Allen&Heath DR66 USER GUIDE - Manual (Page 16)

16 DR66 & DR128 Installer User Guide
Earthing
The connection to mains earth (ground) in an audio system is important for two reasons:
Safety - To protect the operator from a high voltage shock associated with the AC mains supply
feeding the system.
Audio Performance - To shield the audio signals from interference pickup and to minimise the
effects of ground loops which result in audible hum and buzz.
Earth is also used to shield audio cables from external interference such as the hum fields
associated with power transformers, lighting dimmer buzz, and computer radiation. Problems arise
when the signal sees more than one path to mains earth. An earth loop results causing current to
flow between the different earth paths. This condition is usually detected as mains frequency
audible hum or buzz. To ensure safe and trouble-free operation we recommend the following:
Do not remove the earth connection from the DR mains plug. The chassis of the unit is
connected to mains earth through the power cable to ensure your safety. Audio 0V is connected to
the chassis internally. If problems are encountered with earth loops operate the ‘ground lift’
switches on connected equipment accordingly, or disconnect the screens from interconnecting
audio cables at one end, usually at the destination.
Use a clean mains supply for the audio system. Ensure the audio mains supply is run
separately from lighting, motor, kitchen equipment and any other potentially noisy mains supplies.
Use a mains filter and surge protector for the audio supply if necessary.
Connections and Cables
Use professional quality cables and connectors and check for accurate wiring and reliable
solder joints.
Avoid running audio cables alongside mains, computer or lighting cables or near thyristor
dimmer and power supply units. If unavoidable, cross the cables at right angles to reduce the
interference pickup.
Use low impedance sources such as 600 Ω or less microphones to reduce susceptibility to
interference. The outputs of the DR are designed to operate at very low impedance to minimise
interference problems.
Use balanced connections where possible as these provide further immunity by cancelling out
interference that may be picked up on long cable runs. To connect an unbalanced source to a
balanced Mic/Line input link the cold input (XLR pin3) to 0V earth (XLR pin1) at the unit. To
connect a balanced Line output to an unbalanced destination, link the cold output to 0V earth at the
DR unit.
WARNING: For safety it is important that all equipment earths are
connected to mains earth so that exposed metal parts are prevented from carrying
high voltage which can injure or even kill the operator. The installation engineer
should check the continuity of the safety earth from all points in the system including
microphone bodies, cables, racks and equipment panels.