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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
bdp24's avatar
bdp24 OP
9,788 posts
 

The filters are not for tweeters, they’re for high-passing main speakers, the lower frequencies going to subs with their own filters and amps. I guess I assumed the mention of mounting the caps on the amp’s input jacks would imply that. I’ve done it before, but not personally---I had Tom Carione at Brooks Berdan, Ltd. do the installation (on a Bedini 25/25).

So, I came up with the incorrect cap values, ay? I sent the amp designer/maker an email asking for them, but if anyone wants to educate me, great!

The specs:

Amp 1: 100k Ohms input impedance; 80Hz x/o.

Amp 2: 30k Ohms input impedance; 175Hz x/o.

Cap suggestions, anyone? I used a Solen Metalized Polypropylene 35uF (sourced from Madisound) in a very low-frequency low-pass filter, for which it was fine. I always look for the "sweet spot", where diminishing returns kick in big. A completely subjective determination, of course. I don’t discourage opinions, I welcome them! Good value caps at $20/ea? $50/ea?

pragmasi
212 posts
 

Assuming we're talking line level here, there's no need for monster capacitors.

Amp 1: 100k Ohms input impedance; 80Hz x/o.

20n (0.02uF) created from two 10nF Polystyrene 1% Caps in parallel

Amp 2: 30k Ohms input impedance; 175Hz x/o.

30n (0.03uF) created from three 10nF caps

All for less than $10, you can spend more on exotics but I don't think they'd sound any better.

bdp24's avatar
bdp24 OP
9,788 posts
 
Yep @pragmasi, line level. So I was by off by x1000?! I gotta figure out what I misunderstood on the cap value calculator. Thanks for the info.
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.
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