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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
erik_squires
16,243 posts
 
... and what is the crossover point you are trying to achieve??



bdp24's avatar
bdp24 OP
9,868 posts
 
Did I use the wrong abbreviation? The 19.9uF (microfarads) is for 80Hz into 100k ohms, the 30.33uF for 175Hz into 30k. I used the calculator on the learningaboutelectronics.com site to get those cap values.
gs5556
1,236 posts
 
Those frequencies are mHz, not Hz (0.080 and 0.175) . Why you want to cut off below 80 to 175 hz?
bdp24's avatar
bdp24 OP
9,868 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.

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