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MACRO range installation manual. Issue 6
Automatic level control - all units except slave amplifiers. Mustang product code TB/ALC
The standard module fitted to a mixer or mixer-amplifier to provide line driver facilities is a TB.6 and is located second from the
right when viewed from the front. This is replaced with a TB.6A to provide the Automatic level control facility (ALC) and it will
enable the system to be set up such that a pre-settable amplitude cannot be exceeded. All signals that would normally pass
through the amplifier circuitry are subject to automatic level control operation.
The module operates to produce a fast attack so that limiting to a pre-determined level takes place almost instantly, and a slow
decay whereby the sensitivity - or gain - is allowed to increase progressively over several seconds or until another limit is
triggered.
To set the module to give the required performance: Firstly, referring to APPENDIX E, identify the GAIN pre-set control which is
adjacent to the integrated circuit at the left of the board approximately half way up. Then identify the THRESHOLD pre-set control
which is a little below it. Set the Master Level control on the mother PCB to maximum (see Fig.1 on page 7). Set the GAIN pre-
set fully anti-clockwise to give approximately unity gain. Now, whilst running the amplifier at full volume using a test signal, adjust
the THRESHOLD to the desired maximum output level indicated by the output meter. Clockwise adjustment increases the
threshold level. The threshold is adjustable from -24 dBM to +8dBM.
The module will now be operating as a limiter. To enable the Automatic Level Control aspect, the GAIN control should now be
adjusted clockwise until sufficient gain is available to enable the lowest level input signal to attain the THRESHOLD.
The ALC facility will have been specified where -
a) The amplifier output is required to be limited to prevent clipping distortion;
b) The average sound level is to be limited to a specific audible level;
c) Inductive loop systems which may be unattended though still need to operate to the requirements of the current
legislation.
As supplied by the factory, GAIN is set to 0dB (unity) and the THRESHOLD control is set to maximum, and therefore no effect will
initially be apparent. See APPENDIX D to locate the module, and APPENDIX E for identification of the adjustments.
Balanced line input for M/100 and M/250 slave amplifiers. Mustang product code MAC/LT
This option would normally be fitted where difficulty may otherwise be experienced with hum/earth loops. The input signal circuit
has no earth connection, and is referred to as being fully floating.
Fig. 11 Balanced line input connections