The important thing to remember is that hum, buzz, or any other form of unwanted noise can only become a problem if the component or circuit design allows the noise current to be injected into it’s internal circuit conductors. Unfortunately, rather a lot of audio equipment is designed such that electromagnetic interference is directly injected into the internal circuit ground.

Fig 1: The pin 1 problem

Connecting the cable shield conductor directly to the internal circuit ground creates TWO separate “ground loops” that WILL inject noise/interference into the internal circuit. Noise & interference will also be coupled to the power supply. The power supply will circulate the noise to all parts of the internal electronics. The internal electronic circuits will demodulate (convert) the interference into audible signals.

Similarly – if a circuit is producing electromagnetic noise – an audio signal passing through the system will modulate the interference, and the noise will be radiated by the output & input cables (fig 2).

Fig 2: Result of demodulation in audio output

We call this reciprocity:

A conductive structure (eg. Cable, printed circuit board trace) that has a certain antenna behaviour when it picks up an EM wave  and produces currents and voltages in a circuit – is exactly the same antenna behaviour as with an emitter of EM waves when currents and voltages are made to occur in a circuit designed to receive or transmit such EM waves.