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Old 21st May 2012, 03:41 PM   #11
rongon is offline rongon  United States
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I thought the idea behind applying a small-value film cap across a large electrolytic was to decrease the ESR (or impedance?) of those large electrolytic caps at higher frequencies.

If you use an electrolytic cap with very low ESR up to well above high audio frequencies, then a film bypass cap would do nothing, correct?

But if you're using an electrolytic cap whose impedance begins to rise near the upper end of the audio band, then a film bypass cap would help. Right?

Solid state amps use larger value electrolytic capacitors (and lower voltage) than do tube amps. A 10,000uF 100V electrolytic might have much higher impedance with rising frequency than a 47uF 450V cap, no?

What about pulse current?

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Old 21st May 2012, 03:41 PM   #12
Merlinb is offline Merlinb  United Kingdom
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One thing I have not seen addressed anywhere is the fact that different capacitor types have different self resonant frequencies.
Thus, if you put two different caps in parallel then in the range between their resonant frequencies they will behave as a parallel tuned filter, and you could potentially get less good decoupling than you started with! (But only in that range of frequencies)

Last edited by Merlinb; 21st May 2012 at 03:45 PM.
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Old 21st May 2012, 03:43 PM   #13
DF96 is offline DF96  England
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The issue is not the capacitor wire so much as the wiring to get to the capacitor, as SY says. Whether inductance is an issue depends on circuit typical impedances - probably negligible in most valve cases, but more important for SS.

The effect of paralleling two resonators is shown in several places on the web. The effect can be to make things worse, especially if the caps are high quality low loss ones.

Last edited by DF96; 21st May 2012 at 03:46 PM. Reason: expand
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Old 21st May 2012, 03:53 PM   #14
jcx is offline jcx  United States
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Quote:
One thing I have not seen addressed anywhere is the fact that different capacitor types have different self resonant frequencies.
http://www.diyaudio.com/forums/gtsea...D25%26page%3D3
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Old 21st May 2012, 03:55 PM   #15
DF96 is offline DF96  England
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Changing "resonce" to "resonance" might assist the search!
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Old 21st May 2012, 03:57 PM   #16
jcx is offline jcx  United States
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google forum search is smart enough that it works anyway, but I fixed it before seeing your post
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Old 21st May 2012, 04:48 PM   #17
jkorten is offline jkorten  United States
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Got it. Thanks Chrish.
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Old 22nd May 2012, 10:03 PM   #18
data is offline data  Australia
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With the availability of high quality electrolytic caps like Jensen & Mundorf there is not a lot to be gained with film by-pass caps, although I have seen some folk using Mundorf Supreme film caps (not the silver/oil ect) and the film & oil caps like ASC as by-passes in the power supply.

If the voltage and capacitance is low enough, Solen BP series caps are a nice replacement for an electrolytic (they are only 400v) in a power supply.

Personally I'll avoid by-pass caps if at all possible.
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Old 22nd May 2012, 11:47 PM   #19
jkorten is offline jkorten  United States
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Quote:
Originally Posted by data View Post
With the availability of high quality electrolytic caps like Jensen & Mundorf there is not a lot to be gained with film by-pass caps, although I have seen some folk using Mundorf Supreme film caps (not the silver/oil ect) and the film & oil caps like ASC as by-passes in the power supply.

If the voltage and capacitance is low enough, Solen BP series caps are a nice replacement for an electrolytic (they are only 400v) in a power supply.

Personally I'll avoid by-pass caps if at all possible.
Thanks Data - obviously this is a mass produced amp without high quality electrolytics. So I'll roll my own and report back!
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Old 23rd May 2012, 12:52 AM   #20
data is offline data  Australia
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I forgot to mention that I use the Solen BP series caps for the driver stage, as that's low enough voltages and capacitance in my amp.
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