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Old 22nd August 2012, 06:02 PM   #1
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Default 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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Old 22nd August 2012, 06:28 PM   #2
SY is offline SY  United States
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For a lot of reasons, it won't work as drawn, and for many others, fixing the major errors will not give you good performance. I'd start with a better platform than this circuit (which I seriously doubt is CAT's).
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Old 22nd August 2012, 06:39 PM   #3
DF96 is offline DF96  England
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Perhaps that diagram is the result of someone attempting to reverse engineer the circuit? Snag is, the person who did it is obviously not an engineer - either forward or reverse.
Or is it misinformation put out by CAT to confuse cloners?
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Old 22nd August 2012, 08:13 PM   #4
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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.
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Old 22nd August 2012, 09:24 PM   #5
SY is offline SY  United States
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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.
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Old 22nd August 2012, 09:38 PM   #6
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I found a schematic of a clone on the internet at this website. And below is the schematic:

Click the image to open in full size.

To be honest, there's nothing special about the circuit, except it tries to use different tube type in each stage to not have an additive sonic character, I guess.

Last edited by directdriver; 22nd August 2012 at 09:40 PM.
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Old 22nd August 2012, 10:02 PM   #7
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Per that website, I take it that if I listen with this preamp, I'll probably need gall bladder surgery.
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Old 22nd August 2012, 10:05 PM   #8
DF96 is offline DF96  England
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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.
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Old 22nd August 2012, 10:41 PM   #9
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Everywhere I looked, it's essentially the same circuit that is credited as the CAT SL-1.

From a diy site:
Click the image to open in full size.

On the same page, different voltage points.
Click the image to open in full size.

From the same site but showing the phono stage:
Click the image to open in full size.

From an eBay seller:
Click the image to open in full size.

Have at it!
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Old 22nd August 2012, 11:00 PM   #10
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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. 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
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