English: A schematic diagram showing one solution to the problem of a ground loop between electronic components - insert a small resistor in the ground shield of the signal cable. A ground loop is an inadvertent electrical circuit or loop created by the ground wiring of electrical equipment. Ground loops are a major cause of hum and interference in audio, video, and computer equipment, because circulating currents induced by stray magnetic fields from nearby 50/60 Hz equipment can cause AC voltage dropped across signal grounds. In this diagram two AC powered electronic components, C1 and C2, are connected by a shielded signal cable S. The ground shield of S, along with the ground wire ("third wire") P in the components' power cord and the building's utility ground wiring G form a ground loop.
Inserting a 10Ω resistor (red) in the ground shield conductor of the cable S is enough to prevent the ground loop current from flowing. This solution is better than simply creating a break in the cable's ground shield, because if one of the components doesn't have a ground wire ("third wire") P in its power cable the resistance is low enough that the component will still be kept effectively at signal ground potential through S. This prevents it from being totally ungrounded ("floating"), which would cause loud hum or interference. Information and circuit from Larry Robinson, About Ground Loops, MidiMagic website