Dangerous Music 2-Bus Manual
9
To comply with international standards and wiring practices, recording equipment
manufacturers are required to connect all the shield pins of audio and data connectors to
the chassis grounds of their gear. Sometimes, this can cause noise problems in large
systems where pieces of equipment are spread out around a facility because two
‘grounds’ are never quite at the same potential. This can cause ground loops (hums or
buzzing) if the cable shields are allowed to connect two chassis that are at different
potentials due to location, circuit wiring, or induction.
If the audio cables between the racks connect the equipment grounds together via the
shields and the racks are at even slightly different potentials (on different circuits with
different loads, long distance, etc) the shields will try to equalize the potential difference.
Juice will flow down the shields and broadcast hum into the signal wires they
were supposed to protect or wind up imposed on the reference ground of the receiving
equipment. This situation manifests itself as the all too familiar buzz of a ‘ground loop’.
The intensity depends on many variables but can go from unnoticeable to raging. Some
people in desperation resort to using AC plug “ground lifts” to defeat the mains safety
grounds in a random fashion until the system quiets down a bit. This in our view (and the
view of the safety standards organizations) is an unacceptable method of taming ground
buzzes. The simpler way is to make sure that all the gear has a good mains ground and to
lift the shields on the receiving ends of the audio cables. The principals at Dangerous
Music have wired up large, world-class facilities using this scheme and have brought
room after room online with
no buzz problems
from the moment of power up. This is
why many gear manufacturers have shield lift jumpers inside their equipment. If a noise
problem crops up, changing the jumper position will almost always cure the problem.
Planning out the wiring system to minimize the formation of ground loops solves
problems before they happen.
Many powered speakers seem to get their audio reference ground from the input
cable. This means that these shields should not be lifted at the XLR. One favored
technique for trouble shooting buzz issues is to make a short XLR cable with the shield
lifted on the Male connector and use this to test whether or not an interface warrants a
shield lift wiring permanently into its receiving end connector.
