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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
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bdp24 OP
9,868 posts
 

I noticed that too. There are some cap vendors who match them in pairs, and Parts Connexion offers hand picked caps that are 1/2 or 1/4 of the factory tolerance figure, but they sell only "audiophile" brands. I think Michael Percy offers the polystyrene caps he sells (Rel and Multicap, each about $5/ea) matched to 1%, but to each other, not tolerance. The caps you provided the link to are sold on ebay, but I’m not going there!

Something suddenly occurred to me last night: as I mentioned, one of the amps has balanced/XLR inputs, requiring a cap for each leg of each XLR. What I forgot was that while the input is 30k ohms, each leg is of course half that---15k. So on that amp I need not .03uF (three .01 caps) per input jack, but rather .06uF (six .01uF caps!) for each leg of the XLR connection! Oy!

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bdp24 OP
9,868 posts
 

@pragmasi, I took a look at the product description of the B4 cross-over (one of which I own) on the First Watt website, and in it Nelson Pass mentions he used Wima Polypropylene film caps in all the filters. If they’re good enough for Nelson.....

I then went on the Wima site, and I’ll be darned: they offer a .01uF poly cap (model no. FKP 2) with a range of tolerances, 1.5% being the lowest "regularly" available, but 1% available if requested.

Mouser sells the 1% version for cheap ($2.11/ea, $17.10/for 10), so I have my caps. For the 100k/80Hz amp, anyway. I’m thinking I’ll just use the B4 on the 30k/175Hz amp, as soldering .12uF onto each XLR may be just too much!

Thanks again for your advice, it was of great help.

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.
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bdp24 OP
9,868 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.
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