50 Hz suppression is clear.
The zener in the opamp circuit gives unsymmetric feedback. As soon as the opamp limits, this unsymmetry causes a DC component at the output.
.. DSP ... simple...But i am looking for simplest DSP solution
And what do you think about my idea of simply doing FFT and then trigger output based on sum of all bins except those containing hum?
i suppose that filtering will possibly add more distortion than the hum itself.
This filtering in particular do not affect output.
That's true, it will not affect output, because there will be no filtering nor sampling before amplifier. Amplifier gets unprocessed analogue input. DSP is just to turn this analogue amp on and off. I am not sure i made this completely clear. I don't care about quality of digital signal. I just don't want to turn the amp off when there will be actual music playing or turn it on when there will be just hum and no music.This filtering in particular do not affect output
Have a look at Figure bellow, which somehow illustrates the concept behind this technique
somebody in this thread said that mains hum is composed of multiple frequencies
In real world, the waveform of the mains is not exactly sinusoidal as shown at the pictures above, but softened at the peak, and this actually add some multiple odd harmonics of the fundamental frequency, but considering their much smaller amplitude, the humming effect should be negligible.
I am not sure about this
It's actually not the long cable as such rather than an unsuitable single-ended signal connection. Using twisted pair cable with separate ground and differential amplifiers or audio transformers on the receiver side can eliminate the problem.But in some cases where audio cable is too long there is 50Hz (EU) mains hum
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