A steady low hum in your speakers or headphones, usually at the same pitch as the electricity supply (60 Hz in North America, 50 Hz in much of the world), is most often a ground loop. It happens when two pieces of equipment are connected to ground by more than one route, and a small current flows through your audio cables. Here is how to find the source and get rid of it.
How a ground loop forms
Each piece of gear is grounded through its power cord, and your audio cables connect the grounds together through their shields. If the outlets differ even slightly in ground potential, a small current flows through the shield, and the audio equipment picks it up as hum. Cable television boxes, computers and anything else connected to a second ground path are common culprits.
Hum or buzz?
A ground loop is typically a low, steady hum. A thinner, harsher buzz is more likely caused by something else, such as a light dimmer, an LED power supply or a wireless charger. The fixes differ, so identify which one you have before you start.
Step by step: find the cause
- Disconnect everything from the input. Unplug the audio cables from the mixer or amplifier input. If the hum goes away, it is coming from something upstream.
- Reconnect one piece at a time. The piece that brings the hum back is where the loop is.
- Plug everything into the same power strip. Using a single outlet puts all the gear at the same ground potential and often solves the problem on the spot.
- Try another cable. A damaged shield or poor connection can create hum.
- Move computers and monitors. If the hum changes when you turn off a computer or screen, that is the source.
Fixes that work
- Use balanced cables. Balanced connections reject much of the hum. See balanced vs unbalanced audio.
- Use a DI box with a ground lift. A ground lift switch breaks the shield connection on pin 1 of the XLR, which breaks the loop while the audio signal still passes.
- Use an isolation transformer or hum eliminator. These devices put a transformer in the signal path to separate the grounds of two devices electrically.
- Shorten or reroute cables. Keep audio cables away from power cables, and cross them at right angles if they must meet.
What not to do
Never remove or defeat the third (ground) pin on a power plug to stop hum. The ground pin is a safety feature that protects you from electric shock if equipment fails. Use a ground lift on the audio connection or an isolation device, never the power plug.
Is it really a ground loop?
If the hum comes only from a guitar with single-coil pickups and changes when you turn or move, it is probably pickup hum from nearby screens or lights rather than a ground loop. Facing a different direction or moving away from the source usually helps.
If a worn cable is the problem, what makes a good guitar cable explains which features reduce noise.