Electrical – Remove ground loop hum by 180º phase shift


I have a homebuilt subwoofer for my surround sound audio system. 400W constant, 900W peak. However, there appears to be an AC hum on the input line, and it becomes especially noticeable when I connect a preamp inline at the subwoofer end of the line. As in, house shaking 60Hz hum. That hum's gotta go.

Is it possible to remove the effect of a ground loop by feeding an equal amplitude, ~180º out of phase copy of the loop frequency? I've been thinking about this and it seems like there would be some feedback or something.

Would it work to use this schematic? If not, please correct it.


simulate this circuit – Schematic created using CircuitLab

And I'd love to have a simpler solution for active ground loop/AC hum elimination if there exists one.

Best Answer

You would need a very selective (read complicated) band pass filter or you would distort a decent chunk of your bass range. Feedback would not be a real problem since there are no real feedback paths in that circuit.

It would be a lot simpler to take a mains transformer that outputs a low AC voltage, add a potentiometer and a capacitor in series with the secondary winding and mix the output with the line audio.

Even better would be to dismantle the ground loop altogether with an audio isolation transformer, or a TOSLINK S/PDIF optical digital audio cable.