[mudihan]

Happy holidays!
 
So the old potentiometer has become very scratchy. My solution was to bypass it and use an outside digital potentiometer instead (I also changed R102 and R202 from 910k to 50k). The problem, however, is that my DAC outputs a healthy 2.1v and so the amp is already very loud when the digital potentiometer is only at 40%. And that's just for classical music. For those loudness "winners" popular tracks, 25% is enough. If the amp's gain is set by resistors, please point me to them so that I could play with the values to lower the gain.
 
Also the 2w 27k resistors appear to be carbon composite resistors. I am thinking changing each pair to a single film-type resistors of either 51k or 56k rated for >5w for lower noise and better stability. I have found 5w metal dioxides rated for 800v, but just saw Caddock MP925s that are rated for 500v and 25w (if mounted on a heatsink). Any sonic benefit of using the Caddocks vs metal oxides (or even wirewounds)?  
 
Many thanks!

 

[mudihan]

Anyone? It would be especially helpful to know how the gain could be modified. Thanks.

 

[kevin gilmore]

The feedback resistors are 2 x 300k in series.  If you make that 2 x 150k in series you will reduce the gain of the amplifier by 6db.
 
you will also lower the peak to peak voltage from 1284 vppss to 1184 vppss and then clipping will not be symmetrical. the common mode voltage will also need to be adjusted which may require additional parts. if one of the collector resistors opens up with the reduced feedback, likely the j109 will burn up and those are pretty hard to find these days.
 
better idea would be to leave everything alone and put a 2 resistor attenuator in place of the pot.
 
with respect to the collector resistors, if you use standard wirewound resistors, the amp will oscillate. if you can find mills non-inductive wirewound resistors, some have had problems with them in the past overheating, plus there is the size issue in that amp.
 
the caddok resistors are your best bet here.

 

[mudihan]

  The feedback resistors are 2 x 300k in series.  If you make that 2 x 150k in series you will reduce the gain of the amplifier by 6db.
 
you will also lower the peak to peak voltage from 1284 vppss to 1184 vppss and then clipping will not be symmetrical. the common mode voltage will also need to be adjusted which may require additional parts. if one of the collector resistors opens up with the reduced feedback, likely the j109 will burn up and those are pretty hard to find these days.
 
better idea would be to leave everything alone and put a 2 resistor attenuator in place of the pot.
 
with respect to the collector resistors, if you use standard wirewound resistors, the amp will oscillate. if you can find mills non-inductive wirewound resistors, some have had problems with them in the past overheating, plus there is the size issue in that amp.
 
the caddok resistors are your best bet here.

Many thanks. Received Caddock MP925 yesterday, put them in with big heatsinks, and they actually make a big difference over the (1) the old carbon comps and (2) the metal oxides that I installed earlier. 
 
I will add in a 2-resistor attenuator. It sounds like the most sensible and easy solution.