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Coupling capacitors
Coupling capacitors
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Old 16th May 2012, 10:11 PM   #1
ChristianThomas is offline ChristianThomas  United Kingdom
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Default Coupling capacitors

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.


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Last edited by ChristianThomas; 17th May 2012 at 12:31 AM.
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Old 16th May 2012, 10:19 PM   #2
DF96 is offline DF96  England
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Electrolytics are not bad, but they do have to be used appropriately. If someone does not know how to do this then they are best to fall back on simplistic notions like 'electrolytics are bad'. No capacitor is perfect, but some are closer than others. If there is little or no signal voltage across a cap then it can do little harm.
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Old 16th May 2012, 10:20 PM   #3
ChristianThomas is offline ChristianThomas  United Kingdom
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Lets try this image again.

An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.


Can anyone else see this image. I can't for some reason.

Last edited by ChristianThomas; 16th May 2012 at 10:23 PM.
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Old 16th May 2012, 11:38 PM   #4
indianajo is offline indianajo  United States
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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.
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Old 17th May 2012, 12:21 AM   #5
Ian Finch is offline Ian Finch  Australia
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Perhaps the OP would like to read Cyril Bateman's papers on "Capacitor Distortion" published in EW from 2000-3. There will be few questions left to ask after that lot - guaranteed
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Old 17th May 2012, 12:50 AM   #6
Mr. dB is offline Mr. dB  United States
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Quote:
Originally Posted by ChristianThomas View Post
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.
What about tantalum beads?
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Old 17th May 2012, 12:53 AM   #7
jkorten is offline jkorten  United States
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I'm sorry, but unless an electrolytic is bipolar it shouldn't be used at all as a coupling cap. Or am I missing something in this discussion?
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Old 17th May 2012, 02:46 AM   #8
Ian Finch is offline Ian Finch  Australia
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Yes, DC bias, which makes polarized caps as good as electrolytics get.
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Old 17th May 2012, 02:52 AM   #9
gootee is offline gootee  United States
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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.

Last edited by gootee; 17th May 2012 at 02:54 AM.
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Old 17th May 2012, 05:10 AM   #10
jkorten is offline jkorten  United States
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Quote:
Originally Posted by Ian Finch View Post
Yes, DC bias, which makes polarized caps as good as electrolytics get.
Oh I thought coupling meant from the output of the phase splitter to the pentode/triode output tubes.
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