Bypass Caps
May 8, 2005 at 5:50 AM Thread Starter Post #1 of 10

chillysalsa

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I was playing around with a certain circuit that uses a 5.1uF decoupling cap on the output, and decided to try putting a 0.01uF cap in parallel with it.

It sounds like it improved the sound. I feel like the highs are less edgy / grainy, but there seems to be some reduction in the midbass.

But I can't really explain why I hear any changes, I did a microcap simulation of the circuit and it's apparently not affecting the AC response at all that I can see.

Are my listening observations founded on anything other than placebo effect?
 
May 8, 2005 at 9:13 AM Post #2 of 10

amb

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chillysalsa, you didn't say what type of capacitors these were, but it's very possible that doing what you did would influence the sound quality. For example, electrolytic caps are notoriously bad as coupling caps in the signal path, and if the capacitance is so high that one cannot feasibly replace it with a film cap, the common solution would be to parallel the electrolytic with a good film cap. This doesn't solve the problem completely but does yield improvements.

See Richard Marsh and Walt Jung's classic writeup Picking Capacitors and The Sound of Capacitors by Steve Bench.

AFAIK Microcap does not model capacitor parameters such as dissipation factor or dielectric absorption, etc., which would influence the distortions they add to the circuit, thus I am not surprised such simulations yield no useful info on this. These capacitor characteristics do not usually cause frequency response aberrations within the audio band, but the distortion products they add are audible.

By the way, just to straighten a misuse of terminology... the capacitors you refer to are "coupling caps", not "decoupling caps", because they serve to couple amplifier stages, allowing AC signals to pass while isolating DC. Decoupling caps refer to those usually found in power rails of high speed circuits, to lower the supply impedance at high frequencies. See Using decoupling capacitors (an application note from Cypress Semiconductor) for a writeup on what these are. Often the term "bypass cap" is synonymous with "decoupling cap", although the former sometimes has a broader scope.
 
May 8, 2005 at 11:29 PM Post #4 of 10

Garbz

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If I think I understand correctly then bypassing the 5uf cap would definalty have the most impact on the highs. If you removed th 5uf cap completely then I think you may end up with no bass at all. Coupling caps combined with resitance of the parts either side would form a low-pass filter which would explain why the highs changed.
 
May 9, 2005 at 2:06 AM Post #5 of 10

amb

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Nah, paralleling a 5.1uF with a 0.01uF gives you 5.11uF (without getting into tolerance issues), which is hardly a change as far as total capacitance goes. What is changing the sound, if it isn't psychological, then it's the combination and interaction between the two caps (ESR, inductance, dielectric absorption, dissipation factor, and other parasitic characteristics). That said, polypropylene caps are quite good for coupling duty, and I am a bit surprised to find that bypassing it with a small WIMA (which I assume to also be a polypropylene type) would make a significant audible difference.

BTW, the latest issue (May 2005) of AudioXpress magazine has an interesting article about capacitor bypassing, and is worth reading.
 
May 9, 2005 at 2:12 AM Post #6 of 10

chillysalsa

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Yeah, the AudioXpress article was what got me thinking and looking into my parts bin.
biggrin.gif


That's a good magazine!
 
May 9, 2005 at 2:31 AM Post #7 of 10

Garbz

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So wouldn't it's not that high-frewquencies would pass thorugh the the smaller caps more easily? Afterall the smaller caps would have better ESR too. Meh I don't know what i'm thinking anymore.
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May 9, 2005 at 3:21 AM Post #8 of 10

amb

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Quote:

Originally Posted by Garbz
So wouldn't it's not that high-frewquencies would pass thorugh the the smaller caps more easily? Afterall the smaller caps would have better ESR too. Meh I don't know what i'm thinking anymore.
etysmile.gif



You might think that small value polypropylenes are better than the larger ones, but I think the jury is out on that. I tend to think that as long as we're talking about good quality polypropylenes, the difference among them are going to be very small, and for coupling cap duty, such differences shouldn't exhibit sonic differences within the audio band.

The AudioXpress article makes an interesting observation that the physically smaller metallized polypropylene caps have lower inductance than those that are large (but not necessarily higher value in capacitance). This certainly flies in the face of many of the "boutique" caps which are quite big.
 
May 13, 2005 at 6:36 PM Post #9 of 10

Finch&Music

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Quote:

Originally Posted by amb
You might think that small value polypropylenes are better than the larger ones, but I think the jury is out on that. I tend to think that as long as we're talking about good quality polypropylenes, the difference among them are going to be very small, and for coupling cap duty, such differences shouldn't exhibit sonic differences within the audio band.

The AudioXpress article makes an interesting observation that the physically smaller metallized polypropylene caps have lower inductance than those that are large (but not necessarily higher value in capacitance). This certainly flies in the face of many of the "boutique" caps which are quite big.




You are refering to a specific article on caps in Audio-express. I do not have access to that (or only at a price I do not want to pay!). Your remarks trigger some questions.

I observe - and I am sure I am not alone - that there are very much questions and/or doubts on types, (supposed) qualities, values, brandsourses, technical points etc in respect to implementation and sonic results of capacitors. Sometimes diy-ers get quite obsessed by uncertainties, that are fed by some topic-goroes or persons who are economical concerned with vending capacitors. Thist is IMO a particular valid observation for caps in supply functions and in functions for signalcoupling. So the questioning is relevant in the context of building headamps.

For me - and I am sure for a lot of diy-ers - it is all a sort of voodoo.
Some of us are tricked to impelement very expensive one's (when resources allow you); others buy what there is locally available, not knowing if there is some negative relevance in relation to the more expensive one's; others combine different types and (mostly higher) values in the hope to encourage some sonic benefit for themself.

All in all in my opinion a trial and error approach without any real underlying rationality, let alone a real proof of sonic benefit (or it is strict subjective and not repeatable for observations by others).

On the site of AudioExpress I have seen that there is a lot of underlying technical information around bypassing electrolytic caps (with low ESR). I can not interpret that statistics without the context of the article and insigh in their research stratagy and/or conclusions. So I want to ask some general questions.

Is it possible that some experts within the forum members who has access to this article and all the (historical) information on the subject to translate the spread findings in a short list of 'do's and don'ts' on implement capacitors for supply-decoupling or signal-coupling?

I hope they also take the historical articles on 'pick your caps' or something like that into account.

Alle the available references are not always approachable or understandable for the 'simple' diy-er. I do not yet encounter a comprehensive and easily understandable tutorial on this topic. Even Tangent - who's tutorials for me are very informative - present information on this topic in different information sheets and do not deliver a real oversight; that's no critic on him but only an attempt to get a fully oversight from whoever feels he/she is an expert who can present something like that for us 'simple diy-ers'!.

Looking forward to the one expert who take the challenge.

Greetings,

Pieter Vink.
 
May 13, 2005 at 10:19 PM Post #10 of 10

Banfi T.

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"If you removed th 5uf cap completely then I think you may end up with no bass at all."

Please don not do this. You will probably have very deep bass for a short moment then deep silence. The coupling cap is there to protect your cans from DC voltage present at the output as a result of the circuit design.

Although 5uF does not seems to be edaquate for me as a coupling capacitor at the output. If you use a 150Ohm can for example then your frequency cornerpoint would be around 212.206Hz which is pretty high. You will have phase shifts in the Khz region because of this.
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