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Old 4th July 2011, 05:46 PM   #1
BigE is offline BigE  Canada
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Default power supply bypass cap size

What is the correct size of a power supply bypass capacitor?

The idea is for a large capacitor bank feeding the main amp. The amp is an F5.

I heard that the bypass caps should be 1/10th the size of the main bank.

So, if there is 94,000 uF per rail, then the bypass caps (ie. those actually in the amplifier side of the PS) should be about 10,000 uF.

Is this correct?

Thanks!
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Old 4th July 2011, 06:15 PM   #2
AndrewT is offline AndrewT  Scotland
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I don't agree that putting capacitors in the wrong place improve performance.

Decide what you want a capacitor to do. Then select a capacitor that can do that job effectively. Finally select a location to allow the correct capacitor to do it's job well.

That is very different from adding bypasses onto the main smoothing of an amplifier. But before I understood I made exactly that mistake. I have learned.
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Old 4th July 2011, 06:38 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

I'd say for a typical case electrolytics on the power amp board are ~ 1/20th,
and these can also be bypassed with small film capacitors, ~ 1/100 and higher.

rgds, sreten.
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Old 4th July 2011, 06:48 PM   #4
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I find that as I go up to 300 uf there are differences, above that not so much. It may be coincidence that that is equal to 8 ohms at 60 cycles, maybe not. I actually use film capacitors for that. But if you use an electrolytic then consider a small bypass of 1 nf with a good RF capacitor and another of around 20 uf film, but not mylar!
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Old 4th July 2011, 07:21 PM   #5
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I strongly disagree with the "common knowlege" that bypass caps are necessary. The most important thing to consider, as Andrew T alluded to, is the application. In my experience, using bypass caps incorrectly can be worse than using none at all. One popular justification is the supposed inferior high frequency abilities of electroytic caps, not always true. At times they may be beneficial in supressing undesirable high frequency oscillation, but if not implimented properly, might even make it worse. If the circuit is well designed, physical layout done correctly and proper components are utilized, they will generally not be necessary, although there are always exceptions. There is also the unintended consequences of the inherent resonant frequencies of different caps creating problems. So the long-winded short answer is, know what is necessary to get the job done through design analysis, measurement and even experimentation. Guessing, or "standard answers" won't get you there.

Mike
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Old 4th July 2011, 08:26 PM   #6
dazed2 is offline dazed2  Canada
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Well the point of the exercise is to use an external case for the "power supply" and have an umbilical cord to the amp case.
In this case, the question is what size of capacitor if any, is necessary
at the amp case (ie the amp board).
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Old 4th July 2011, 08:49 PM   #7
BigE is offline BigE  Canada
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Yes, I was not clear -- the bank is supposed to be external to the main amp case.
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Old 4th July 2011, 09:19 PM   #8
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Measure and make sure there is a problem, attempt to correct and measure again.
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Old 4th July 2011, 09:57 PM   #9
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Quote:
Originally Posted by BigE View Post
What is the correct size of a power supply bypass capacitor?

The idea is for a large capacitor bank feeding the main amp. The amp is an F5.

I heard that the bypass caps should be 1/10th the size of the main bank.

So, if there is 94,000 uF per rail, then the bypass caps (ie. those actually in the amplifier side of the PS) should be about 10,000 uF.

Is this correct?

Thanks!
94,000uF per 1 RAIL?

Did you already take out the mortgage?
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Old 5th July 2011, 04:54 AM   #10
BigE is offline BigE  Canada
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Aren't there many of these freakshow sized power supplies in the gallery?
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