That amp will only be used 20 minutes a week, so cheapo NP would not a problem I suppose. However, polyprophylene won't fit in the chassis in the 68 uf size in an Allen S100 amp I'm upgrading. I use my main entertainment amp 2000 hours a year, so I look down at my nose at NP. While I can buy NP rated caps, they come with no life specification and are probably sealed with red gum rubber for at least 500 hours of trouble free service. The organ circuits are much crisper than equivalent circuits using 10 uf polyester caps, which is cool on glockenspiel envelope for example. The ST120 with djoffe OT idle current mod sounds at 1.0 v pp base level just like my Peavey CS800s which is a straight through no caps in signal path amp. Both applications were 50 v rated caps on 2 v max signals, which IMHO reduces the non-linear effect of the dielectric on the signal. The input caps were X7R and sounded much better than the tantalum caps I bought at the local store that had frying pan popping noise from day one. The 10 uf were CPO, which was a one off find and no longer stocked anywhere, though still in the Aerovox catalog. The organs have considerable internal pressure from the built in speakers at 70 watts. I've used Aerovox gold 4.7 and 10 UF ceramic caps, as input caps to my 1966 design dynakit ST120 amp, and as coupler caps in organ envelope shaping circuits (Hammond H100). How commonly is it encountered in real life? That is, when the effect is severe enough to be a problem in low-level stages? I've come across it 2 or 3 times during the course of a long career in electronics, but I'm not an audio specialist. My concern is about the microphonic effect. Both of these effects could be largely swamped out by using large capacitances to get cut-off frequencies well below the audio range. Secondly, the distortion introduced by the nonlinearity with voltage swings <<5V would be apparent only at very low frequencies. This is my thinking: First, for a 50V capacitor, the reduction in capacitance with DC bias shouldn't be significant where the bias is no more than a volt or less. However, I'm wondering about their use in one-off constructions for personal use, in particular when the aim is not to achieve the highest hi-fi quality possible. I'm aware of the shortcomings of ceramic capacitors of high dielectric constant like the X7R types and I wouldn't consider using them in signal paths in production designs. The subject of ceramic caps in audio must have been discussed more than once before, but I'd like to have your inputs on a specific aspect.
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