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In Reply to: Question for Mark posted by Allan on March 20, 2001 at 02:11:07:
The cap in question is only there to keep audio out of the power supply rail. Without the cap in place, V4 will modulate the power supply a bit. As frequency increases, capacitors become more inductive, so they are not as effective at higher frequencies. Since we're trying to shunt audio to ground and not just 60 or 120 hZ in a power supply, the capacitor is asked to do a bit more for us at much higher frequencies. So, to calculate a good bypass cap value would be a most difficult undertaking. Not all 100 uF caps are created equal. As frequency rises, differing caps become inductive at differing rates. There are many, many variables that need to be known before a bypass cap value can be calculated. A 100 to 1 ratio almost always works well in practice. So that's probably where the 100 uF and 1 uF values came from. Since only we're dealing with audio frequencies, it is very possible that the bypass cap isn't really needed. I've never measured what 1 typical 100 uF IE cap does at 20 khz. Of course, harmonics can go much higher than 20 khz, so the 1 uF bypass is still cheap insurance. If you're really an anal audiophile, then bypass the 1 uF with a .01 uF.This topic is one of great complexity. I could probably do a whole soap box on it. I hope this is enough to start. There's lots of text books that cover this topic in extreme detail.
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- Re: Question for Mark - mark 10:17:17 03/20/01 (0)
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