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In Reply to: Re: Bypass caps, a little more information posted by Jim McShane on March 24, 2007 at 08:51:14:
Hi Jim,From what I read of the article, if I understand the model correctly, matching caps isn't that big of a deal if you use parallel capacitors. For a couple of reasons. First, the more capacitors in parallel you use, the more likely you are to get an overall capacitance very close to the average of the manufacturer. So, if you are matching the coupling caps for 2 different channels, for instance, and use 10 random parallel capacitors for each channel, your likely to have much better channel to channel matching than if you used 1 randomly chosen capacitor per channel. In this case, random distribution and statistics work in our favor.
The other thing is, as I understand the model, even if you have large variations in the individual capacitor, say +- 10%, the amount of the total that contributes to the time smear is much smaller, so if you cut down a 4uF cap to individual 1uF caps, you will be cutting the latent storage amount as well, so the amount of variance in the latent storage should be small too. Again, if anything, random variations would work in our favor.
However, when dealing with only 2-4 parallel capacitors, it's probably not a bad idea to match them anyway. You could always have a cap that is well out of whack which can throw the whole thing off, especially if only using 2 per channel.
What I'd be curious to know now is whether anyone wants to take up the challenge of actually measuring the relative time smear of some high-end capacitors, say a Solen, Auricap and Multicap, so we can compare. Also, to see how well say a single Solen works compared to 4 parallel, or one in parallel with a teflon cap and why?
If I wasn't in school all the time, I'd consider doing it myself. I wonder how many "high-end" capacitors would actually come out as being better than some mid-end electrolytics or worse? ;)
Take care,
Follow Ups:
"From what I read of the article, if I understand the model correctly, matching caps isn't that big of a deal if you use parallel capacitors. For a couple of reasons."Hmm, okay, let's continue.
"First, the more capacitors in parallel you use, the more likely you are to get an overall capacitance very close to the average of the manufacturer."
Sure, that's absolutely true. Although that may be a moot point in audio, as rarely do we need such precision as relates to the absolute value.
"So, if you are matching the coupling caps for 2 different channels, for instance, and use 10 random parallel capacitors for each channel, your likely to have much better channel to channel matching than if you used 1 randomly chosen capacitor per channel. In this case, random distribution and statistics work in our favor."
Again, that's true - but does it matter? If one channel uses a .4699 uf coupling cap and the other channel uses a .4582 uf cap (just picking numbers out of the air) the difference is insignificant. It may also be swamped by the differences in the resistance of the next stage, etc.
"The other thing is, as I understand the model, even if you have large variations in the individual capacitor, say +- 10%, the amount of the total that contributes to the time smear is much smaller, so if you cut down a 4uF cap to individual 1uF caps, you will be cutting the latent storage amount as well, so the amount of variance in the latent storage should be small too. Again, if anything, random variations would work in our favor. "
I'm not sure I agree. Dielectric absorption is measured in percentages. As a result, the distortion caused by D/A would be additive in multiple caps, not the average. Since the D/A % is always positive, there is no cancellation it seems to me. The net effect would be that it is entirely possible that the four caps could increase the "smear".
"However, when dealing with only 2-4 parallel capacitors, it's probably not a bad idea to match them anyway. You could always have a cap that is well out of whack which can throw the whole thing off"
My point exactly. My concern (and I don't really know the answer, although I suspect I do) is that small differences in D/A among individual paralleled caps may be worse than the single cap D/A.
Somebody a lot smarter than me (that means almost everyone reading this!) care to comment?
And thanks for your reply to my post too - you got my brain in gear again!!
If less "time smear" is the goal I can't see how more caps are better. But maybe we could make an argument for how quickly the multiple smaller caps would recover from blocking?For coupling, I’d use one cap and deal with blocking by other means. But I can see the wisdom of unequal value power supply caps in parallel. Maybe like a 1/3 and 2/3 size to equal desired amount?
Funny how we get so wrapped up in this in a tube asylum. Loads of stuff is forced to use something like a 100uF/25V electrolytic coupling cap. In a push pull amp we are blessed with the ability to use great caps if we have the room.
Sorry Russ, what is blocking?
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