Two or more interconnected (via a grounded audio line) pieces of audio equipment plugged into electric circuits with different ground connections. Both units are grounded by the AC power cord as well as the shield wire that runs between them. This creates a circular path through the ground and shield lines, resulting in a ground loop. The resultant loop can produce low-frequency (60Hz) hum and high-frequency buzzing sounds. Solving such a problem isn’t an easy task.