Another Adcom Amp Question

[fmaxwell]

I've recapped a couple of GFA-555 amps with Panasonic FC series and I'm getting ready to do a pair of GFA-545 II amps.

In all of these, there is a 4.7uF cap in parallel with each bias trimmer pot. I'm thinking of replacing these with Wima MKS 2 Metallized Polyester (PET)
Capacitors:

Any thoughts from the experts as to whether this is a good idea from the standpoint of reliability and sound? It seems like a good idea to replace the electrolytics with something more long-lived, but I don't know if there are other characteristics that might make this swap less than completely successful.

 

[djk]

The point of the cap is to prevent the signal from modulating the bias.

I would use a larger value, you may want to listen to it before putting it back together for good.

I have seen Nelson Pass use 4.7µF, 10µF, and 220µF in that location in some of his designs.

 

[fpitas]

The point of the cap is to prevent the signal from modulating the bias.

I would use a larger value, you may want to listen to it before putting it back together for good.

I have seen Nelson Pass use 4.7µF, 10µF, and 220µF in that location in some of his designs.

I agree with the bigger cap principle here. Rod Elliott, who is usually a reasonable fellow, also likes big caps across the bias circuit, and claims it makes a difference:

Headphone Amplifier

About halfway down the page, under Description.

 

[fmaxwell]

I have seen Nelson Pass use 4.7µF, 10µF, and 220µF in that location in some of his designs.

Thanks for the replies.

I'm hesitant to second-guess Nelson Pass when it comes to component values (I view using improved quality parts of the same rating as being a more conservative approach I am comfortable with).

Since he used different values of caps (as well as different values of trimmer pots) in different amps, I have to wonder why. I can't believe cost was the driving factor, given that small value el caps are pennies each to a manufacturer. I also doubt that the choice was made randomly.

P.S. I'm not one of the listen and judge types. I know too much about human psychology to believe that I, alone in the human race, am immune to expectation bias.