Electrical
Ground Loop
01 — Definition
The low-frequency hum (50 or 60 Hz and its harmonics) that appears when two devices are joined both by a signal cable and by different earth paths sitting at different potentials. A current loop forms and injects noise into the signal. The cure: an isolating transformer (DI box) on the offending path, or a single-point earth for all the equipment.
02 — In the field
Hum that disappears when you unplug the XLR but not when you mute the channel? That is a ground loop. As a diagnostic, lift the shield (pin 1) at one end of the cable: if the hum goes, you have found your loop.