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Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
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Take the power supply.If you are already using a film cap,why would anyone bypass it thinking you will lower ESR or ESL? You won't.It's already ultra low by nature of film caps..Even ESL of capacitors by nature,are only a few hundred pico henries so their rising effect on impedance, only emerges when system frequencies exceed 100 MHz.In RF circuits that would make sense. If you bypass a lytic with a film cap that is 10% or more of the value of the lytic,that will be effective because much of the energy delivery duties at static levels will be handled by the film cap as well as storage duties..It will dip into the lytic under higher dynamic conditions.Using a .1uf to a .22uf will do little to nothing IMHO.I guess some think that tiny value film caps can lower ESL to the point where higher freq riding on the rail can't get filtered out.That may be somewhat true,but I have never witnessed it with measurement.
Now,in the case of the photo where the caps in question are used for signal,the same principles apply as far as bypassing in this case.Bypassing with that many caps does nothing but cause phase shift issues. Some companies bypass with a single small value film cap across a larger film cap.This is why you might see a 4uf bypassed with a .47uf..Remember the 10% rule I like to use even applies to film caps in this case.Typically,I don't bypass film caps in the power supply but here in the case of signal caps,it might make more sense to some people. They think that larger value caps don't pass higher frequencies as well so they bypass it to reinsert the higher frequencies.It may or may not be a tad faster,depending on design parameters. Now,have you ever wondered why they use such a large cap in the preamp output stage of tube preamps compared to years ago? It's to be able to drive a lower impedance. A lot of people that buy tube preamps,will be using them with solid state amps which are a much lower impedance than a tube amp.Many will also be driving subwoofer amps from the preamp along with their tube amp.If you are just going to drive a tube amp and nothing else,you don't need a 3uf to 4uf cap in the output of the preamp to drive a 250k to 500k load.I have people take out the large caps and put in a .47uf and they hear all the same bass as before, only with much more clarity on mids and highs.Try it sometime.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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