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DIY CAT SL1 preamp

I build the CAT SL1 preamp with the attached schematic... Noted that the gain of this circuit has about 26dB. I noted that the new SL1 have an option of reduce the gain to about 15dB only and does any one can share how to do the modification? just changed the resistance of the feedback (20K ohm) or the cathode resistor at the input stage as well? What should be the value of the resistor(s) to be changed.

Open for other method recommended.

Thanks!
 

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Hi the problems with the circuit as drawn are not that many, the grid of the bottom 12AX7 has no grid leak to gnd, that is not that big a flaw, but a 1Meg to gnd there might make the circuit work a little better. Without that 470k to 1Meg to gnd it won't work at all. I tend to want to fix circuits with a minimum of modifications, instead of declaring the circuit junk. Sometimes that may be true, but with minor mods one can make a bad circuit work pretty well.

The bias of the input stage, the 12AU7, is a little bogus. I would add a 10K to gnd from the cathode of 6922/ECC88, and rather connect the 20k feedback capacitor to the other end of the 10uF output capacitor. That way the input stage will self servo to be in the linear region.

There may be a few problems with the totem-pole coupled 12AX7, but since I have not analyzed that configuration yet I can not comment in a rational way.

Hans J Weedon.
 
It's not really a totem pole, it's a misconfigured mu follower. You're free to polish doo-doo (to slightly paraphrase an Americanism) if that's what you like to do, but why not just start with a good circuit instead of trying to fix all the mistakes in a poor one?

What's worse is attributing it to a manufacturer using their brand name.
 
Valves don't have a sonic character. Valve-circuit-signal triples may have sonic characters. Assuming the designer knew what he was doing, he would have chosen each valve to match the requirements of each stage.

That official-looking circuit still contains some of the strange features, such as an active load stage masquerading as a mu-follower. It is still possible that it is an anti-clone spoiler.
 
Everywhere I looked, it's essentially the same circuit that is credited as the CAT SL-1.

From a diy site:
An externally hosted image should be here but it was not working when we last tested it.


On the same page, different voltage points.
An externally hosted image should be here but it was not working when we last tested it.


From the same site but showing the phono stage:
An externally hosted image should be here but it was not working when we last tested it.


From an eBay seller:
SL-1%20schematic.JPG


Have at it!
 
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. I saw this circuit first in about 1955 as an application of the new output power tube EL86 to drive an 800 Ohm loudspeaker manufactured by Philips in Holland without an output matching transformer. Maybe the first transformer-less output amplifier.

Regarding different tube-types for different stages, I would prefer to use 12AY7 for the low level amplifiers and more conventional resistive load resistors. Why fool around with vacuum tubes as active load resistors. All they do is to increase complexity and reduce faithful reproduction of the original sound. Myself, I prefer wire-wound resistors, although inductive, the inductive effect only shows up in super-sonic frequencies in the 50kHz range and above. For the output stage I would use a 12B4A single low mu triode, very similar to one section of a 6AS7/6080, but a lot more convenient.

But then again, I am a professional circuit design engineer that designs electronic circuits for a living and as a hobby for a loving.

I have been doing this since about 1950 or maybe even before. (a living legend?)

Hans J Weedon

Salem MA
 
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
 
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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. :eek: :D

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

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

Kevin wich one schematic did you refer, if it's not ask too much what do you change?
 
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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.
 
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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!).
 
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 .
 
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.
 
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 .
 
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<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.. :D

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. :p