Why use the Kimmel mu-stage © 2001 Alan Kimmel

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.


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.


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.


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.


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.


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.