Counterpoint SA-100 again!

[AxFe]

I have a Counterpoint SA-100 power amplifier of the 1994 year. This device exposes various problems:

1. Bias power supply 36V zeners (5W) overheating. I consider change the 470 ohm 1w resistors and the zeners for a constant current source (CCS). The CCS would be a transistor with a base red led and 270 emitter resistance connected to 50V and a 10K base resistance connected to ground. Therefore, the zener from the collector to ground would be of 1 watt with a best voltage regulation. Are there best ideas?

2. Instability problems. The DC offset and bias current changes with the temperature. Actually I adjusted the bias current (via a 1 ohm resistor in place of a fuse) to 400 mA. Is this correct?

3. Motorboating and/or ripple noise. This is very disappointing. Should I change decoupling capacitors and tubes?

4. Should I change the design of the tubes section? Is there a schema of the changes?

This my first post. Thank you very much and sorry for my English. If anything is not clear please just ask.

Andres Fontalba.

[anatech]

Hi Andres,
Yes, those amplifiers have many design problems.

1. Yes, use a CCS to bias those zeners at a lower current. You will find that this solves a few problems. Those 10 uF caps are bad also. I love it when people think about things! Repace the zeners also.

2. The bias will never really settle down on these. I can't remember the exact bias off-hand, but look for about 100 mA per output. For two devices this would be 200 mA. Your power transformer will last longer.

3. The heater filter caps may be defective now. Just change them. Motorboating may be due to excessive gain. If you have VR3 (pots), short them out. They go noisy.

4. You could. The amplifier should run as it is after doing steps 1 through 3. There are no official schematics for a modified tube section on these.

-Chris

[ilimzn]

To be brutally honest, I'd use the same parts (except a better power transformer and build a better amp with them. Something like john broskie's Aikido for the tube stage would probably work very well here, and you even have all the necessary tubes

[anatech]

Now ilimzn!
You know I can't disagree.

It really depends on how comfortable Andres is doing this.

-Chris

[Wavebourn]

Gentlement;

is it possible to find some schematics of that Counterpoints?
I am 100% positive that some slight modifications may improve it drammatically.

[anatech]

Hi Wavebourn,
Look up Michael Elliot. He is the designer of this product. He does tend to guard his designs for a price.

But you are correct, there are some fundamental errors in this design. Ilimzn gave a very good suggestion for a start. The initial post exchange pointed out a couple others. The incremental improvements would result in a complete revamp of this design, not just a modification.

-Chris

[AxFe]

The schema of the Counterpoint SA-100 is in the post #4 of this thread:

http://www.diyaudio.com/forums/showt...?postid=852504

Searching for the broskie's Aikido:

http://www.tubecad.com/2004/blog0011.htm

I will study it. Thank you very much.

[anatech]

Hi AxFe,
Never could read that schematic. What program is it for?

-Chris

[AxFe]

¡Hola!

The schema is a postscript file that can be converting to pdf: http://www.ps2pdf.com/convert/convert.htm

The circuit of the SA-100 hasn’t power mosfets source degeneration resistors. Would be advisable include then? And feedback?

Thank you very much. Andres Fontalba.

[anatech]

Hi Andres,
Thanks, that is the later model.

Source degeneration resistors will not sound very good. Also, this design does have some feedback through V2a. I don't know if it's intended or not. It is a no overall feedback design, so adding feedback will change the sound.

One of the big deals about this design was the very close matching between the outputs. This is due to the lack of source resistors. Michael did find that source resistors had a negative impact on sound quality with this design. He is correct as far as this design is concerned.

I think ilimzn's ideas have merit, or leave it stock save the zener biasing.

-Chris