mica capacitors in amplifier

[sam9]

I have used ploystyrene for small values ( 75pF -300pF), but those may be almost as expensive as silver-mica unless you can find good source.

One alternative would be to decide which of the silver-micas are the most critical to sound of the amp and bear the cost for those while sustituting film or ceramic for the others. Although I have not built a Leach, I have faced this dilema with other amps. The rule i used, (which may be not entirely valid) was that very, very small values 1p-20p are passing signals so far above the audible range so they couold be replaced by ceramic (I'm not sure there are film caps with such low values) and anything not in the signal NFB return can be replaced (film being prefereable to cermic, if possible). This is strictly a cost strategy, if cost were not an issue I would just order SMs.

[pooge]

Exactly. The silver micas in the Leach amp are very small lag compensation caps. Probably won't have much, if any, impact on sound. In a correspondence I had with Dr. Leach, he said that microphonics of ceramic disks gave the amp problems. NP0s don't have as big a problem with microphonics, since they aren't made of the same piezoelectric ceramic. However, I have found difficulty finding any good data on NP0s and microphonics. But other than that possible problem, they should be an excellent cap in that application.

[Eva]

The Miller [C-B] compensation capacitors on the leach amp determine open loop frequency response of the circuit above 1Khz [first pole] but these suffer some degree of distortion correction due NFB

Capacitor nonlinearities on the split-feedback network may also start to show some contribution to overall THD at high frequencies but here there is no NFB to correct that

[Jocko Homo]

Only at the surplus stores...........!

Jocko