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Old 13th August 2005, 02:16 PM   #1
Bean is offline Bean  Australia
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Default Hate all those electrolytic bi pass caps

We all know that those cheap electrolyic coupling caps are no good. Most of them are ranging 10u to 47u in value. It is hard to replace them with the same value with MPK caps becasue there will not be enough space. Can anyone help let me know what is the min value if I like to replace those elect cap with MKP...is 1u enough?

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Old 13th August 2005, 02:49 PM   #2
sam9 is offline sam9  United States
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Quote:
what is the min value if I like to replace those elect cap with MKP...is 1u enough?
First it depends on what you are "decoupling". If you mean power supply rails I think you are in for disapointment. For opamps just follow the info on the datasheets. Usually 10uF is the minimum, less is not enough. For power amps 100uF is more like the minimum.

If you mean input and output caps, there are a few 10uF and greater caps available, but not MKT (I think). You can use the big ones that are typically used in speaker crossovers such as Solen, Auricap, etc. Check www.Madisound.com or www.parts.express.com for a selection. However, they are rather large, but I have seen some people use them.
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Old 13th August 2005, 05:30 PM   #3
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And, as mentioned on other forums here, sometimes a cheap electrolytic is exactly what you want for decoupling, as low-ESR parts might resonate with the series inductance of the power trace.


Francois.
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Old 13th August 2005, 05:59 PM   #4
Magura is offline Magura  Denmark
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For power amps the rule of thumb AFAIK is 1000/1, so if you have 47.000uF PSU caps, you will need 47uF bypass caps.

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Old 13th August 2005, 06:16 PM   #5
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It depends on what you are trying to bypass, and why. You have not given us an application, frequency range that it has to be effective over, or anything. Just that you do not like cheap electrolytics.

Tell us something that we don't already know.

Francois is right........and depending on whether or not this is for a 3-terminal regulator........you may get the same answer for a slightly different reason.

Jocko
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Old 13th August 2005, 06:42 PM   #6
sam9 is offline sam9  United States
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You probably don't like ceramic caps either, yet for some purposes they are exactly what you need. There are far too many urban legends floating around about caps.
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Old 16th August 2005, 01:36 PM   #7
macboy is offline macboy  Canada
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Quote:
First it depends on what you are "decoupling".
From what I read, the word "decoupling" wasn't mentioned at all. Beam was asking about coupling caps, not decoupling caps.
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Old 16th August 2005, 02:13 PM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by macboy

From what I read, the word "decoupling" wasn't mentioned at all. Beam was asking about coupling caps, not decoupling caps.
He does say "bypass" though, which is often used interchangeably with "decoupling" (although they don't mean quite the same thing).
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Old 16th August 2005, 03:06 PM   #9
AndrewT is offline AndrewT  Scotland
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Default input coupling caps

Hi,
for my two pence worth, I think he is asking about input coupling caps.
For low bass he needs an RC time constant of between 80mS and 200mS. But even a big film cap here will need a bypass (22nF to 220nF) to help the high audio frequencies through without phase errors.
Bipolar or back to back electrolytics also need a bypass but of a higher value maybe 470nF to 1.5uF.
MKP (polypropylene) is probably the best at an affordable price.
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Old 16th August 2005, 04:26 PM   #10
sam9 is offline sam9  United States
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Quote:
He does say "bypass" though, which is often used interchangeably with "decoupling" (although they don't mean quite the same thing).
That's why I asked. The terms are interchanged more or less loosey-goosey. I would rather get clarifaction babble on about the wrong thing.
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