A ground loop forms when two pieces of equipment share multiple ground paths, creating a hum-inducing loop antenna

A ground loop occurs when there is more than one electrically conductive path between two points in a system — typically because both pieces of equipment are grounded to earth at different physical locations plus connected via the audio cable shield. The duplicate paths form a closed loop that acts as an antenna. Electromagnetic interference (mainly 50/60 Hz mains frequency) induces a small current in this loop; that current flows through the resistance of the conductors and produces a voltage fluctuation on the audio signal reference (ground), heard as hum. Ground loops are the most common cause of hum in audio systems. Breaking one of the loop paths (usually at the cable shield) eliminates the problem while preserving safety earth connections at the chassis.

Examples

A guitar amp plugged into a stage outlet and a mixing console plugged into an FOH outlet both have safety earth connections. Their audio cable shield creates a second ground path — a loop — between those two earth points. The loop picks up mains hum.

Assessment

An engineer lifts the ground pin on a power amplifier’s AC plug to eliminate hum. Why is this dangerous? What is the proper solution?