[not again ?]bypass cap in singal circuit


2009-01-21 4:05 am
I searched the forum in regards of quality and brand of bypass capacitor. Eveyone discuss the affect it did in PSU and speaker only but none of them are talking about how the brand of a bypass capacitor did affect when installing in the signal circuit.

Anyone mind share their opinion, I don't want to waste too much money on this issue.

The pre amp I am going to build is this
and I mainly concern in C9 - C20
My opinion is that the best way to avoid wasting money on bypass caps is to avoid wasting money on bypass caps! That is, don't use bypass caps. It is rare that they do anything useful in audio circuitry, but they can sometimes create problems of unwanted parallel resonances. If you must use them (e.g. in order to follow fashion) then make sure that either they or the electrolytic they are bypassing is sufficiently normal and cheap that there is enough resistive damping to kill off any unwanted resonances.

Monte McGuire

Paid Member
2013-07-28 2:58 am
Here's another data point to support the "use good electrolytics" suggestion. I've cobbled together a test jig to measure the impedance of a power supply capacitor, and found that most high quality electrolytic caps have a very low impedance, even at somewhat high frequencies. I measured a 470µF 35V Panasonic FR cap using a swept oscillator and a series resistor, and found that, while the cap's self-resonance is somewhere around 4-500kHz, the low impedance region around resonance is broad and wide, unlike a ceramic or film cap's narrow and high-Q resonant region. This cap has a far lower impedance magnitude at 1MHz than any ceramic bypass will have, even though this is well above its self-resonance. Using this jig, it's also instructive to see just how little a typical small bypass cap does when added in parallel to a quality large-ish electrolytic cap.

So, my suggestion is to spend your money on quality electrolytics, and if you're concerned about low impedance at extremely high frequencies, put some small SMD bypasses right near the finicky component, not across the large electrolytic.