You plug everything in, turn up the monitors, and there it is: a low hum that will not go away. Noise gates will not fix it, and moving cables only helps until you touch them again.
What a ground loop actually is
A ground loop happens when two pieces of equipment are connected to earth ground through more than one path. Because no two paths have exactly the same resistance, a small voltage difference appears between the two chassis grounds. That voltage drives current through the shield of your audio cable, and that current is what you hear as hum.
The loop is not in the cable — the cable is completing a circuit that already existed through your mains wiring.
How to confirm it
- Disconnect everything except one source and one monitor. If the hum disappears, add devices back one at a time.
- Turn the volume all the way down. If the hum stays at the same level, it is entering after the volume control — that points to a ground issue, not gain staging.
- Listen to the pitch. Mains hum is 50 Hz (most of the world) or 60 Hz (North America), plus harmonics.
Fixes, in order of preference
- Isolation transformer — a 1:1 audio transformer breaks the galvanic connection while passing audio. This is the correct fix for balanced and unbalanced lines alike.
- Balanced connections — XLR or TRS reject common-mode noise. Use them if your gear supports it.
- Star grounding — power everything from one outlet strip so all grounds share a single path.
- Lift the ground — a ground-lift switch or adapter. Effective, but it defeats a safety feature; use it last and understand why.
Where the noise enters
Common culprits: a USB audio interface sharing ground with a computer on a different circuit, a guitar amp in the same chain as a DI, or a monitor controller fed from two sources. Diagnosing means isolating — pull connections until the noise stops, and you have found the loop.