ALLEN
&
HEATH
48 AP10733_1_XONE:PX5 User Guide
The connection to earth (ground) in an audio system is important for
two reasons:
SAFETY - To protect the operator from high voltage electric shock,
and
AUDIO PERFORMANCE - To minimise the effect of earth (ground)
loops which result in audible hum and buzz, and to shield the audio
signals from interference.
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. It is
recommended that a qualified system engineer check the continuity of
the safety earth from all points in the system including microphone
bodies, turntable chassis, equipment cases, and so on.
The same 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’ (ground
loop) results causing current to flow between the different earth paths.
This condition is usually detected as a mains frequency audible hum or
buzz.
To ensure safe and trouble-free operation we recommend the following:
Have your mains system checked by a qualified electrician. If
the supply earthing is solid to start with you are less likely to experience
problems.
Do not remove the earth connection from the console mains
plug. The console chassis is connected to mains earth through the
power cable to ensure your safety. Audio 0V is connected to the
console chassis internally. If problems are encountered with earth loops
operate the audio ‘ground lift’ switches on connected equipment
accordingly, or disconnect the cable screens at one end, usually at the
destination.
Make sure that turntables are correctly earthed. A chassis earth
terminal is provided on the console rear panel to connect to turntable
earth straps.
Use low impedance sources such as microphones and line level
equipment rated at 200 ohms or less to reduce susceptibility to
interference. The console outputs are designed to operate at very low
impedance to minimise interference problems.
Use balanced connections for microphones and mix output 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 console input, link the cold input (XLR pin 3 or jack ring) to 0V
earth (XLR pin 1 or jack sleeve) at the console. To connect a balanced
XLR output to unbalanced equipment, link the cold output to 0V earth
at the console.
Use good quality cables and connectors and check for correct
wiring and reliable solder joints. Allow sufficient cable loop to prevent
damage through stretching.
If you are not sure ...
Contact your service agent or local Allen & Heath dealer for advice.
EARTHING
