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iDR-8 User Guide 15
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 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 any iDR unit 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 use ‘ground lift’ switches or adaptors
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. Take care to ensure good earthing in
distributed systems such as those with audio units linked over long distances.
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 or run them in separate
screened trunking to reduce the interference pickup.
Use low impedance sources such as 600Ω or less microphones to reduce susceptibility to
interference. The outputs of the iDR 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 iDR unit.
Backup Power Supplies and Safety Standards
The iDR system is designed to conform to internationally accepted safety standards IEC60065 and
UL6500. It uses a built-in mains voltage power unit to supply the various internal power rails. It is
recommended that the iDR system be backed up using a UPS (uninterruptible power supply) in
installations where public safety is at risk. This is a requirement when an alarm or voice evacuation
system is integrated.
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.
Check which safety standards apply to your installation. A backup UPS
supply may be required. The standards may also apply to the grade of cable and
installation methods required.