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Old 2nd September 2012, 08:13 PM   #11
MGH is offline MGH  United States
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Quick question. Are mica capacitors similar enough to ceramics to be used as high freq bypass capacitors?
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Old 2nd September 2012, 09:17 PM   #12
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Frank, how about this? At least this way, most will be ceramic caps.

0.001 microF ceramic cap, 0.1 microF ceramic cap, and 10 microF film cap (1/100 ratio)
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Old 3rd September 2012, 12:22 AM   #13
fas42 is offline fas42  Australia
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Haven't used any mica caps, but they should do the trick if you have them lying around. A premium product, that's again overkill, really, for the task at hand.

The trouble with using a 0.001uF cap is that something like that has to go absolutely precisely where it's needed, within a couple of mm's or so; else it's close to being worthless. The trace, lead, or wire length between it and the circuit will add too much inductance otherwise for that relatively tiny bit of capacitance in parallel to the others to do any significant good. To put things into context, 0.001uF still has an impedance of 10 ohms at 10MHz ...

If you have these values actually lying around then there should be no harm in trying it, see what difference it makes.

There is another technique, you could call it a swamping method, where you add vast numbers of caps adjacent or very close in value: 1, 1.5, 2.2, 3.3, 4.7uF, and on and on, say -- this gets very messy, and expensive. It's used in very high speed circuits and if done correctly can create a uniform, very low impedance right across the frequency spectrum. But it shouldn't be necessary in audio products, at least, I hope not!!

Frank

Last edited by fas42; 3rd September 2012 at 12:27 AM.
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Old 3rd September 2012, 09:56 AM   #14
MGH is offline MGH  United States
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Hmm...you're giving me some good ideas. Something like this.

0.01, 0.05, 0.1, 0.5, 1, 5, 10, 50 microF caps in parallel.

There must be some mathematical sequence of cap values that will give a even overlap of frequencies. I can find 0.01, 0.05, and 0.1 in ceramic, not so sure about 1.

Wouldn't be better to get 1, 5, 10, and 50 caps in metal foil, not metalized film?

Last edited by MGH; 3rd September 2012 at 10:13 AM.
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Old 3rd September 2012, 01:28 PM   #15
fas42 is offline fas42  Australia
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Quote:
Originally Posted by MGH View Post
Hmm...you're giving me some good ideas. Something like this.

0.01, 0.05, 0.1, 0.5, 1, 5, 10, 50 microF caps in parallel.

There must be some mathematical sequence of cap values that will give a even overlap of frequencies. I can find 0.01, 0.05, and 0.1 in ceramic, not so sure about 1.

Wouldn't be better to get 1, 5, 10, and 50 caps in metal foil, not metalized film?
The sequence is an approximate multiple of 3; so 0.01, 0.033, 0.1, 0.33, 1, etc. These are standard values ...

Be very careful if you try the "overlap" method; the values need to be quite close together, something beyond a gap of 5 times or so of the next value starts not to be good, that resonance issue begins to rear its ugly head again!

There shouldn't be any significant difference between foil, and metalised film.

Frank
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Old 3rd September 2012, 09:40 PM   #16
MGH is offline MGH  United States
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0.01, 0.033, 0.1, 0.33, 1, 3.3, 10, and 33. OK this is what I'm going to settle on. I might not go with 33 because this is getting expensive! I need two of each value for left and right channel.
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Old 4th September 2012, 01:05 AM   #17
MGH is offline MGH  United States
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Originally Posted by fas42 View Post

Be very careful if you try the "overlap" method; the values need to be quite close together, something beyond a gap of 5 times or so of the next value starts not to be good, that resonance issue begins to rear its ugly head again!

Frank
Frank, I'm a little confused. You state gaps beyond 5 times could cause resonance, but you suggested values 50x or more in your initial response. Am I missing something? Is there some equation that predicts where the resonance will be at a particular spacing of values?

Last edited by MGH; 4th September 2012 at 01:11 AM.
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Old 4th September 2012, 03:32 AM   #18
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Frank, I'm a little confused. You state gaps beyond 5 times could cause resonance, but you suggested values 50x or more in your initial response. Am I missing something? Is there some equation that predicts where the resonance will be at a particular spacing of values?
No, there is a critical range of one value being a certain size greater or smaller from another that can cause resonance issues. The top end of this range is about 50 times, the bottom end is around 5 or so, it's a while since I looked at this closely, so I haven't got an exact number here at the moment. But 3 times is fine ...

Another way of looking at it, if the next value is spaced this number of times that of the next closest value then the combination is:

2 times - good
5 " - OK
10 - not good
15 - really bad
25 - not good
40 - OK
50 - good

Does this make more sense? Those values are very rough and ready, just to give you an idea what's going on ...

And remember this is worst for film caps, almost as bad for ceramic. Electrolytics are totally benign, never a worry in this regard. And can be combined with film and ceramic without any problems, no matter what the value. In fact, the solid polymer variety, the Sanyo OS-CON and their bretheren would be brilliant to use everywhere, but they can't take any reasonable voltages ... bummer!

And I can't emphasise too strongly that the real benefit of doing this exercise is for all leads, wiring to be as absolutely as short as possible. If you can't physically add this bypassing exactly where it needs to go, to have as close to zero wire length as you can possibly organise, then don't do it, it won't do the job!

Frank
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Old 5th September 2012, 04:08 AM   #19
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Seems difficult with so many caps to keep the wire length minimal. May be it's simpler and practically just as effective if I go with your original recommendation of 0.01, 0.47, and 22. But it's hard to find a 22 microF film cap that is small.
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Old 5th September 2012, 05:01 AM   #20
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Seems difficult with so many caps to keep the wire length minimal. May be it's simpler and practically just as effective if I go with your original recommendation of 0.01, 0.47, and 22. But it's hard to find a 22 microF film cap that is small.
Exactly. Just throwing in cap's willy nilly may give one an improvement. Or it may not. Depending on how carefully and precisely one positions them. And again, it is most critical to keep the lengths shortest on the smallest value caps, and they should be the ones most precisely positioned in the circuit where they're needed to do the job. The larger caps can be allowed a bit more lead length to fit them in, and I've created little sculptures in space at times just to have the ideal configuration for minimal lead length .

I agree that it will probably be just as effective for an audio environment to use the widely spaced combination. Ultimately, the trick is just to try out how much of a difference it makes doing this sort of bypassing; if the change in sound is very significant then it is probably worthwhile seeing if going the overlapping values route is even more beneficial ...

Don't get paranoid about the larger values being physically large - it just equates to having a slightly longer lead length, and then see above! In simple terms, size and length means more residual inductance, which cancels out the effectiveness of the capacitance at higher frequencies. Unless, the manufacturer has gone to great lengths to minimise this, which he will surely crow about ...

Frank
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