ALLEN & HEATH ML4000, Service Manual

Page: 13 / 91

ML4000 Service Manual
13
Earthing
The connection to earth (ground) in an audio system is important for two reasons:
1.
SAFETY
- To protect the operator from high voltage shock, and
2.
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 the engineer check the continuity of the safety
earth from all points in the system including microphone bodies, guitar strings,
connector cases, equipment panels 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 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:
Use a clean mains outlet for the audio system
. Keep the audio
equipment mains feed separate from that powering ‘dirty’ equipment such as air
conditioning and lighting systems, motors and vending machines.
Use star point earthing
. It is best to install a ‘star point’ system where the
individual earths to the equipment racks and equipment areas are separately run from a
solid central reference earth point.
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’ switch on the power supply
or connected equipment, or disconnect the cable screens at one end, usually at the
destination.
Avoid induced interference .
To prevent interference pickup keep audio cables
away from mains power units, cables and distribution boards, motors, lighting and
computer cables and equipment, and any other heavy duty electrical equipment. Where
this cannot be avoided cross the audio and ‘dirty’ equipment cables at right angles to
minimise interference.
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 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 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 console output to an unbalanced destination, 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.