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Capacitors for HP filters in tube amps recommendations?

I’ll soon be installing a capacitor on each of the input jacks of two tube power amps, to create passive 1st-order high-pass filters. Cap values are 19.9uF (20uF will do) and 30.33uF (let’s say 30uF). I’ll need two of the former and four of the latter (balanced/XLR input jacks), and I don’t want to spend more on the caps than the amps cost me ;-) .

I’m all ears for nominees. I don’t need or want any flavoring, "just" neutral transparency. The amps are powering fairly transparent loudspeakers (ESL’s, and Magnetic-Planars with Ribbon tweeters), which will pretty well reveal the character of the caps. In spite of that fact, "most-bang-for-the-buck" nominees are of particular interest, not cost-no-object ones. Thanks y’all.

128x128bdp24
pragmasi
212 posts
 
Wima is a good choice of PP capacitor, at line level I tend to use NP0/C0G for 10pF to 10nF, Wima MKP for 10nF to 10uF and try to avoid higher values in the signal path. Be aware the leads are short on the Wima caps so you may need to mount them on strip board as point to point soldering might be a bit fiddly although do-able.
bdp24's avatar
bdp24 OP
9,821 posts
 
I'll be putting just a pair of .01uF (10nF) caps (.02uF apparently isn't available) on each RCA jack, so those short leads will be long enough. I wasn't able to find any 1% polystyrene .01uf or .02uF caps. I'm going to research NPo/CoG (with which I am unfamiliar) right now.
pragmasi
212 posts
 
I think you've found a solution that will work for you at a sensible price point... I didn't realise Wima made PP caps in 1% so I've learned something new too. I usually go for 5% as I use them for high pass filtering at sub-sonic levels, knowing they are available in tighter tolerances will come in handy I'm sure.
bdp24's avatar
bdp24 OP
9,821 posts
 
Thanks for coming to my aid! I’d still be floundering around if you hadn’t, ’cause I haven’t heard back from Modjeski. I figured he might have the appropriate caps in his parts bins, but I think he uses polyester caps in his amps.
dhl93449
455 posts
 
pragmasi

You need to be cautious when recommending CGO/NPO ceramics for audio (particularly coupling not bypassing) applications. I know from experience that some NPO/CGO caps (particularly those from TDK ceramics) have a voltage coefficient of capacitance that is quite significant (sometimes as high as 0.5 x C) at DC voltages near rated max. This is more evident in the larger values (and .01 uF is large for an NPO).  Remember, the CGO/NPO spec relates to temperature coefficient only, and while meeting the low TC, are still quite nonlinear when stressed at various voltages. Folks have slammed the use of tantalum caps for years because of this characteristic, and some NPOs are just as bad or worse. Also, you may be hard pressed to find a ceramic cap manufacturer of NPOs providing dielectric absorption specs. Due to the large variety of ceramic compounds, it is risky to extrapolate the product of one manufacturer with an NPO compound of another.

Also, there is data that shows polypropylene has pretty much the same dielectric absorption as polystyrene. With the same metallisation, ESRs and dissipation factors are identical. Polystyrene has a major weakness in that these caps can be easily damaged while soldering, due to the low temperature ratings of polystyrene. I am not a fan of the cheap Xicon polystyrene caps, but those made by RelCap are quite good.   
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