Coupling capacitors

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The question here, which comes as a spin-off from discussions on the Doug Self pre amp thread, is whether film caps really are that much better than their electrolytic alternatives. Or, the other way around, whether electrolytics are quite as bad as people make out.

Here are some impedance measurements done on Clio of 3 caps. A Wima MKP10, 10uF, which is about as pricey as they come, a plain LL Alcap 16uF as supplied by Expotus, and a Rubycon ZA (now alas defunct as a line) but regarded quite highly in general. (Well, at least by me at any rate).

So they are all good in their ways. But the foil cap is already showing inductive tendencies at 5kHz, where we see the phase notch down one pixel from what is, up until that point, pretty much a ruler flat line on 90 degrees. It ends up on 92.5 degrees. So there is no loss to speak of, but there is inductance.

Of course, if this were distortion (which I may get round to doing), the big Wima would probably win hands down, though the others might fare surprisingly well. But what I'm suggesting here is that all may not be quite so rosy in the film/foil garden.


clioCapsZ.jpg
 
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No small image, just words "click the image to see in full size". When I click the image, it downloads forever.
Glad to hear I wasn't missing much by only using 10 uf film caps in my organ, which tops out at 7000 hz. I'm really tired of changing electrolytic caps every decade or so. In the power amp input last year I used 50V CPO 4.7 uf ceramic caps. The **** 1985 tantalum caps were the source of the popcorn noise I heard, which I had blamed on "NTE" non-audio transistors until the recent cap change.
 
It seems like the frequency is not nearly high-enough to show any inductance for a well-made film capacitor, unless maybe the leads (and/or test leads) were left very long, or unless it's one of the crazy types that have the internal lead connections only at the ends of the foil rolls instead of all along one edge of each roll (but a WIMA should be made correctly).

Also, the phase angle for an inductive impedance would be positive, while that for a capacitive impedance would be negative. With a mixed capacitive and inductive impedance, how do you know what a small change in the phase angle versus frequency is telling you? There should be rising inductance with frequency (a net positive-tending phase angle), at least eventually. But how would a negative-tending phase angle ever indicate rising inductance? (i.e. I don't think it could.)

Disclaimer: I am not an expert (any more) and might be looking at it wrong.
 
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It looks like there are two camps in this discussion (audio - always two sides to a story! ;-)

I used a large feedback coupling cap in my Ovation 250 Amp and that amp has a great sound IMO (I have heard many amps). I don't believe taking it out and replacing it with a servo would necessarily take it up into a totally different level - and before I get flamed, I am not saying that its a perfect design - far from it.

Edmond Stuart and Syn08's PGP used an electrolytic for feedback coupling and they got under 1ppm distortion. So, if its problems with distortion or bandwidth (other than very low LF of course) then its clear that a decent quality and suitabley sized (see Cyril Bateman) electrolytic is not the cause.

That said, on small signal circuitry, I do have an aversion to using electrolytics for coupling (sorry Doug!) and I know Bob is also not a fan. To put this thing in perspective, if you took two very good amps (or even the same amp) and used a feedback coupling cap in one, and a servo in the other and then did a listening test on them, do we think we would all say that one sounded terrible while the other one sounded great? There may be a difference, but my guess is it would be pretty nuanced and so it is with all sorts of other things in this game as well.
 
jkorten said:
Oh I thought coupling meant from the output of the phase splitter to the pentode/triode output tubes.
That is just one example of coupling. An electrolytic would be inappropriate there because of leakage current. A coupling capacitor is any capacitor which couples the AC output of one stage to the input of the next stage. Also sometimes called a DC blocker.
 
...There may be a difference, but my guess is it would be pretty nuanced and so it is with all sorts of other things in this game as well.
I agree bonsai. I often 'worry' about where the nuanced or subjective audible changes border on esoteric or even the ridiculous. :eek: Don't get me wrong... little improvements(?) all along the audio chain will - at least by some definition - all add up to a bigger gain and possible sonic improvement. Where and when do we say, "enough is enough" though in the quest for that often illusive, sonic holy grail? I personally believe that there are other, more important factors that play into what we ultimately hear from our speakers. :)
 
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