Allen&Heath GL3000 - Manual (Page 5)

GL3000 U SER G UIDE ISS .1 5
EARTHING THE AUDIO SYSTEM
The console chassis is connected to mains earth via the DC power cable. Audio 0V is connected to the
chassis and hence mains earth internally. In this way all signal returns and connector shields are connected
to mains earth at the console.
To ensure operator safety do not remove the earth connection
from the power supply mains plug.
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For best performance it is important that the earth system is solid, clean and noise-free. To prevent mains
born and external interference pickup from lighting equipment, motors and other mains powered equipment
it is recommended that a separate 'clean' mains distribution outlet is used for the audio system.
All signal cables should have their screens connected to earth at the connector. If earth (ground) loop
problems result in audible hum or buzz then disconnect the cable screen connection at one end, usually the
destination end. This may be done within the cable plug or by operating the 'ground lift' switch if available
on the connected equipment. Many DI boxes and power amplifiers include this facility.
PLUGGING UP THE CABLES
Where possible use balanced connections to prevent noise and interference pickup especially on long
microphone cable runs. Avoid running audio cables next to mains, computer or lighting cables, or near
thyristor dimmer units, power supplies etc. The use of low impedance sources such as good quality
microphones of 200 ohms or less significantly reduces interference pickup. Many problems can be avoided
by taking time to check that all cables are correctly wired (in-phase) with professional quality cable and
carefully soldered joints. The following wiring convention is used:
XLR pin 1 = 0V earth shield 1/4" jack tip = hot / signal / left / send
pin 2 = hot (in phase signal) ring = cold / 0V / right / return
pin 3 = cold (out of phase signal) sleeve = 0V earth shield
To connect an unbalanced source to a balanced console input, link the cold input (XLR pin 3 or jack ring)
to 0V earth (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.
Switch off channel +48V when the inputs are connected to non-
phantom powered, line or unbalanced sources.
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ADJUSTING THE LEVELS
For best performance it is important that the connected source signals are matched to the "normal operating
level" of the console. Similarly the console outputs should be correctly matched to the operating levels of
the connected amplifiers and destination equipment. If too high the signal peaks will be clipped resulting
in a harsh distorted sound, and if too low the signal-to-noise ratio is reduced resulting in excessive
background hiss.
For best results operate the console with the meters averaging '0' or just below and allowing the loudest
passages and occassional peaks into the 'yellow'. Reduce the gain if the peak indicators flash (red). The
GL3000 produces a standard XLR output level of +4dBu for a meter reading of 0VU. It is advisable to
adjust the power amplifier input gain or fit an attenuator pad if normal console operation results in an output
level too high for the connected amplifier. Normal operation should result in fader levels around the '0' mark.
The GL3000 has an advanced PFL (pre-fade listen) / AFL (after-fader listen) and channel metering
system to let you listen to and check the level of signals at different points in the signal path without affecting
the main outputs. Use the channel PFL switches to set up the input GAIN controls to read an average
'0' (yellow LED). Signal activity is always shown on the channel meters regardless of fader position. The
green 'SIG' LED lights at -20dBu to indicate signal presence, the green '0' LED indicates normal level, and
the red 'PEAK' LED warns of potential overload 5dB before clipping.