Allen&Heath GR1 SERVICE MANUAL - Manual (Page 12)

GR1
USER GUIDE 9
EARTHING THE AUDIO SYSTEM
The chassis is connected to mains earth via the power cord. FOR SAFETY REASONS NEVER REMOVE THE
EARTH WIRE FROM THE MAINS PLUG.
Audio 0V is connected to mains earth
by the internal ground lift link op-
tion. Selecting ground lift discon-
nects 0V from the chassis (mains)
earth. This avoids ground loops in
situations where the chassis metal-
work is in physical contact with an-
other path to earth, (often the case in
19" rack installations), or if audio 0V
connects to mains earth elsewhere in
the system.
Multiple earth paths cause
earth (ground) loops which
may result in audible hum
and interference. These may
be avoided by making sure
that there is only one path to
earth from each piece of
equipment. An internal link
option is available for each
XLR output to disconnect
0V from the pin 1 cable
screen if necessary.
Shown ground linked
Shown ground lifted
MIC INPUTS
6 electronically balanced XLR inputs each with a high
performance pre-amplifier and option links:
GAIN CALIBRATION TRIMMER
20-turn preset trimmer accessable with a trim tool or small
screwdriver through the rear panel. Matches a wide range
of microphone levels to the operating level of the unit.
Adjust with the channel level control fully clockwise for
an output meter reading averaging '0dB'. Reduce the trim
level if the channel PEAK indicators flash.
OPTION LINK - +48V PHANTOM POWER DISABLE
Remove link (set on one pin) to disable the internal +48V
phantom power supply from the XLR. Always disable
+48V when connecting to non-phantom powered micro-
phone or line level sources. The rear panel +48V switch
turns on phantom power to all enabled XLR inputs when
pressed.
OPTION LINK - 20dB PAD
Remove 2 links (set on one pin) to enable the 20dB
attenuator pad when connecting to high output micro-
phones or mono line level sources.
OPTION LINK - LO-CUT FILTER
Remove link (set on one pin) to enable the lo-cut filter to
reduce low frequency interference such as microphone
proximity noise, rumble and hum. This reduces frequen-
cies below 70Hz.