[Herald1360]

Ref post #226.....

Unwanted feedback can occur if an input circuit and an output circuit share a common path from where their grounds join to the actual 0V point in a system.


If you have a common ground impedance of 10milliohms, say, (not at all unlikely) then an output circuit current of 1A will cause a feedback voltage of 10mV to the input circuit. Add in enough gain from input to output and a few unpredictable phase shifts and anything can happen. One of the anythings (in real life the most likely one) is that the circuit now hoots.


One way to avoid this is to return every single ground in the circuit independently to the famous "Star Point". This may be overkill for audio if you can quantify things a bit and just keep sensitive circuits away from power ones, and at higher frequencies stray inductance/capacitance in longish ground connections may be a worse problem than just tying everything down to a ground plane or chassis. You really do need to think about where any ground currents are going on their way back to the PSU.