Modified Altec 1567A

Modified Altec 1567A
Quick-Start Guide
Revision 3 (July, 2013)
Please give extra attention to red text, which contains the most important cautions.
Clark Huckaby
http:www.clarkhuckaby.com
©2012-2013 Clark Huckaby. All rights reserved.
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Unpacking
“This Side Up”: The modified Altec 1567A is shipped with its front panels facing up. This
seemed like the best and most stable way to fit it into the cartons and give it a gentle ride (it is
not the normal operating position). Open the top of the outer carton and remove the inner box.
Open the top of the inner box by cutting tape at the places marked with dashed lines. Further
unpacking instructions are on the inner cardboard cover that protects the front panels.
Before lifting the unit out of the inner box, look for the following accessories which are among
the packing material next to the unit’s plywood base:
One AC mains cable, IEC type
Two short XLR-to-XLR patch cables, ground-lifted at female end
One makeshift XLR- to-RCA cable with 600-ohm load
One package of three spare 1-amp slow-blow fuses
Normal operating position: After lifting the unit out of the inner box (please read paragraph
below on “Handling the modified unit”), sit it down on its wood base so the front panel is vertical.
This orientation is best for heat flow (and also operation of the VU meter without sticking).
Remove transformer stabilizer: Before turning the unit on, remove the stabilizer blocks for the
audio transformers. The stabilizer system is pictured here:
As instructed on the stabilizer blocks, support the block while cutting the four zip-ties, so that the
blocks do not fall against tubes or other components when freed.
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Leave plywood base attached for now: The unit’s plywood base should not be removed until
you are ready to mount the assembly in an equipment rack (see below). This base helps protect
the output driver boards and other components.
Power Up
Hook up AC cable: Make sure main power switch (on auxiliary panel) is turned off. Make sure
standby switch (on vintage panel) is on standby. These are the down positions for both of these
toggle switches. Connect the AC cable to IEC connector in the rear of the unit. It is located on
the enclosure behind the main power switch. (Note: Please avoid excessive wiggling when
inserting or extracting the AC cable. A broken connector or wire to one of its terminals will be
very difficult to repair. It will require separating the vintage unit from the auxiliary unit. I want to
consider the marriage of these units permanent. When unplugging, watch your knuckles as the
IEC pulls free.) Plug the male end of the AC cable into a good, grounded 120-VAC outlet.
Main switch on: Leave the standby switch in its down position but turn the main switch on (up
position). (Note: Please be gentle with all toggle switches, grasp the handle between thumb and
forefinger then use a smooth motion. Don’t “flip” or “throw” them so they click loudly. That loud
click represents a jolt of excessive mechanical force to the terminals, where wires are soldered,
causing these connections to fatigue sooner than they should. The thumb-and-forefinger
approach helps dampen this jolt.) The two yellow LEDs to the left of the main switch should light
up with equal brightness. The red LED above the standby switch should flash, indicating
standby mode. The two VU meter lamps should illuminate the meter; their brightness is
adjustable using the knob above the flashing LED. (Note: Less than full brightness will allow
these lamps to last longer before burn-out.) Make sure illumination on each side of the meter is
equal. If not, it’s possible that a lamp socket dislodged during shipping. If so, turn the unit off,
swing the vintage panel open, and re-seat the socket; see below, under the Rack-Mounting
heading, for remarks about opening and closing the vintage panel. If a lamp socket is dislodged,
it needs to be fixed because it may cause a short circuit.
Standby switch on: Proper use of the standby switch should extend the life of the preamp’s
three 12AX7 tubes by providing them a soft start-up. Details about how this works are given in
the Technical Report. After powering on the unit in standby mode (above paragraph), allow
about 30-60 seconds before moving the standby switch to the up position for normal operating
mode, in which the flashing LED is turned off. Powering off the unit follows the reverse
sequence: first go to standby mode, wait about a minute, and then turn off the main switch.
Fuse replacement: The new fuse holder is unlike the vintage type. It is not a knob to be turned
with fingers; that could loosen or break the housing. Please use a screwdriver on the central
insert to access the fuse. Do not over-tighten.
Overview of Modified Preamp
To help you get to know the modified Altec 1567A, the following block diagrams show the signal
paths of the original and modified systems. Many more details are presented in the Technical
Report. The first block diagram represents the original vintage unit:
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Original design: The original unit’s central feature was the mixer stage--the summing amplifier.
It mixed signals from the four active input channels (microphone preamps) and one passive
input channel. This mix fed a single master channel. All outputs came from the master channel.
This general architecture is typical of the microphone preamp/mixer heads of 1960s.
Modified design: The modification trades the mixer function off for channel independence. This
makes the preamp more versatile in today’s multi-channel studio environment, while retaining
the classic tube character of the original vintage unit. It also allows two or more channels to be
linked in many different configurations, offering many possibilities for distortion and coloration
effects. The block diagram on the next page depicts the signal paths of the modified unit.
Channels one through four (CH1-CH4) are built around the original unit’s four microphone
preamps. Channel five (CH5) is derived from the former master channel.
Channels one through four share a common basic layout, except that CH1 and CH2 have
additional circuitry in the high-impedance (hi-Z) link between the input transformer and triode
stage (more about this below and in the Technical Report). The transformer-coupled lowimpedance (low-Z) balanced input circuits of CH1-CH4 have pad, polarity and Z-select switches.
The normal position for each switch is down. These toggle switches operate relays installed in
the vintage chassis near the input transformers. These sealed relays with gold-plated contacts
handle the signals for enhanced reliability. The balanced outputs of CH1-CH4 are driven by
solid-state circuitry linked to the wiper of each channel’s fader pot. The direct wiper signals are
available at these channels’ unbalanced hi-Z outputs.
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Channel 5 has a hi-Z input, with a pad switch and attenuator pot. Otherwise it is similar to the
old master channel (retaining its all-tube signal path), except variable feedback is provided
using the pot which was formerly the “mix 5” fader. Less feedback (clockwise) yields more gain
and harmonic distortion. Details are covered in the Technical Report. CH5’s transformercoupled low-Z balanced output can be set for nominal 600- or 150-ohms (series or parallel
secondary connection, respectively) using a toggle switch on the rear of the vintage unit.
Special Hi-Z Circuitry in Channels 1 and 2
Compared to CH3 and CH4, the additional hi-Z circuitry in CH1 and CH2 has these features:
Hi-Z ground-referenced input: (Note: “ground-referenced” means “unbalanced.”) This ¼-inch
jack bypasses the input transformer to patch signals directly to the triode grid circuit. At 1 KHz,
the input impedance is 840 KΩ when the Hi-Z pad is off and the pre-triode attenuator is full
clockwise (the recommended settings; see below). This is a “normalled” jack, meaning it
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normally links the input transformer to the triode. Inserting a plug there automatically opens that
connection and replaces it with the plug’s signal. Although logical and simply implemented, such
jacks a bit risky for long-term reliability. If CH1 or CH2 ever develop intermittent signal drop-outs
with the low-Z input, test whether a few insertion/withdrawal cycles of a ¼-inch plug in the hi-Z
jack clear the problem. I selected good-quality Neutrik jacks with gold-plated contacts, but like
all phone jacks, the contacts are exposed to the environment and can get dirty over time.
Pre-triode attenuator pot: The main application for this pot (and the pad switch described
next) is to reduce a signal’s amplitude when experimenting with input transformer saturation as
a distortion effect. Without significant pre-triode attenuation, the triode would be far too overdriven to hear transformer saturation. The recommended attenuator setting is full clockwise (and
high-Z pad off) for nearly all other applications; but when trying out transformer saturation, this
knob must be nearly fully counter-clockwise. This is fortunate, because the in-between settings-from about 10:00 to almost full clockwise--cut the bandwidth (high-frequency response)
significantly, as shown in the Technical Report. This is due to stray capacitance. It’s one reason
I wanted to keep some channels (i.e., CH3 and CH4) free of the hi-Z build-out.
Hi-Z pad: The normal position of this toggle switch is down. The up position engages a -20 dB
pad that expands the useful counter-clockwise range of the attenuator knob. Use it if the input
signal has extreme amplitude. With the pad engaged, the bandwidth loss in the attenuator’s
clockwise range is more severe, and it extends to the full clockwise position.
Notes on Inputs and Outputs
Hi-Z inputs: Available on CH1, CH2, and CH5, the unbalanced hi-Z inputs are suitable for
patching directly to any of the unit’s hi-Z outputs. They can also accept instrument signals such
as from guitars, basses and keyboards. They can handle unbalanced line-level signals from
other studio equipment provided that all of the gear is powered by the same grounded AC line to
avoid ground loops. Generally, keep pads off and pre-triode attenuators fully clockwise, and
adjust the output level on the source equipment or channel.
Low-Z inputs: The balanced inputs on CH1-CH4 directly interface with dynamic microphones
and the balanced output of any of the unit’s other channels, as well as that of most external
gear. Condenser mics requiring phantom power will need an external power supply and
adaptor. The actual impedances of these inputs depend on both channel and frequency, and
are detailed in the Technical Report. Generally speaking, as presently configured, CH1 and
CH3 have the same characteristics and offer the lowest Z. CH2 has the highest and CH4 is
intermediate. CH2 has the best input for transformer saturation experiments.
Low-Z input pad, polarity, and impedance switches: The normal setting of these toggles is
down. If necessary, pads may be engaged (up position) for hot mics on loud sources. Pad loss
is nominally -20 dB, but differs depending on the channel, its impedance switch setting, and the
source Z. Engaging a pad changes the input impedance in most cases (making it 1200 ± 50 Ω).
Engaging a polarity switch (up position) simply reverses the polarity of the balanced input signal.
More specifics about polarity are discussed below. The up position of an impedance switch
reduces a channel’s input Z by switching its transformer from the full primary winding to its tap.
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Unbalanced outputs: The unbalanced output Z on CH1-CH4 depends on fader setting (see
Technical Report). Generally, it is too high to drive “line-level” unbalanced inputs of modern
studio gear (which are about 10 KΩ), but can feed instrument-level inputs (i.e., guitar or bass
amps). They can also feed any of the hi-Z inputs on the modified unit (i.e., those of CH1, CH2 or
CH5). Short ¼-inch to ¼-inch patch cords are recommended. Since these outputs are not
appropriate for external “line-level” gear, instead use the balanced outputs with cable types
suggested below to feed such unbalanced inputs. On the other hand, CH5’s unbalanced output
has a lower Z (about 2 KΩ) so it can patch to any “line” input; just watch for ground loops.
Balanced Low-Z outputs of CH1-CH4: These outputs are driven by solid-state THAT1646
balanced line driver chips operating on +/-18V rails. Nominal and measured output impedances
are 50 and 57 ohms, respectively. These are powerful outputs that emulate transformerbalanced outputs, except in five important respects:
1. Warning! Patching the CH1-CH4 balanced outputs to a mic preamp input whose
phantom power is turned on can damage the driver chips. Please carefully avoid
applying phantom power to these outputs!
2. Output impedance is essentially independent of frequency.
3. These outputs work well into a very wide range of input impedances (albeit with
different clip thresholds) from low (about 100 ohms) to very high (infinity, or open
circuit). In contrast, a load resistor is often recommended when transformer-coupled
line outputs feed high-Z inputs (see remarks about CH5’s output).
4. As with most solid-state circuits, when saturated, the distortion mode of these drivers
is hard-clipping. The clip alert feature attempts to address this issue. As presently
adjusted, the alert LEDs light up exactly at the clip threshold only when driving
balanced 600-ohm loads. This occurs at a very high RMS output level of 25.5 dBV
(that’s 53 volts peak-to-peak across a floating load). In normal operation, it should
only be a concern when driving another channel to transformer saturation. Of course,
more detailed information is in the Technical Report.
5. Ground-referencing one output leg for single-ended (unbalanced) operation can
reduce the clipping threshold by as much as nearly one-half (6 dB), depending on
the load (which determines whether the clipping threshold is set by the device’s
voltage or current limit; see Section 5.9 in the Technical Report).
Balanced Low-Z output of CH5: This is a tube-driven transformer-balanced output (no hard
clipping possible!). Nominal impedances given in the vintage Altec 1567A manual are 150 or
600 ohms, which is now selectable using the toggle switch on the rear of the vintage chassis. At
1 KHz, actual output Zs measured 42 and 169 ohms, respectively. Altec’s impedance figures
may refer to recommended load Zs, not the actual source Zs which are unsurprisingly a bit
lower. In any case, transformer-balanced output drivers are optimized for a fairly narrow load
range, compared to solid-state drivers. CH5’s balanced output can be patched directly to any of
the modified unit’s other channels’ balanced inputs (and low-Z balanced inputs of external gear).
However, when patching to any internal hi-Z or external “line-level” (typically 10 K-ohm)
unbalanced input, a load resistor is recommended. See schematics of suggested patch cables
below.
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Polarity of Outputs Relative to Inputs
Defining terms: Unfortunately, audio engineers, who should know better, frequently conflate
the terms “polarity” and “phase.” It doesn’t help when polarity switches on many mic preamps
and mixing consoles are inaccurately labeled “phase.” Phase refers to timing differences
between signals; the possible number of these is infinite and the consequences are frequencydependent. In contrast, there are only two possible polarities (“+” and “-“), and these are
independent of time and frequency. Reversing the wires hooked to one monitor, for example,
changes its polarity relative to the other; it does not put the monitors “out-of-phase” as it is toooften called. However, depending on frequency, sound reflected from a wall in your control room
may arrive at the ear in-phase or out-of-phase with a monitor’s direct sound, due to the time
delay caused by increased path length.
By convention (industry standard), air compression at a microphone capsule results in a positive
voltage swing at XLR pin 2 and a negative swing at pin 3. Rarefication does the opposite. So pin
2 is called “+” or “hot” and pin 3 “-“ or “cold”. In the opposite case, polarity is considered
inverted. This standard enables preserving “absolute polarity” throughout a recording process.
Polarity in the modified Altec 1567A: The balanced outputs of CH1-CH4 have the same
polarity as their balanced inputs unless a channel’s polarity switch is in the up (“invert”) position.
For channels with unbalanced hi-Z inputs (CH1, CH2, and CH5), balanced outputs have the
same polarity as the hi-Z input (using the standard of XLR pin 2 = “+”). The polarity switches
have no effect on hi-Z inputs. The polarity of the unbalanced hi-Z outputs is inverted relative to
pin 2 of the balanced inputs of CH1-CH4, unless the polarity switch set to “invert” (it is also
inverted relative to the hi-Z inputs in CH1, CH2, and CH5, regardless of polarity switch position).
To summarize, the balanced low-Z outputs normally preserve the polarity of all inputs (using the
pin 2 = ”+” standard), but the input signal is inverted at all unbalanced hi-Z outputs. In other
words, normally pin 2 of all XLR jacks has the same polarity, and this is the same as all
unbalanced inputs, but all unbalanced outputs have the opposite polarity. The reason the
unbalanced hi-Z outputs are inverted is that each channel has an odd number of commoncathode triode stages, each of which inverts the signal. The balanced low-Z outputs are simply
hard-wired to preserve the polarity of the inputs, which seemed like the logical approach.
Polarity is relevant for at least two reasons: (1) You may need to know whether a series of
channels cascaded in a particular way inverts your original input signal. (2) Non-linear
amplification causes waveform asymmetry, so multiple triode stages can cause different
distortion effects depending on signal polarity. It’s possible to conceive of conditions where the
second of two identical non-linear inverting stages cancels out harmonic distortion caused by
the first. Inverting the signal between them would compound the distortion rather than cancel it.
But no two stages are perfectly identical, especially in this modified Altec 1567A. This is by
design; we chose color over conformity.
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Suggested Patch Cables
Balanced output to balanced input: The recommended cable for balanced patches (except to
microphones) is diagrammed below:
Note that the cable’s shield is grounded at the male end only (pin 1 = ground), so that the shield
is supplied (“telescopes”) from the input jack. Such a hookup prevents ground loops and is most
important when patching to an input on an external piece of gear. It doesn’t matter so much for
patches between channels of the modified preamp, but it certainly doesn’t hurt; the two short
XLR-to-XLR cables shipped with the unit are wired as shown. Standard mic cables, however,
are wired with the ground connected at both ends to extend the shield to the microphone body.
Balanced output of CH1-CH4 to ground-referenced input: As mentioned before, the solidstate balanced line drivers work well into unbalanced inputs with a wide range of impedances.
Just be sure to tie one of the output pins (either 2 or 3) to ground. Grounding pin 3 keeps the
polarity of the patch the same as that of a balanced link (since pin 2 is “+”; see above), and a
simple example of such a patch cable is shown here:
To invert the polarity while patching to an unbalanced input, the equivalent cable is shown here:
Notice how the connections to pins 2 and 3 of the female XLR are simply reversed to invert the
polarity. This simple rule applies to all balanced-to-unbalanced cables. I’ll show only the noninverting patches henceforth. Also note that RCA plugs can replace ¼-inch units if appropriate.
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To emphasize that neither pin 2 nor 3 should “float” under any circumstances, here is an
example of an incorrectly made balanced-to-unbalanced cable:
Each output leg (pin 2 and 3) needs to be hooked to something. This also applies to a
transformer-balanced output (see below).
Except for that previous one, the above cables are simple and work fine for internal patching
between the modified preamp’s channels. But if you must patch to an external unbalanced input
on a separate piece of gear, the following connection might help prevent a ground loop:
Here, the balanced line driver gets its ground reference from the external gear. It’s important
that the AC power for all gear in the studio is on the same well-grounded circuit. But ground
potentials can differ between outlets and this connection might help. I would not recommend it
for isolated ungrounded gear such as most wall-wart or battery-powered devices; in that case,
the modified preamp must supply the ground so pin 1 should be connected to the shield.
Balanced output of CH5 to ground-referenced input: The impedance of most unbalanced
inputs is too high for CH5’s transformer-balanced output to work with directly. (A typical “linelevel” input is 10 KΩ, for example.) A load resistor is recommended. The following simple cable
will work in many applications (it is like the cheap example shipped, except for the ¼-inch plug):
A 600-ohm load is shown; perhaps a 150 ohm resistor should be used if the CH5’s output Z
switch (on rear of chassis) is on the nominal 150-ohm setting, but this is not critical.
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In cases of driving unbalanced line-level (or guitar amp) inputs on external gear, perhaps
referencing the ground at the destination and lifting the shield at the source (CH5 balanced
output) is advisable, to help eliminate any ground loops. This is like the corresponding patch
described above for CH1-CH4, except for the added load resistor:
Using CH5’s unbalanced output: The vintage Altec 1567A manual says that the original (i.e.,
master channel) unbalanced output can be used with or without the output transformer plugged
in, and that the unbalanced and balanced outputs may be used simultaneously. I agree, and
suggest that you simply keep the output transformer plugged in, no matter how you are using
CH5. When using the unbalanced output alone, you can terminate the unused balanced output
with a 600-Ω resistor, but this is not necessary:
Or you could just plug in one of the above loaded patch cables into the balanced output and
leave the other end open. You could clip the cheap RCA cable off of the makeshift patch I
shipped with the unit and use the XLR female with the resistor across pins 2 and 3. You can
operate channel 5 with no dummy load on the transformer without hurting anything. This is
experimental gear; so play with it and see what sounds best. Don’t be afraid of blowing anything
up on channel 5. More insight about CH5’s outputs may be found in the Technical Report.
Rack-Mounting
General requirements: The modified Altec 1567A needs seven standard rack spaces, good
ventilation, and about nine inches of depth. Avoid installation near any equipment that could
magnetically couple its power transformer to the Altec’s audio transformers and thus induce
hum. The main concern is gear immediately above the modified preamp.
Removing plywood base assembly: Once a suitable rack location is found, remove the
plywood base: In the rear, cut the two zip-ties holding the lower horizontal brace to the wooden
base assembly. In the front, unscrew the bolt associated with the lower rack-ear hole at either
side of the auxiliary panel. Carefully lift the unit off the base; store the base and re-use when the
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unit must be moved outside of an equipment rack. It helps protect the exposed output driver
boards, output jacks, and associated wiring.
Handling the modified unit: Plan your movements carefully and remember that the modified
unit weighs about 25 pounds. The best way to grasp and lift it is by the horizontal braces in the
rear, the rack ears in the front, the plywood base (if still attached), or some combination of
these. (One-handed moving should only be done using the top rear horizontal brace.) Notice
that the flange at the top of the auxiliary panel extends outward enough to help anchor your grip
when lifting by the rack ears. With the base removed, be especially careful not to wrap fingers
around the bottom of the auxiliary panel or set the unit down on an object that could smash an
output driver board or output jack. While inserting into an equipment rack, before fastening
screws through the rack ears, make sure there is adequate support. This may require carefully
placed blocks and shims to support the side panels and/or an assistant working behind the rack.
Opening and closing the vintage panel: For a unit this heavy, you will need to use all eight
available rack-mount holes. (And I recommend using a flat washer with each mounting screw to
help distribute the load over the maximum rack- ear area.) Of course, accessing the four holes
in the vintage unit requires swinging open its front panel. Make sure there is nothing plugged
into the hi-Z input jacks. Loosen all four screws that secure the vintage panel, and then fully
remove the bottom ones first. Support the panel with one hand while removing the top screws,
and then gently swivel the panel open. When opening and closing the panel, make sure the
meter illumination lamp holders clear the chassis so they are not dislodged nor their leads
pinched. The arrows in the photo below show the critical points where clearance is required:
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Always open and close the vintage panel carefully. And try not to do it needlessly, because
there are a limited number of such operations available before mechanical fatigue will cause the
insulation or conductors of the cables to fail, or one of their support points to break.
As the panel is nearly closed, a gentle downward pressure on the vintage panel may be
required for its mounting holes to line up. The big arrow in the following picture suggests the
direction of this gentle force:
Apply only as much force as needed, because cables running to the panel are the target of this
force. Insert the top screws first, but don’t tighten them down yet. Insert the bottom screws but
leave them loose also. Finally, as you tighten all four screws, apply some gentle downward
pressure to the panel (mostly on the left side) so that vintage panel appears neatly parallel
(level) with the auxiliary panel. Tighten it down, but do not over-tighten.
Conclusion
Hopefully this “Quick-Start Guide” has enough information to get you up and running with your
new, experimental, one-of-a-kind, modified Altec 1567A. As I mentioned, some more technical
documentation is available in a separate PDF document (“Technical Report”), including
schematics, circuit descriptions, some performance measurements, and how it all relates to this
equipment’s applications. Of course, you may contact me at any time with all questions.