L2SM2
ALLEN & HEATH
4
TECHNICAL DESCRIPTION
The ALLEN & HEATH GL2 and GL2-S offers the flexible solution to the many requirements of the live sound and
recording environment; whether for conference sound ... in a club ... theatre .. or church ... for touring ... contract
hire ... as well as stereo and multitrack recording. Both versions offer 6 Aux sends switchable in combinations of
pre or post fader, a powerful 4-band 2-sweep equaliser, dedicated stereo channels, and comprehensive monitoring
facilities combine in a compact all metal rack mount chassis to allow total and reliable control of the mixed sound
whatever the application. The GL2 offers 4 sub groups each with 100mm faders, inserts and balanced outputs. It
has a unique switching arrangement which configures the console for Front-of-House or On-stage Monitor
operation, or a combination of both. The GL2-S offers 3 individual stereo outputs each with level control, 6 full
feature stereo inputs and 6 mono inputs.
CONSTRUCTION
All metal chassis for 19" rack mount in 11U space. Comprises a 16swg steel front panel housing individual channel
circuit assemblies interconnected by means of a plug-on IDC harness and single soldered copper wire earth busbar.
The connectors are rear (base) mounted for easy access and space saving in rack or plinth mounted applications.
Access to the internal assemblies is by removal of the 18swg folded steel base. The channel assemblies may be
removed for servicing. The internal power supply unit is a separate module which may be removed for servicing
without the need to remove the console base. Although the GL2 and GL2-S is designed for rack or plinth mounting,
the base is angled at the front and includes two rubber feet to allow the console to adjust for table-top operation using
a suitable rear support. This provides a slope of 14 degrees which allows the rear connectors to clear the mounting
surface whilst minimising the surface area used and providing a hidden or tamperproof connector system.
THE CIRCUIT COMPONENTS
GL2 and GL2-S consoles are manufactured using high performance industry standard linear op-amp, logic gate
and discrete semiconductor circuit designs. The same high quality components as found in top range consoles are
used. In particular the switches, potentiometers and 100mm faders have proven to be durable and problem free.
When operated correctly the normal performance of the console introduces no noticeable audio signal degradation.
THE POWER SUPPLY AND CONSOLE EARTHING
The GL2 and GL2-S uses a universal low noise regulated linear DC supply system for all amplifier, logic and
indicator subsystems. The internal power unit operates from single phase 50/60Hz AC mains input to provide the
three regulated DC outputs required for console operation: +16V (0.5A), -16V (0.5A), +48V (0.05A). The +/-
16V supplies are filtered into separate feeds for the audio and logic/indicator circuits on the individual circuit
assemblies. A high quality low radiation toroidal transformer is wired to suit the local mains supply voltage: 100,
110, 120, 220, or 240 V.AC. The power unit rear panel green LED indicator monitors the pre regulator +/-16V.DC
voltage while the console front panel DC ON indicator monitors post regulators. It is normal for the rear of the
console to run slightly warm as the regulator ICs are insulated and bolted to the power unit chassis which acts as
a heatsink. The console DC input wiring harness is soldered direct to the power unit circuit assembly. There is
enough harness loop to allow removal of the power unit from the console for inspection and servicing. A ground
(earth) lift switch is provided on the rear of the power unit to isolate the console audio 0V from mains earth to avoid
earth loops in situations where multiple earth paths are present, for example in 19" racks where the console is in
contact with earthed rack metalwork. The GL2 and GL2-S chassis metalwork is always connected to the mains
power lead earth connection to ensure operator protection from mains shock. DO NOT REMOVE THE
CONSOLE POWER LEAD MAINS EARTH ! The console audio performance is optimised by using an internal
distributed earthing system that separates audio and logic earths. A copper wire busbar is soldered across the circuit
assemblies to provide a solid low impedance earth path for the connector 0V returns.
AUDIO INPUTS AND OUTPUTS
The channel mic and line inputs, and main group, L-R and Mono outputs are electronically balanced (differential).
The SSM2142 differential output driver is used to balance the outputs. To use these with unbalanced equipment
the -ve signal should be linked to 0V in the cable or input connector. All console outputs are low impedance and
thus capable of driving several high impedance inputs simultaneously. All inputs and outputs except the group
inserts are in phase.
THE PFL/AFL SYSTEM
The console PFL (AFL) switches send pre-(post)-fade signal to the pfl (AFL) mix busses. This is switched within
the Phones/Monitor circuit by 4053 CMOS gates biased from a +/-7.5V DC supply generated locally from +/-16V.
These gates are switched from the selected monitor source when the PFL or AFL DC buss is switched to 0V. Note
that on GL2 only PFL overrides any selected AFL source.
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