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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
 
No problem... even if you are tempted to try expensive capacitors, buy ten of these and compare. The only dialectric that may work better than polystyrene for this application is C0G/NP0 ceramic (definitely not any other type of ceramic) which is fine if you can solder surface mount but the through hole versions are ridiculously expensive.
atmasphere's avatar
atmasphere
11,800 posts
 
@bdp24 , @pragmasi is giving you good advice. I've had a number of customers do things like this and we've used polystyrenes for the job as there's very little out there that is small enough in any other dielectric.
bdp24's avatar
bdp24 OP
9,788 posts
 

Thanks fellas. I figured out what I did wrong on the calculator, and with the help of Danny Richie at GR Research got the correct figures. 100k and 80Hz requires .02uF, 30k and 175Hz .03uF. But caps aren't made in those exact values, .022 and .033 being as close as one can get.

The resulting x/o frequencies are 72Hz and 161Hz respectively, close enough considering the shallow slope of the 1st-order filter. Small value caps are priced relatively modestly, even the better ones being not THAT expensive.

But you're right Ralph, some cap designs are huge, even in small values. To fit them onto the amp input jacks will take some doing!

pragmasi
212 posts
 
100k and 80Hz requires .02uF, 30k and 175Hz .03uF.
 You parallel up .01uF caps (for caps in parallel you just sum the values), so five for each channel means $4.20 in capacitors.

erik_squires
16,057 posts
 
On the inexpensive, I like Cornel Dubelier a great deal.

If you want to try pricey ... both Audyn (True Copper Max) and the 600V versions of Jansen copper foil caps come in 0.01 values.

Best,

Erik
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