Quality Input Cap

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Hey All,

I'm currently building a single ended amp that uses +9 volts bias on the grid of the voltage amplifier. Therefore, I need an input cap to contain the DC. But what to use? Obviously, I don't need a 600 volt cap in this position so the obvious choices used as coupling caps would be overkill. I was thinking of using a 200 volt Wima .22uf MKP but are there better options?
 
The best choice here would be a teflon capacitor like the russian FT-1 or FT-3 - they have lower distortion and sound better than other plastic or PIO caps.

My second choice here would be a polystyrene cap (russian K71 for example).

K75 sounds OK but I think it adds something to the music. K40 PIO sounds good but has less micro-details than plastic caps (it doesn't matter in a power amp, but it matters in a preamp or headphone amp, where resolution is important).

Wima MKPs sound just average (that is, not very good).
 
Plastic caps are good, but polyprop' is the worst of the bunch....

that's not supported by measurement evidence, or even audiophile guru "wisdom"

Mylar/polyester would be in the running for "the worst of the bunch" of plastic dielectrics, polypropylene is usually ranked right next to polystyrene by both measrured properties and golden ears

polycarbonate is now rare and suspect in "audio quality" anyway due to the polar molecular structure

polypropylene is the default "quality audio cap" dielectric until you want to spend stupid money to impress


and as mentioned a big step for distortion reduction is minimizing the signal V across the cap - so "supersize" - increase the value to drop fc another order of magnitude below audio
 
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I'm not really a tube guy, but for tube amps with output transformers, wouldn't a coupling cap need to provide the proper high pass roll off to prevent LF distortion in the output transformer? In this case the type and quality of the cap would probably be important. But that isn't in any way an endorsement of expensive boutique parts. On the contrary, I see no reason to use $100.00 caps made of some kind of blessed who knows what, that are 3 inches long and 2 inches in diameter, when superior devices of known and consistant quality are available from commercial suppliers.

Mike
 
I found that caps could make a difference. However some ordinary caps turned out to be fine compared to boutique and expensive caps. In fact some polyester caps were as good as polypropylene caps.So you need to test them yourself and don't be surprised if you find it hard to hear any difference between them!;)
 
The coupling cap which matters (a little) is the one which defines the LF rolloff, as that one may have significant signal voltage across it. Hoever, if 'significant' is still only 100's of mV then little harm will be done whatever cap is used.

The strange biasing of the E180F (suggested by the datasheet IIRC) suggests to me that it suffers from high sample variation and/or high grid current (and conceivably thermal runaway - although unusual in a small signal valve).
 
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