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In Reply to: Re: Bypass caps, a little more information posted by Michael Samra on March 24, 2007 at 19:11:03:
Hi!I think it's important to understand that there are two issues, one of which is frequency domain, and the other time domain.
Bypassing a large cap with a very small cap is a great idea for frequency domain issues. When you are doing this though, what you are bypassing is not the capacitor, but the inductance in the capacitor. :)
Think of a real capacitor as being a series LC network. Larger, electrolytic caps tend to have the most L. Now if you draw this in paper, you can see how the bypass effects work. You aren't interested in adding capacitance, so much as creating a low L path. Very much like a crossover.
Say you have a big ass 4,700uF electrolytic capacitor with 120 mH of parasitic, or unintended inductance. What this inductance is going to do is eventually cause a roll off of high frequencies. So, if you can add any capacitor around it, with say 10 mH of inductance, you have just increased your high frequency response significantly. BUT, even with this system, you have to choose your bypass cap well. You can end up with a "hole" or "hump" in the response where the bypass cap's effects doesn't match well with the main. This would definitely cause the sound to be worse than before.
Where small bypass caps don't work, at all, is in the time smear, or Dielectric Absorption problem. For this, it seems you are stuck with either banks of caps, or using higher quality caps to begin with, or maybe new caps with multiple leads will work best. Or, if you want to get really really really outrageously tweaky, you can design an op-amp based circuit with a model of the coupling cap, which outputs an inverse signal to that produced by the DA. Hah. :) That would be a never ending story. :)
Regards,
Erik
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Follow Ups
- Bypassing the large with the small - erik_squires@hotmail.com 12:53:51 03/25/07 (0)
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