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In Reply to: RE: Oscillations posted by djk on July 03, 2017 at 14:23:44
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...
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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