[sdiy] Hum & Buzz

Ingo Debus igg.debus at gmail.com
Tue Sep 19 19:14:12 CEST 2017

> Am 18.09.2017 um 22:13 schrieb Michael Zacherl <sdiy-mz01 at blauwurf.info>:
> OTOH I had a very close look at the signals with the oscilloscope and the oscillation would have to  be in the mV range to get cover in noise (hiss).
> To understand correctly: your question is implying the oscillation only would occur with a signal present at the input, i.e. the ins not tied to ground?
> The audible hum is a very prominent 50Hz signal.

Ok, I was thinking of an oscillation with higher voltage, not something in the millivolts range.
A long time ago I had something that oscillated in the ultrasonic rage, and the only thing I noticed first was a slightly increased hum from the speakers. Then I noticed that the heat sink of the amp became hot although there was almost no signal to be heard. Obviously the amp happily amplified that high frequency signal and loaded its unregulated power supply so much that the mains hum became more audible.

I don’t think that the preamp oscillates or not depending on the input signal. But it can depend on whether the input is shorted directly or via some impedance to ground and, more important, where this ground is connected. I recently built a preamp for a piezo guitar pickup. Standard noninverting circuit with a gain of 5 or so, with an 1 meg resistor to ground at the noninverting input, in parallel with the piezo. That thing oscillates without the piezo connected but it doesn’t when the piezo is in place!


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