Capacitors for HP filters in tube amps recommendations?

@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.

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!

Excellent @pragmasi, exactly the kind of information I was hoping for! And lots of it!! Pure ee considerations and concerns, very much aligned with the kind of thinking that went into the design of the amps. No intent or desire to disparage audiophile language and concerns, but you gotta get the ee stuff right first ;-) .

Yes, having the caps matched to 1% is a very high priority (perhaps THE priority). I’ve seen the term "time constants" many times over the years, but never took the time to educate myself on what it means. Perhaps I should do so now! I’m also familiar with the term "parasitic", and DO understand it. It’s bad, right? ;-) . I’ll look up Doug Self (I’m familiar with the name) and his research.

I’m going to try and reach RM (the designer and maker of the amps), He may have the caps I need in his parts inventory. He encourages the installation of caps on the input jacks of his amps as a way to create a 1st-order filter for partnering the amp/speaker with a sub. I’m sure he views the question of cap "sound" just as do you---a matter of their electrical characteristics in any given application.

Thanks again for all your time and help @pragmasi. It is very much appreciated, and of great value and use to me---Eric.

@ramlabs, Roger, baby, where are ya?! When you have time, could you look for my email from last week? I'll call you Monday morning.

@pragmasi, I was hoping I would see a response from you to the above! Polystyrene, ay? I take it you don’t share the perception that caps made of plastic materials impart a "plastic" coloration? Can you expand on your contention that polystyrene caps will outperform polypropylene or copper foil/film/paper/oil/wax, etc. caps IN THIS APPLICATION (that interests me greatly---see below)? Thanks.

I haven’t received a response to my email to the designer of my two amps, but I already know he "believes" in using the right part for a given application---which doesn’t necessarily require an audiophile-approved boutique part, and that higher cost (or audiophile cache’ ;-) does not necessarily buy one higher sound quality.

Like Frank Van Alstine (another "sensible", old school ee), Roger Modjeski has seen his amps butchered by well-intentioned audiophiles, who replaced the stock parts (chosen and used for very specific reasons) with boutique parts which actually degrade the sound of their designs, not to mention their reliability. Some boutique parts are designed with a specific parameter prioritized, but also possess characteristics such that when (mis)used in an inappropriate application can result in not just degraded sound quality, but also unforeseen and unfortunate consequences.

Thanks again for the tip on polystyrene in general, and to those you above provided the link to. One advantage they afford is small diameter, making possible using 2 or 3 stacked .01uF caps side-by-side, to get exactly .02 and .03 combined values, rather than the .022 and .033 compromises. I’ll take another look at them, and google this polystyrene of which you speak. ;-)

It finally occurred to me (duh ;-) to do an Audiogon Forum search on the subject of capacitors, and holy cow---what a goldmine of information!

I also followed the link to the Humble Homemade Hi-Fi Cap Test, and double holy cow!! That was a lotta work!

Some caps stood out in both the above, and I now have it narrowed down to what appear to be the caps that provide the highest sq-to-price ratio. I’ll share them here, both for anyone else researching the subject, and any reactions.

- Jupiter Copper Foil. Though considered at the very top-o-the-heap, Sonicraft (a favorite vendor of mine) sells the .022uF for $28.50, relatively cheap. Parts Connection sells the same for $38.00. ?

- Clarity CMR (thanks Erik). Sonicraft sells the .022uF and .033uF for even less than the Jupiter.

- Miflex KFPM-01. Recommended to me by Danny Richie at GR Research, who sells .022uF and .033uF matched to 1% for about 20 bucks each. Sonicraft also sells them..

-Audyn True Copper. Parts Connection sells the .01uF, don’t remember the exact price, but as Marge Gunderson says in Fargo, it’s reasonable.

Any of the above are good enough for my needs, so I can’t loose. The only question remains will any of them fit in my amps?!

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!

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?