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In Reply to: RE: Which 0.1uF polypropylene bypass caps? posted by petercapo on July 02, 2017 at 18:18:19
Small film caps for bypass may induce oscillations, depending on the layout of the wires and traces.
A low value resistor may be needed in series with the bypass, and a high-speed oscilloscope to make sure it is not oscillating. You will also need to check it on music, under various loads.
Follow Ups:
maybe I should have mentioned it, but it has been rare in my experience. On the other hand, sometimes adding a series resistor to a capacitor bypass will improve the sound in subtle ways, which may be an indicator that there was an undetected oscillation going on and it's been damped by the resistor. Typically, I would use 10 to 1000 ohms in series with .01uF to 0.1uF capacitance.
How might such an oscillation be manifested, sonically?
A touch of edginess to the sound, if the oscillation is very far above the audible range. Can get worse if lower in frequency, such that you wouldn't want to listen. In one extreme case having nothing to do with a bypass capacitor, one of my Beveridge amplifiers was set into oscillation (by the output from a CDP) and promptly blew a fuse.
But the only time I can recall with a bypass capacitor, it was not very audible at all. I added the resistor empirically, whereupon I did hear that a slight tinge of edginess was gone. I don't think one can generalize; it depends upon the circuit, the values, etc. For sure, bypass capacitors do not always cause oscillation.
First time I have heard of this...
It's common.
Back in the day (30 years of hi-fi) I used a 100Khz oscilloscope (ignorance was bliss).
For audio I generally use just a good electrolytic between 22µF~47µF and leave it at that.
(I currently work in L-band RF and chooses bypass caps with great care and use a spectrum analyzer to monitor the results. Where the cap is grounded makes a huge difference, and series resistors on the order of 1Ω~10Ω added as well)
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