Allen&Heath GL4000 SERVICE MANUAL - Manual (Page 7)

ALLEN & HEATH Section A - 2 L4DSM2
TECHNICAL DESCRIPTION
The ALLEN & HEATH GL4000 is an 8 bus Front-of-House and 10 bus On-stage Monitor mixer. It can also be configured
for full 8 track, stereo and mono recording. Standard formats are 24, 32, 40 and 48 channels with optional SYS-LINK expander
system and RPSD-2 dual supply combiner / monitor.
CONSTRUCTION
All metal chassis with aluminium extrusions to provide rigidity along the console length. The front panels are non modular
with 8 channel input blocks and a master panel incorporating 8 groups, left, right, mono and master sections. The
connectors are rear panel mounted with input connectors mounted on circuit boards in blocks of 8. The connectors in
the master section are mounted on a single circuit board. The chassis base can easily be removed to gain service access.
The soft touch front armrest is incorporated into the front extrusion running along the length of the console.
THE CIRCUIT COMPONENTS
The GL4000 is manufactured using high performance industry standard linear op-amp, digital and discrete semiconductor
circuit devices. In particular the switches and potentiometers have proven to be durable and problem free. When operated
correctly the normal performance of the unit introduces no noticeable audio signal degradation.
AUDIO INPUTS AND OUTPUTS
All XLR connector inputs and outputs are balanced. All unbalanced connections are line level 3-pole ¼" jack sockets with
low impedance outputs and high impedance inputs. The channel mic and line inputs, and the L-R and Mono outputs are
electronically balanced (differential). The outputs can be used with unbalanced equipment by linking the -ve signal to
0V in the cable or input connector. All outputs are low impedance and thus capable of driving several high impedance
inputs simultaneously. All inputs and outputs are in phase except for the 2 track Return inputs.
THE PFL/AFL SYSTEM
The console stereo PFL (AFL) switches send pre- (post)-fade signals to the PFL (AFL) mix buses. These signals are
switched with 4053 CMOS gates located on the MASTER circuit board (PCB No: AG2626). The supply for the 4053 is ±7.5V
DC and is derived locally from the ±16V. The gates are switched when a PFL or AFL switch is selected.
EARTHING THE AUDIO SYSTEM
The console chassis is connected to mains earth via the DC power cable. FOR SAFETY REASONS NEVER
REMOVE THE EARTH WIRE FROM THE POWER SUPPLY UNIT MAINS PLUG. The console audio 0V
is connected to the console chassis. An additional chassis 0V terminal is located on the console rear panel for extra earth
bonding if required. 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, disconnecting
audio cable screens at one end if necessary.
INTERCONNECTIONS
Where possible use balanced connections for the CHANNEL inputs, AUX SENDs, L/R and MONO outputs to minimise
noise pick-up. Avoid running audio cables near to mains or lighting cables, thyristor dimmer units or power supplies etc.
These may cause audible hum and buzz. The use of low impedance sources significantly reduces interference pick-up.
Check the cables for correct wiring to avoid problems with phase reversal and unreliable connection. The GL4000 follows
the convention for XLR pin 2 and jack tip = signal hot (+).
Always use balanced cables when connecting to phantom powered microphones.
MAKE SURE THAT THE +48V SWITCHES ARE OFF WHEN THE CHANNEL
I N P U T X L R s A R E C O N N E C T E D T O N O N - P H A N T O M P O W E R E D
MICROPHONES OR LINE SOURCES.
If ground loops cause problems, connect the cable screen at one end only. Balanced outputs may be connected to
unbalanced inputs and vice versa by linking the signal cold (-) to 0V ground.