disc ceramic HF feedback cap

Just today I started building the Silicon Chip Ultra L-D amplifier (the 100W version) from a kit and one of the feedback caps is a disc ceramic. Now I know for a fact that non-NPO ceramic caps generally vary their value all over the place with changes in voltage so I would have thought that using a ceramic here would have been a really bad idea. What should I replace it with, if in fact I should replace it? It is the one from Q8 collector to Q7 base.

The switchover from NP0 to non-linear ceramics takes place at different values for different capacitor ranges. There is a colour code - I think a black band indicates NP0?

You could try silvered-mica or polystyrene - these are the other common low capacitor value dielectrics, but I don't know what effect they may have on sound.

Ian Finch

Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
When you have recovered from the cost and lead time to buy 100pF caps in better than non -NPO/COG ceramic in Oz, you will find it hard to get assurances or indications like the black top on NPOs from retailers or distributors any more. That is my recent experience anyway.
Rockby Electronics might be your best bet in Melbourne, though as surplus resellers, they may have little stock of such popular values.
I'd just use the cap provided. Considering the lengths the circuit designer went to to keep distortion low and quality high, they must have also considered that cap's value and type. Plus, that cap's value only comes into play way into the KHz range, so you'll never hear it or even a better part anyway.

Ceramic can have poor capacitance stability with respect to voltage. Some ideas: I've seen mention of silvered mica said to sound sweeter/sharper and polystyrene should be the most neutral but it can't resist high temperatures, so carefull with soldering. Polypropylene is a good alternative.
However, I've yet to see a measurement of distortion spectrum vs type of feedback cap... some measurements would be interesting.

You can always try to swap parts to see if you can hear a difference.