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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
9,821 posts
 

@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. ;-)

pragmasi
212 posts
 
I guess the important factors in this application are that we're working at line level and we are setting time constants in the audible frequency range.

Line level means we don't need to worry about dealing with high currents, voltages or significant power dissipation (large packages are better at dissipating heat than small ones). But it also means that any distortion will be amplified by the following gain stages.

Setting the time constant means that we're attenuating a selection of frequencies, in this case ones we can hear so we're looking for precision and linearity (high linearity = low distortion).
If the actual capacitance deviates from the nominal capacitance then we'll find that the -3dB point is at a different frequency to that specified. In this case, assuming you can adjust the roll-off of your subs then the precision of the pair of filters (left and right) is not overly critical. However the precision between the filters (the difference between left and right) is critical and may well be audible if one channel rolls off at a different frequency to the other.
So we want to know that the capacitors on both channels are close in value but not so worried about how accurate that value is (in an RIAA filter both values are critical).

Linearity is a lack of distortion so the output of a linear component will closely resemble the input (in this case we are intentionally distorting the input by filtering out LF, but we want the audible frequencies to be unaffected). Linearity is measurable in Total Harmonic Distortion and there have been studies into the distortion characteristics of dielectrics,  if you want to learn more (and I completely understand if you don't) Doug Self has researched the topic in depth. C0G/NP0 and polystyrene caps can be considered free of distortion, polypropylene is very low distortion which can probably be said to be negligible, electrolytic is another matter.

So bearing this in mind...

Jupiter Copper Foil: I don't know the linearity of wax and paper but it can't be better than unmeasurable, so let's be generous and assume it's unmeasurable. Precision... the ones I found were 5%, okay but not great.

Clarity CMR: These are polypropylene so we know what we're dealing with. Precision is 3% which is better.

Miflex KFPM: Polypropylene and 2% precision.

Audyn True Copper: Couldn't even find out what the dielectric is, so let's assume it's unmeasurable again. 2% precision.

I couldn't find (admittedly I didn't try too hard) any detailed datasheets for the above. One thing they all have in common is that they are large, which means they can dissipate heat (which we don't need) but the size also means that the parasitic properties of the leads, foils etc are greater.

Polystyrene is available in 1% in a small package and has no distortion to speak of. So it's not streets ahead of the caps above but it will be better for this application (and that's without taking into account cost). Some of the exotics will probably be available in matched pairs which is an improvement... but still not better unless the matching is less than 1%.

I wrote more there than I meant to... I hope it's useful.
bdp24's avatar
bdp24 OP
9,821 posts
 

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.

erik_squires
16,142 posts
 
Modern film caps are usually spot on in terms of measurements, but you can always order matched pairs.

I've not bought a film cap in ages that wasn't actually within 1%.
pragmasi
212 posts
 

I just realised the capacitors in the link above are 5% tolerance, shouldn't be too hard to find 1% ones though... in the UK they are available from both Farnell and RS Components.

I’m also familiar with the term "parasitic", and DO understand it. It’s bad, right? ;-)

Parasitic properties are the characteristics of the 'real world' component that deviate from the ideal. An example in this case would be the capacitor's inductance, that is directly related to lead length so a large axial resistor will present greater inductance than a small radial cap with short leads. Whether anyone can hear the difference is another matter but if you can avoid it then why not.

In reality you could get a good result using 1% polystyrene or 1% polypropylene, the former are smaller and less expensive... it's less clear whether the better spec in terms of parasitics would make an audible improvement.

Hope the project works out well whichever you choose.

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