tive bias supply is in·
orc:rative.
The
existence
of
nt·,r.::llive
voltage at the: grid should be chl'l.kc:·,1
with the rccrifier removed bc:fore �r·
mitting the B+ voltage to he a p p l ied .
Then the rectifi�r should be i nse " l·d and
the bias set to 3' vo lts from �rid to
g round ../In the cubes ha v e had timc to
warm up. Do no t remov� the secon,1
6SN7 while the amplifier is on, because
this will cause additional positive volla,r.:e
to be applied to the grids of the out
put stage, upsetting the bias and possihly
ha rming the cubes.
AUDIOCRAFT Test Results
The
amplifi e r
wh ich
shown in Fip. 2 and
I,
the basic
Williamson c on ·
verted in accordance with this a"icl�·.
produced 0.6% IM disto"ion a t �o
watts output. 0.2% at 36 watrs, and
0.1 % at U watts. Below I � wa tts dis
was
to"ion was in the residual range of the
m eter and could reasonably be called
negl igible if n ot unmeasurable. Test
frequencies we� 60 and 7,000 Cp5.
passed the cathodes can be used for
filtering the bias supp ly. The schemat ic
calls for 40 p-fd, but th e difference in
hum l e vel is only 2 db.
Jacks formerly used for meter ing plate
current can st ill be used to c heck equal
ity nf the output tubes. The former con
",:cti on to the 2 '0-oh m bias resistor
must now be: g round ed in ord�r co com
plete the circuit path for the cath-xle cur
rent. 'At the same time the No. 1 p ins
( sup pressor grids of the output cubes)
should be connected at the socket to
the No. 8 (Cilthode ) pins.
In o rd er to inse" the 10,OOO-ohm
paras itic suppressor resistor directly at
the input grid, the 2. 2 - megoh m resistor
should be reconnected directly from the
input socket to grou nd . The 10.000uhm n:sistor can then be inserted from
the i n put connector to pin 1 of the
first ('SN7 tube. This replaccs the .05p.fJ capacitor which is used as p art of
Ihe bias supply iD the powe r chassis.
Make sure that the pre:amplifier used has
an output coupling capacitor since the
amplifier now has none in the input.
Practically all preamplifil.'cs are so
mixed in a 4-ro-l ratio.
These: readings could have been im·
proved slightly by using pans matched
mo� precise:ly, or by add i ng to the
power supply filtering. It is d oubtfu l
that such smal l improvement would he
apparent audibly. On the o rh�r hand. if
the phase inve " er balance w�re off, Ot
certa in other �Iements had drifted in
v alue, the disto"ion figures might be
doubl ed - still exceptionally good per·
f ormance.
frequency rnpnn� was perfl'Criy 1Ii11
within the range of our test equipment.
Calc ulated response, w ithout the 100p-p-fd f�baclr: capacitor at the driver
stage plat�, is ± I db from 2 to 200.000
cps; with this added, response: above
80,000 cps slopes off smoo t hly. This
capacitor causes a very slight increase: in
distortion at 20,000 cps, which is quite
i nsignificant compared to th� 12 d b in·
crease: in the high-frequ�ncy stabili ty
margin. Stability at the low end, deter·
min ed by recove ry characteristics f corn
a sharp overload pluse, was. apparentl}'
perfect.
Squa�-wave response: at low frequcn.
cies
was exce llent, and �xceptionalh'
at high fCl'quencil.'s. Power re·
s ponse: was completely flat from 20 III
20,000 cps at '0 watts. Toral COSt for
all parts required to ma k e Ihe com plt·te
conversion: less than $·10.00.
good
AlIl>IO(RAI'T MAGAZINE
♦
Verso Filler Page
♦
IMPROVING
the WILLIAMSON
AMPLIFIER
By T ALBOT M.
WRIGHT
Changing a few resistors in the driver section results
in reduced distortion in this hi-fi amplifier design.
HE Williamson amplifier, whether it
be of the classic triode type or the
clium to high levels and instability. We'll
discuss instability later-but first to the
distortion.
T "Ultra-Linear" vel-sion, has to some
extent fallen into disfavor with audio
philes since it is now possible to build an
IM Distortion
If you run an intermodulation distor
tion test on an avemge Williamson am
plifier using the classic drive system you
amplit\er at lower cost and still equal or
better the Williamson's performance.
Many Williamson owners have dis
carded their trusty old amplifiers in
favor of 50 or 60 watts in the never
ending chase after unmeasurable dis
will find that the 1M exceeds 2'ir at only
slightly over half the rated power. By
the time you reach rated power you
tortion. Many Williamsons have been
converted to higher powel's by the use
find the distortion is well past the toler
able level. The result of this testing
usually leads to a substitution of all the
tubes in a wistful search for lower dis
of newer tubes and heftier output trans
formers. In some cases the proud owner
tortion. Sometimes this helps a little.
Basically {ne voltage amplifiers are
has realized an audible increase in defi
nition and an accompanying drop in
distortion.
Frequently such an experimenter will
add a high-power output stage to his
Williamson drive system-in most cases
paying for 50 watts in order to get a
clean 20 watts. Even then he doesn' t
producing much greater amounts of dis
tortion than they should. "But," you say
to yourself, "I followed the design
scrupulously; even the voltages are
what they should he." The point over
looked. however, is that the American
6SN7, 6CG7, or 12AU7 will not work
always get the improvement he expects.
The fault lies not in the power stage but
well with the circuit values specified by
Mr. Williamson in his original paper on
in the drive system. The two main faults
of the Williamson are distortion at meFig.
1.
the subject.
aeductlon In Intermodulatlon distortion of the amplifier shown in Fig.
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dicate that the 6SN7 should be operated
with 250 volts on the plate and an 81J.a
volt bias.
All you have to do is change that
cathode bias resistor from its present
value to 1000 ohms. A resistor of 1200
ohms works well, too. With a lOOO-ohm
bias resistor, there .will be about 250
volts at the driver plates and the bias
will rise to approximately 9 volts. Not
only does this cut distortion but it
means you can produce more drive volt
age. Naturally, too, you can now put
more signal voltage into the 6SN7 driver
before it starts to draw grid current.
The driver will stay class A, in fact long
after the output stage has finally given
cause of the d.c. coupling between them.
Let's start with the first stage. The
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around 560 ohms. This produces a bias
of around 51,2 volts with about 175 volts
on the plates, assuming a supply of
around 440 volts. The tube manuals. in
up.
rhis change alone, of course. will not
eliminate the distortion fed into the out
put stage. The first two stages are still
underbiased and they hang together be
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2.
A careful inspection of any U.S. tube
manual will disclose the fact that the
6SN7 or 6CG7, as used in the William
son amplifier circuit, is badly under
biased.
Take the push-pull drivel' stage fO/'
instance. Usually the plate loads arc
47,000 ohms and the cathode resistor is
30
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40
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bias resistor usually found there is 470
ohms. This should be increased to twice
its value-simply add another 470-ohm
resistor in series with the fil'st. This
raises the bias to around 3 volts, but
incl'easing this bias causes the plate
voltage of this stage to go up, thereby
decreasing the grid-cathode bias aCI'oss
the' phase-inverter-the next stage.
There al'e now two things to be done:
lower the supply voltage to the first
stage and increase the supply voltage
to the phase-inverter. If each of these is
changed in the right proportion, both
stages will be biased correctly.
The average WiIliamson circuit uses a
33,OOO-ohm resistor from the 450-volt
supply point to decouple the first stage;
this must be increased to about 47,000
ELECTRONICS WORLD
ohlll�. This will cut the current through
the stage somewhat, producing a cath
nde uias of around 21,.. volts, and it will
also lower the plate voltage. The de
coupling resistor to the phase-inverter
is usually 22,000 ohms from the 450 - volt
point. This resistor should be l'ec1uced to
3900 ohms,
pulling
more
current
through the phase - i n v e rte l' and raising
the cathode uias,
\\'hen these last two ch a n ges have
b(,l'1l made there should be about a 5 %
to i-volt bias from grid to cathode of
t Il!' phase-in\'el'ter. Of couI·se. i f t he ini
tial supply voltag\? is much below 450
you will not ge t exact Iy the ri ght bias,
but it will be close enough to allow the
/irst two stages to bias each othel' cor
rectly. Typical voltages for this circuit
are found in Fig, 2.
The net result of all these changes is
to almost double the drive voltage avail
able to the grids of the output tubes.
Fig,
2.
With a 450-volt supply you can get well
over 100 volts to each gdd. Wit h a 400volt supply that drops to somet hin g near
350 volts due to lack of regulation in the
power supply, you should still be able to
get 80 vo l ts of drive per grid.
There is one thing more which must
be done before you have the amplifier
operating correctly. The outp u t trans
formers used in Williamson a m plifi e rs
will not usually hane-lIe more than abou t
20 db o f feedback, but increasing t.he I'e
sistor to the cathode of the first stage
has
already
increased
the
feedhack.
Since the value of the cathode resistor
has been doubled, it is necessary to
double the value of the feedback l' es istor
to put the feedback at its previous value.
With the changes outlined, any "UI
tra-Linear"-type Williamson should not
produce 1 ';', distortion until it reaches
overload. If the drive volt ages to the
gri ds of the output tubes are balanced,
the distortion should be well below 0.5";' .
Performance of the amplifier of Fig, 2
is illustrated in the curve of Fig. 1. For
all IM tests, the gene rator supplied fre
quencies of 60 and 6000 cps at a ratio of
4 to 1. Readers should not be too con
cemed about the absolute values of
power indicated nor a comparison of the
power in one amplifier we have worked
on with another. The important thing
heing shown here is the definite im
provement in performance that occurred
with the changes incorporated.
Amplifier No. 2 (see Fig. 3) u s es four
5881's in a push-pull parallt'1 triode out
put stage. In this one only the b iasing
modifications have been made, The
tubes were chosen at random and no at
tempt was made to balance the drives
beyond the tolerances provided by the
5-year-old 5"/� resistors already in the
circ uit, A 450-volt capacitor-input
power supply was used in this circuit.
Circuit of one of the amplifier, modified by the author. The value, of five re,i.tors have been changed,
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Improvement
plifier ulin, four
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way to opUmize the high-frequency re·
sponse of the amplifier is to use an oscil
loscope when making adjustments. In
practically all cases, bowever, the l 'e will
be no audihle dilTel'ence between resu l ts
derived from u!!ing the given values and
those elerived fmm inclividual trim ming .
The low-frequency transient response
of thc Williamson is less critical and
e8!lier to !!traighten out. Many ampli·
fiel'!!. tend to get "hloflpy'� when .hit hy
tiTgh--intensiiy. Iow-frequency sounds,
clue mainly to inadequate dceoupling of
the "8 + ." It is recommended that all of
the stages he dccouplccl hy at least 40
,.f., although in these amplificrs one of
the (sul'pl'isingly) critical points is the
phase invel·tel' which should he mOl'e
heavily decoupled. The now 3900-ohm
l'csistOl' decoupling the phase splittel·
should he hypasscd with an elcctmlytic
whose value is in the neighborhoocl of RO
to 100 ,.f. Thesc chan�es will pl'o{luce a
stahle amplifier if you have uscd a go{){l
outpu t transfonnel'.
:-Jow that the nmplifiel' has heen sta
hilized, let's try to furthel· impl'fI\·e thc
low-frequency t nlOsient I·cspnnsc. Thc
capacitol' bypassing the cathodc resistor
of the output stnge is usually sOl11e
whcre hetwecn 20 ,.f. and 250 ."f. Use of
the 2;)()-,.f. (�apacitOl' hypasses the staJ:e
to at·ound 6 cycles, that is, the stagc is
fiat to about 10 cycles. Low-pitched mu
sical waveshapcs contain nC1\l'-d.c. com
ponents and in orelel· to hnnelle these
ndequately you must have the output
stage bypassed to approximately 1/10th
of the lowest r.·equency you wish to re
produce. If you want to have go{){l tran
sicnt response helow 60 cycles, you have
to bypass the output stage more heavily
than 250 /.f. Capacity of 500-600 /.f.
(mnrle up of a couple of 300-,.f. elec
trolytics in pamllell will hypass the
output. stage to ahout 3 cycles nnel will
20
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24 ZII
of am
in triode connection.
Amplifiel' No. 3 (sce Fi�. 41 uses two
EL34/SCA7"s connected ns pentodes
with a 250-ohm self-hias resistor and
5000-ohm plate-tn-plate load. A�nin. no
special elTOI't WllS made to hlllnnce the
drives. A 450-volt choke-input filter wns
used hel·e.
Transient Response
All of the fm'egoing will cut distortion
dramatically, hut. we have yet nnother
pl'oblem to face: tl'Hnsient I·espo nse . Im
provement in an amplifier's lI'ansient
response is usuully more striking than
impmvement in distortion. Now, the
high-frequency I·esponse of many \vil
Iinmson amplifiel's is ragged at hest and
it is nearly impossihle to give n set of
values to smooth the high end of all
these units. However, the values given
in Fig. 2 will be about right for an "UI
tra-Linear"-type \Villiamson using
either Stmu:or or D.I",a(:o lI'ansformel·s.
The RC values bypassing the plate load
of the first stage aren't too critical. The
small capacitivc fccdback loop fmm thc
plate of thc lower drivel' to the cathode
of the first stal!c is also non-critical.
However, the value of the capacitor
should not exceed 100 JIIlf. Thc only
value that must be jugglcd with ex
treme care is the capacitor acmss thc
feedback resistor. About 50 N'f. to 150
p,p,f. is usual in this position. The only
give good transient response to nrounel
30 cycles. It is n l'at·C speaker system
thnt can proeluce 30 cycles and it is n
rare I'oom that will sustain that low n
note even if the speaker is capahle of
Fig. 4. Performance of another amplifier modifi.d by author. This one utiliz.d a
pair of El34/6CA7', with cathode bias and a choke-input filter in the power supply.
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prodUCing that note In the ftrat place,
One word of caution: the use of vt'1')
large bypassing values, 1000 flf, (m' ('X,
amp l e, may again lead to instability.
Other Improvements
A Wi lli amson amplifiel' incorpcmll inl:
the modifications already out1incc I Idll
sounrlgood indeed but thel·c i!l one alllJi,
tiona l thing you may do if you want to
be "elegant." Any "Ultm-Linear"-t�·
Williamsoil- -even onc with a choke-in
put. powel' supply -will display It dl'Op
in "n +" dUI'inJ: extremely loucl pns.-
sages. Thill hns a limiting elTeet on the
powel' output amI the amplifiel' may not
recovel' fl'om ovcrloads ,'Ill quickl,\- ;15
you'd like. The remccly ill to dccou pl(' 1 he
output stn�e 11100'e heavily. It hns 11C'�n
found that foUl' 9O-"f. elcetl'lllytics '·on
nceted across the center-tap of the out·
put. h'anil fOl'mel' will pmvide supel'b
transient I'csponse. These capacitors
hold enough of a chat'ge to supply the
output tubes with plenty of current ,lur
i ng short·eluration, high-intensity
sounds and prevent n dmp in "B ·i-." The
amplifier shown in Fig. 2 does nnt ,lis·
play the slightest change in "8 -I·" until
it is opcmting almost continuously in
overload, Thc elTeet of this is most no
ticeable on low-fl'equency transi('nls.
Also, it will give the amplifier practi
cally instantaneous overload l'cellvel·Y.
Changing the bypassing of the out put
stage cath{){le and incl'easing the out·
put-stage decoupling will likely pl1Kluce
no audible changes unless you have n
really finc.spenkel' lIystem. \Vith such a
system, the impl'Ovement is readily ap
parent.
Of course, neither of the last 111"0
m{){lificntions_ is o( much value if :hc
phono preamp doe� not equalize ,·or
rectly at the very low frequencies. t 'n
fortunately, many preamps don't. The
boost applied below 100 cycles in the
RIAA curve is a help, but many record
companiell do not always apply the pre
Pe
sumecl low-fl'equency pre-emphnsis. A
dillcussion of equalization nnel tonc-<:nn
tl'lll is heyond the scope of this a t 'tielC'. �n
we will assume that you can get thc
lows into yOUl' amplifier.
The modifications outlineel can Ioc
made for around $10 anel, in view of I he
improvement in pel'fOl'mance, an� 11 ,,"
worth the money, It is not the aiitholi··�
intention to disparage the OI'iginaV"'i1Iiamson nmplifier elesign-it has much
to recommend it, It is easy to build anll
by I·e-biasing the voltage-amplifiel� it
becomell non-critical of tube selcetinn.
Furthel'mOl'e, the "Ultra-Linenr" \·C'i'·
sion is cnpahle of elelivering all the c1C';In
powel' nceded to drive any speakel' sy:<
tern except fOl' some of the less effid('1I1
---although excellent-bookshelf tYIII':<_
The use of a chokc-input power supply
combined with the adrlitionnl hypassing
and dec()upling will permit the "Ultl'l\
Linear"-type Williamson to cleliver
several watts mOl'C than ordinal'y cir
cuih'y permits. The distortion won't cx
ceed 1�; until you l'each overload and
will sound as though yo u have doublc<1
the power output when what you havc
clone is improvc the overload pel'Corm·
ance.
ELECTRONICS WORLD
♦
Verso Filler Page
♦
The "Williamson Type" Amplifier
Brought Up to Date
M. V. KIEBERToOI
Attention to detail in the construction of the Williamson-type am
plifier, as well as in the choice of components and tube types, re
sults in an amplifier with low noise level and a minimum of "bugs."
HILE BUILDING a new "super" am
plifier, it became necessary to pro
vide an interim power amplifier
for use with the pr.e-amplifier-equalizer
unit to be described by the author next
month.
The unit was to be rack mounted on
a standard 19-inch assembly with a
chassis 17 by 10Yz inches. The power
supply was to be integral with the am
plifier with the provision for self-con
tained output-stage metering and plate
current balancing.
ElectricaHy, i t was required that the
unit have a l6-ohm output; a calibrated
input attenuator, a 6OO-ohm input or
an unbalanced high impedance input of
100,000 ohms; a power output level of
approximately 15 watts with negligible
intermodulation distortion; a relatively
flat power-frequency characteristic, a
W
• Eclipse-Pioneer Division, Bendi; Avia
tion Corp., Teterboro, N. I.
high output damping factor; a gain of
60 to 80 db, and a noise and hum level
held to at least 80 db below a one-half
watt nominal output level. Combined
with these specifications was the re
quirement that the unit have excellent
transient response not only when work
ing into a resistive load but also under
the condition of driving a speaker load.
To cross-check the latter requirement,
square-wave tests were to be made un
der both conditions of operation-re
sistive load and speaker-and these
measurements were to be compared
with the speaker output as sampled by a
calibrated microphone.
Preliminary Desig"
A number of circuits were investi
gated and checked. The basic William
son design looked like a good starting
place, and accordingly a rather compre
hensive investigation was made of the
various circuit literature published on
this general type of circuit.
Initial tests soon indicated that four
major sources of noise, distortion, and
poor transient performance limited the
optimum capabilities of the generaHy
used circuit. The difficulties were found
to be as foHows:
1. The input stage was too noisy, had
hum. and required careful selection of
plate and coupling resistances in order
to provide a balanced, "clean" output
from the second half of the tube.
2. The driving power to the 807'5 was
marginal at peak levels.
3. Typical coupling capacitor tolerances
and variation of distributed capacitances
tended to cause circuit unbalance at both
high and low frequencies with attendant
transient difficulties.
4. Composition resistors as normatly
used in the feedback circuit gave distor
tion from the frequently overlooked volt
age co-efficient characteristics of the
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Fig. 1. Over-ill schemitic of the iuthor's up-to-dite venion of the Williamson amplifier. Note use of tube types which differ from those
origi nilll' chosen for IIse in the Musician's Amplifier-the most populu American version to date.
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feedback.
units generally specified for
position.
use
in this
It was also believed to be desirable to
have a time delay provision so that
plate voltage would not be applied to
the sy'stem until after cathodes had
reached a satisfactory operating temper
ature. Either an Edison or Amperite
thermal relay tube would do the job in
a satisfactory manner.
Initial tests also disclosed that the
use of 10- or 20-"f decoupling capacitors
were inadequate at low frequencies if
good transient characteristics were to
be maintained down to 5 or 10 cps when
a considerable amount' of feedback was
used. Accordingly, 40-"f units were
used in this position, which then pro
vided fairly good frequency and phase
characteristics well down into the lower
audio frequencies.
Study of the complete schematic, Fig.
1, will show the differences in detail
from the standard design. In order to
avoid the four major difficulties ob
served in basic Williamson circuits, all
stages were carefully checked and tested
as individual elements and the optimum
configuration found prior to inclusion in
the final assembly_ Likewise, the entire
amplifier was carefully tested and re
checked prior to inclusion of any feed
back loops-that is, care was taken to
assure a good triode design and per
formance even before feedback was in
corporated in the final circuit. Tests in
dicated that this was a sound design
procedure any way you listened to it
or looked at it on an oscilloscope.
First Stage Improyements
In order to circumvent the first listed
difficulty-hum due to cathode-heater
leakage, and noise arising in the first
stage-it was necessary to take two cor
rective measures. First, a I2AY7 tube
which was specifically designed for low
noise-level audio service, was used to
replace the conventionally employed
6SN7 and care was taken to follow
carefully the manufacturers' recommen
dation as to input grid resistance, heater
connections, and voltage. By-passed and
isolated heater bias was then employed
with circuit parameters selected to pro-
AUDIO ENGINEERING
•
vide minimum hum when an unbypassed
first-stage cathode resistor was utilized
as required when feedback is connected
in at this point. Second, it was found
that the 47,OOO-ohm plate-load resistor
did, as would be expected, cause increased noise unless wire-wound or low
noise level units were used.
With respect to noise level in the firststage load resistor, a number of resistive
elements were checked in order to secure
a relative evaluation of each type and
value. The following were tested:
.
I
1) �
watt composltton resistor
2) 1 watt compos!t!on res!stor
3) 2 watt comP?SltlOn resls�or
4) 1 watt deposited . film reslst.or
5) 1 wa�t S.S. White 10w-nOlse-type
resistor .
,
.
6) 1 wa�t non-mductlve, wire-wound
resistor
.
•
The use of a I-watt composition resistor in lieu of a �-watt unit improved
the noise level by approximately 3 db
with a like improvement occurring when
a 2-watt unit was substituted for the
I-watt unit. The I-watt 5.5. White unit
was about 6 db better than the 2-watt
composition resistor. The I-watt deposited film resistor was slightly inferior
to the 5.5. White unit while the I-watt
non-inductive wire-wound resistor was
about 6 db better than the 2-watt composition unit. Substitution of the I-watt
non-inductive wire-wound resistor for
the generally specified � watt carbon
unit thus provided about 12 db improvement in noise level, while the I2AY7
with a biased and properly by-passed
heater circuit also cleans up the hum
level by about 12 db .and accordingly
helps to meet the original design requirement.
Phase Splitter Improvements
Following the clean-up of the first
stage, the inverter was investigated. Re
sistors matched to :!: 0. 25 per cent were
utilized. Both S. S. White units and l
and 2-watt carbon units were checked as
regards to noise level. As might be ex
pected, however, the noise level was fixed
by the first stage and no significant im
provement was found when changing
from one type of resistor to another. The
AUGUST, 1952
relatively low resistance of these units,
20,000 ohms, minimizes the problems of
high-frequency roll-off and asymmetri
cal phase charactt:ristics sometimes
found in this type of inver�er �ircuit.
.
The splttload, c!'-thodyne clrcul! w.as
found to be supenor to all other circuits
t�sted for possible use in this applicatlOn .
.
.
The dnver stage was next considered
and a number of tube types were evalu
ated and tested for use . in this position.
Tubes tested and considered were the
6S N7, 6N7, 6K6s and 6V6s operated
both as triodes and as pentodes 6AG7s
2C21, 6J6, 12AU7, and the 5687. It wa �
soon evident that the 5687 was considerably superior to other driver configurations although pentode connected 6K6's
were also 'excellent except for the fact
that the considerable variation in tube
characteristics between tubes required a
�areful balancing of tubes and a re-.ad
Justment of the screen vo!tage each hme
a tube . was changed. ThiS w�s not the
case W lth the 56 87 (a dual tTlode) and
.
.
accordmgly thiS tube was selected as the
driver. It is also obvious that the use
of a single tube considerably simplifies
the circuit layout and saves on both
components . and space.
After gomg through the preceding
studies, a preliminary circuit was set up
on an experimental chassis, and fre
quency and phase characteristics were
run with the feedback loop left open. It
was soon noted that rather peculiar
things were happening at low frequen
cies.:!: 20-per cent coupling capacitors
�ad been used (without actually check
m� to see. what they were) while the
.
gnd coupltng resistors had been held to
:!: 10 per cent. The coupling capacitors
were then tested and found to have a
wide capacitance spread. As soon as
matched units were employed, the sides
of the push-pull circuits maintained bal
ance and the frequency and phase char
acteristics smoothed out the way theory
said they should.
The use of cross-neutralization or
phase correction in the output stage ma
teria1iy helped to extend and smooth out
the hig�-frequency end of the spectrum
but agam, watch out for balanced capa-
19
/
citors and a symmetrical circuit layout·
which is most important in this part of
the circuit because this is one of the lim
iting points in the high-frequency end
of the spectrum.
At this point, the amplifier, even with
out feedback, "listened" good and would
appear to satisfy even the most discrimi
nating user. In order to find out what
was available, a series of characteristics
were measured. The frequency response,
gain, noise level, mid-range output im
pedance and power characteristics were
measured.
'With something already good to work
with and an available 20 db of gain to
utilize, the feedback loop was closed.
Initial steady-state measurements were
made and it looked as though the arith
metic were right. It being early in the
morning and we being anxious to test
the listening quality of this new unit, the
,
18
I
I
I
t6
,
••
,
./
I
3 sus Of'
TllSES
010 FEEDBACK
-
'0
•
I
6
I
I
•
0
D
I
,
12
2
I
'"
.'
I'
,
/
3 $£TS OF Tll8ES
20 db FEEDBACK
1'1
..
-+-t
..... ko
I--'
I
III
III
rll
m
,)
10
12
6
WATTS - OUTPUT LEVEL
1.
Fig. 4. Intermoduliltion distortion curves, with
ilnd without feedback, tilken with three sets
of tubes in each condition.
system was set up and a test tape of
known exceIJence fed into the system.
And was it terrible!
Additional tests were now made. The
feedback loop was opened, and a square
wave generator was fed into the ampli
fier. The transient performance looked
good under this condition of operation.
The feedback loop was closed, the circuit
again checked and curves and bumps
appeared where and when they shouldn't.
The input cathode resistor was a �-watt
carbon ,unit as was the feedback resistor
-so remembering a bit about voltage
coefficients of composition resistors, 2watt carbon units were substituted for
the �-watt units and an immediate im
provement was noted. The transient per
formance, however, did not completely
clean up until these two resistors were
replaced by non-inductive I-watt wire
wound units. The p erformance of the
amplifier was now remeasured and is
compared-with and without feedback
-in Figs. 2, 3, and 4.
'\'ith the final version of the improved
and up-to-uate \Villiamson unit con
nected into the complete system, it ap
pears as though this new a mpli fi er may
become more th an the "interim" ampli
fier that it started out to be.
At this point attention is invited to
Fig, 4 which presents the IM tests on
the circuits both before and after closing
of the feedback loop. In each case a fam
ily of curves has been presented. These
families of curves represent, measure
ments made with eight 807's, four of
which were new, two of which had over
1000 hours of operation, and two of
which had o ver 3000 hours of operation
in a voltage regulator. The best curves
in each case resulted from optimum tube
selection, t h e poorest curves-were made
w it h the poorest combination of tubes.
Tests with various combinations of
I2A Y7s and 5687s indicated that the
IM was negligibly influenced by these
stages but rather was almost entirely
determined by the characteristics of the
particular type 807 tubes u s ed in the
output stage,
Past experience has indicated that all
too often the proud parent of a circuit
presents the brain child in Sunday-go-to
meeting cloth es rather than in the day
to-day attire and environment of run-of
the-mill tubes which are generally th e
only ones normally available to the aver
age experimenter or th e unsuspecting
public. In this instance it is believed t ha t
a representative and fair evaluation of
the circuit has been made.
Construction
Care was taken in construction to
avoid circulating ground currents and
accordingly, an electrical connection is
made to the chassis at only the one point
next to the input. Twisted and capaci
tively shieldecl filament leads and as
balanced and symmetrical physical and
electrical layout as possible with this cir
cuit further simplifies wiring and assem
bly while keeping capacitances as they
should be.
C" C,
C., C,
C.,C.
C7
C.
C" CII, CIl
Cu, CII
L,
M,
R,
R.
R.
R"R.
R., R7
R.
R ., R "
R II , R"
RII
RII, RII, RI/, R ..
R",R"
R"
R",R".
RII,
RII, R"
R ..
RIO
T,
T.
TD,
V,
V.
V" V,
V.
PARTS LIST
.05 "i, 6OO-v. paper,
matched ± l4 %
025 "f. 6OO-v. paper,
matched ± l4%
4O-4O/4SO-v. electrolytic
8 "i, 6OO -v. oil filled
1.0 "f, 4OO-v. paper
0.1 "i, 6OO-v. paper
5.1 ""f, 6OO-v. mica
12-H, 120-ma filter choke
I-ma meter, lOO-ohm resistance
0.1 meg audio taper pot
330 ohms, I watt, wire
wound, non-inductive
47,000 ohms, I watt, wire
20,000 ohms, I-watt. wire
wound, non-inductive.
matched ± l4%
0.47 meg, �-watt.
matched ± l4 %
1200 ohms, I-watt, composition
50,000 ohms, 10-watt.
wirewound, matched
±l4%
0.1 meg, �-watt, compo
sition, matched 1%
lOO ohms, 2-watt pot.
wirewound, linear
lOO ohms, I-watt, compo
sition
1000 ohms, JI,-watt, com
position
250 ohms. 10-watt, wire
wound
1.05 ohms, wirewound
shunt for meter
0.1 meg, I-watt, composi
tion
22,000 ohms, I-watt, com
position
430 ohms, I-watt, compo
sition
18,000 ohms, �-watt.
composition
5000 ohms, I-watt. wire
Output transformer, 10,000 ohms plate-to-plate/
4-8-16 ohm secondary
Peerless S-2650 (Simi
lar
transformers
of
other makes are: 1\cro
-TO-290;
T r i a d
HSM-89
or S-481\;
UTC-LS-6Ll.
Suit
able results should be
obtained, although au
thor has
not
made
measurements
with
these types.)
Power transformer�OO0-400 v. at 200 ma ; 5 v.
at 3 a.; 6.3 v. at 6 a,
1\mperite thermal timedelay switch
I 21\Y7
5687 (Tung-Sol)
807
5U4G (or 5V4G)