DIY CAT SL1 preamp

[7N7]

Unbelievably complicated for a pre-amplifier I think. I thought that my own pre-amp was complicated (955 diff pairs with 6AW8A as cathode followers with integral pentode loads)

And anyway as soon as I see a 12AU7, something goes ker-plunk in my head. I would never use one of them except perhaps in a guitar amplifier.

I agree that 12AY7 is an excellent valve - measurably excellent at least, which cannot be said for 12AU7!

7N7

[kevinkr]

Having both listened to and worked on an original CAT SL-1 about 22yrs ago I can at least confirm that the clone circuits shown are reasonably close to what I remember as the real deal of that time.

And I can also confirm that it sounded a lot better than most of its competition. Measured OK too. Well regarded in its time, I wanted one then based on my listening experience.

Not however the way I would design a similar appliance, knowing what I know today..

[kevinkr]

Originally Posted by HJWeedon

Hi I do not know anything about a mu follower to me it looks like a classical totem-pole output amplifier, why give it a new name.
<snip>
Hans J Weedon

Salem MA

The topology has been known as a mu-follower as long as I can remember 30+ years, no one here invented the term and it's in common usage planet-wide, but I will say there is no mention of it in my Radiotron Designer's Handbook 4th Edition.. It's not really a totem pole, think of it more as a cathode follower stacked on top of a common cathode stage, the cathode follower bootstraps the common cathode plate load resistor - properly implemented it will give you a gain of mu. Output impedance is not as low as you would expect either, not PP either - quite single ended unlike a properly loaded SRPP. (Which has neither the gain nor linearity of the mu-follower) Morgan Jones has an excellent analysis of this circuit in Valve Amplifiers. (4th edition)

[merlin el mago]

Originally Posted by kevinkr

Having both listened to and worked on an original CAT SL-1 about 22yrs ago I can at least confirm that the clone circuits shown are reasonably close to what I remember as the real deal of that time.

And I can also confirm that it sounded a lot better than most of its competition. Measured OK too. Well regarded in its time, I wanted one then based on my listening experience.

Not however the way I would design a similar appliance, knowing what I know today..

Kevin wich one schematic did you refer, if it's not ask too much what do you change?

[kevinkr]

Originally Posted by merlin el mago

Kevin wich one schematic did you refer, if it's not ask too much what do you change?

No longer remember the specific version, there were minor design changes from time to time - and it was a very long time ago. I recall it did have a discrete solid state regulated supply. All of the clone variants presented in this thread will perform quite similarly.

My vague recollection is all I did was replace a bad coupling capacitor and retube the unit, do some quick measurements and listen to it. One of my clients of the time owned it..

From today's perspective I think there are better designs for the diy'er to build. The Aikido comes to mind immediately, and any number of other designs discussed on the forum. Gain is high, significant levels of global feedback are used, complexity is high for its functionality - very current thinking in its time frame.

[DF96]

But of course that line stage does not use a mu follower or a totem pole, because the wrong value grid resistor has been used for the upper grid. It is actually an active load (ECC83 with 3k cathode resistor), with a parallel CR combination beneath it. I assume the designer had problems with loop stability, so had to sprinkle compensation networks around (such as the 150pF cathode bypass!).

[Dimitris AR]

It's mu follower of course , but instead of taking the output from the upper triode's cathode ( as usual ) he ( the designer ) took it from the lower triode's anode the result is high output impedance and in combination with unbypassed high value of cathode resistor he forced to put the 150pF capacitor to increase the high frequencies gain .

[DF96]

Two reasons why it is not a mu-follower: output taken from lower anode rather than upper cathode, upper grid resistor far too low in value so it acts almost like a short. This means the capacitor which appears to drive the upper grid actually merely shorts out the 47k resistor.

The 150pF capacitor will boost gain above around 1MHs so it is clearly an HF compensation network to adjust phase and so ensure stability. It carries out a similar function to a cap across the feedback resistor, but does not change the closed-loop gain at HF.

Of course, we don't know how accurate the circuit is. Either the designer was incompetent, or the circuit contains deliberate errors to fool cloners.

[Dimitris AR]

I can't see any reason of unstability in this circuit , IMHO the circuit is 100% stable without the 150pF capacitor due to "Large" amount of cathode ( topic) feedback in the second stage and it's well known that cathode feedback reduce a little bit the gain in high - very high frequencies , so adding an 150pF there may have the opposite result if large amount of overall feedback is used ( due to phase shift e.t.c ) .
But even now ( with the use of the 150pF ) the circuit work well and this proves even more that the circuit is stable , this capacitor has nothing to do with those described below in your post """"It carries out a similar function to a cap across the feedback resistor, but does not change the closed-loop gain at HF. """ it is simply an bypass capacitor and I belive that the designer wanted to have a "brighter sound" in his preamp or a wider bandwidth , In conclusion the opposite of what you describes .

[kevinkr]

Originally Posted by DF96

<snip>

Of course, we don't know how accurate the circuit is. Either the designer was incompetent, or the circuit contains deliberate errors to fool cloners.

The schematics shown are incorrect in various ways, not the least of which is that the 1K grid resistor shown in the upper stage of the "mu-follower" was IIRC 1M and there was a 1M grid resistor at the input of that stage as well. I hope this is fairly obvious..

WRT to the 150pF cathode cap I believe this was intended to compensate for the considerable miller capacitance of the 12AX7. Cathode current feedback [large cathode resistor] in a mu-follower has no effect on gain, but it does raise rp significantly which in conjunction with miller capacitance does result in really poor HF response. (The effective rp in this circuit is not constant, decreasing slightly with frequency due to that cap.)

As I've said there is more here than meets the eye, and the end result was not at all bad sounding compared to other competing products. I quite liked it at the time, although I liked what I was doing at the time even better.