Why use the Kimmel mu-stage © 2001 Alan Kimmel

Transcription

Geschreven door Peter
vrijdag 26 september 2008 11:11 - Laatst aangepast vrijdag 11 september 2009 10:02
Alan Kimmel
From the orginal TubeLabs site
Copyright © 2001 Alan Kimmel. All Rights Reserved
Why use the Kimmel mu-stage
The short answer: I like to use my mu stage in my amps and preamps
because sonically and performance-wise it's the best driver stage I've
encountered. It gives me more music, more resolution, more speed, more
performance, and more voltage swing ability than any other kind of tube
stage, mu-mode or otherwise including SRPP, cascodes, etc.
The driver stage probably does more to make or break the musicality of an
amp or preamp than anything else. The main purpose of a driver stage is to
develop the necessary voltage gain to deliver to the output stage. The whole
purpose of my mu stage is to give the voltage-amplifying triode maximum
freedom to do exactly what it wants to do. This liberates the music, for if you
liberate the triode you liberate the music, as explained in my Introduction.
1/6
Intrinsically good performance enhances the ability of a circuit to reproduce
music. Poor performance can hinder, because to a large extent the
performance determines how well the circuit can interface the music to the
real world. Even if you had the most musical circuit on earth, if it had very
feeble performance the music would have a hard time reaching you intact.
Of course, ultra performance (as measured conventionally) is no guarantee
of pure sound. Components capable of amazing (conventionally-measured)
performance can be, and often are, amusical. The ideal is to have
performance that is intrinsically as good as possible, i.e., performance that
does not require or abuse NFB or other aids. An aid is one thing but a
band-aid means something is wrong and people will hear it.
Intrinsically/inherently good performance is the kind of performance that
won't get in the way of the music, in fact it can enable the music to reach us
intact.
As for specific performance parameters my mu stage has high speed, wide
frequency response, low output impedance, and low distortion.
A quick review: A mu-mode tube gain stage is a stage whose voltage gain is
equal to or very close to the amplification factor (represented by the Greek
letter µ, called "mu") of the tube. Mu-mode stages obtain this gain by
allowing the voltage gain tube to operate at constant current. The terms
"mu-mode" and "constant current mode" are equivalent.
2/6
A mu-mode stage usually consists of two active devices-- as viewed on a
schematic the "lower" device is the voltage gain tube and the "upper"
device provides the
current
gain
.
In all mu-mode stages the lower device should be a triode whereas the
upper device can be anything. Having a
high gain follower
for the upper device gives my mu stage its intrinsically excellent
performance and musical freedom plus it gives the stage its large voltage
swing ability. The excellent performance and musicality of my mu stage
makes it the ideal choice as the heart of amps and preamps.
Single Ended tube amplifiers can present a very different situation from any
other kind of amp. A typical SE tube output stage produces many kinds of
distortions and sonic blunders. Such amps remedy this by having a driver
circuit that produces more or less COMPLEMENTARY blunders and
distortions; the errors of the driver circuit then cancel the errors of the SE
output stage, resulting in good sound. This cancellation effect occurs to
some extent in many good sounding SE amps. If you were to take such an
amp and separate its output stage from its driver circuit, and listen to each
section separately without the other, you may not especially like what you
hear. But put them back together and voila-- good sound returns. A few
designers have learned how to refine cancellation techniques to produce
very good SE amplifiers that sound excellent and have very low THD without
negative feedback. More power to them.
3/6
I prefer to use a linear driver circuit as when I designed the Laurel single
ended amplifier for Welborne Labs. To sound good, a SE amp with a very
good and clean front end requires an output stage that's also good.
The Laurel's mu stage driver circuit is transparent - it will not inject errors
into the signal path. The result is that the Laurel lets you hear what your
output tube is really doing. It therefore follows that the more linear the output
tube the better the results in this amp. Peter Breuninger of TAS said that
when he inserted the VV3OB tube into the Laurel he was "shocked at the
improvement". The VV3OB was the first tube from KR Enterprise. Today
there are other superb tubes from KR Enterprise, including more very linear
upgrades of the 300B (available in the USA through Welborne Labs.)
The Laurel sounds good with all 300B type output tubes. The inexpensive
Sovtek 300B sounds quite good in the Laurel. You will most likely have your
own preferred output tube. See
Jeffrey Silverstein's exciting
review
of the Laurel in Positive
Feedback Vol. 7, No. 3, page 98.
There are basically two ways to design SE tube amps:
1. The most common way - Use a distorting driver circuit to drive a distorting
output stage and if their errors cancel sufficiently it will sound good.
4/6
2. Or the way I prefer - Use a driver circuit that's intrinsically pure and linear
and mate it with an intrinsically pure and linear output stage.
You wouldn't want to mix method #1 with #2, i.e., a linear driver stage and a
distorting output stage, or vice-versa. The worst approach would be to
misuse method #1, that is, a driver circuit and output stage whose errors
ADD rather than cancel; I think that by a small miracle this doesn't happen
very often. Method #2 is what I chose for the Laurel, so the more linear the
Laurel's output tube the better. A clean and pure driver stage mated with a
clean and pure output stage delivers pure MUSIC.
Some people use inductively loaded or transformer coupled gain stages and
such stages sound good because they operate in the mu mode. If such a
stage is working properly its voltage gain will approach the mu of the tube
(measured at the tube's plate) if you're not loading it too much. Such a stage
is another type of mu-mode stage and does essentially the same thing my
mu stage does except that my mu stage has several advantages including
its speed and broad bandwidth, good PSR, etc.
Last but not least is the subject of PSR (power supply rejection). PSR is how
well the audio circuit rejects power supply noise and variations. Most tube
audio circuitry has poor PSR; poor PSR is yet another gremlin that will color
the sound. There are two ways to deal with PSR: one way is to regulate the
power supply. Another (and simpler) way is to build high PSR into the audio
circuit. Good PSR is another feature of my mu stage. Most mu-mode gain
stages have better PSR than ordinary tube stages but PSR is maximum if
the upper device in a mu-mode stage is a high gain follower as in the
Kimmel mu stage. Having a high gain follower for the upper half of my mu
stage is the major design difference between my mu stage and other
mu-mode stages.
5/6
All my designs have only the absolute minimum complexity required to bring
home the music
. Except for the two-stage phono preamp I designed for Welborne Labs, all
my amps & preamps have only
one
voltage gain stage which of course is the Kimmel mu stage. My mu stage is
very fast, has very high resolution and detail, and above all it's very Music
Friendly. I've tried every kind of tube stage out there and some were good
but when I tried my mu stage it was only then that I was satisfied, for here at
last was the only gain stage that did not disappoint me in some way. It
looked great on the test bench but how did it sound? When I heard the
music flowing out of this music stage in un-shackled 3D glory I then knew my
R&D and testing had been successful; since then many other listeners
agree. My mu stage gives the voltage gain triode what it really wants - plenty
of room to BREATHE. When you treat a triode RIGHT, giving it plenty of
room to breathe, you have a liberated triode, a "triode in paradise"; the triode
returns the favor by transporting you to that musical paradise. Give a good
triode what it really wants and it will give you what YOU want. This is why I
use the Kimmel mu stage in my amp and preamp designs.
6/6

Why use the Kimmel mu-stage © 2001 Alan Kimmel

Transcription

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