Ground Loop
Definition
A ground loop is a closed path formed when two devices are connected both by the shield of a signal cable and by their protective earth conductors, while the earth potentials at the two points differ. Current flows around the loop and, where part of it passes through a signal reference, adds hum at the mains frequency and its harmonics: 50 Hz in Europe and most of Asia and Africa, 60 Hz in North America and part of South America. Balanced interconnections, with the cable shield bonded to the chassis at the connector as AES48 requires for pin 1, reject most of this interference. The usual remedies are an isolating transformer in the signal path, such as the one in a passive DI box, and supplying related equipment from the same power distribution.
In the field
Hum that disappears when the XLR cable is unplugged, but not when the channel is muted, points to a ground loop through the cable shield. A common diagnostic lifts the shield (pin 1) at one end of the cable, usually with the ground-lift switch of a DI box or an adapter made for the purpose. The protective earth of the mains supply is never disconnected to cure hum, because it provides protection against electric shock.
Sources
- AES48-2019 (révision de l'AES48-2005) : contact de blindage (broche 1) et coquille du connecteur reliés à l'enveloppe de blindage par le chemin de plus faible impédance · aes.org
- Rane, RaneNote 110, Sound System Interconnection : boucles de masse, problème de la broche 1, transformateurs d'isolement ; ne jamais retirer la terre de sécurité · ranecommercial.com
- Mains electricity (Wikipedia, pointeur secondaire) : 50 Hz et 60 Hz selon les régions · en.wikipedia.org
- IEC 60364-4-41 : protection contre les chocs électriques par conducteur de protection et coupure automatique