User guide

C475 User Guide Version 1.0 9
PRC Indicators (green LED)
These indicate when the PRC circuit, for the specific pair of channels, has been selected. (Either
manually via the switches on the rear panel or automatically via the impedance sensing circuit)
Temperature Control
The cooling fans respond to temperature sensors within the unit to maintain a safe operating
temperature. In the event of excessive temperature, the protection circuit will operate, disabling the
output. The red ‘AUDIO-PROTECT’ (A/P) LED will indicate this condition. (See fault indicator).
There are 4 fans connected permanently with variable speed and a jumper link to enable them from
cold.
Normal dynamic signals will not cause the amplifier to overheat unless the ventilation is
inadequate. (See installation section and maintenance section.)
Fault Indicator (Audio Protection – red LED)
If the outputs are shorted or if DC is present, the protection circuit will disengage the outputs and the
A/P LED will illuminate. The amplifier will continue to be monitored and depending on the type of
fault, will either reset after the fault has cleared or require manual resetting by switching off at the
mains switch and then on again after a few seconds. (See also temperature control above.)
Maintenance
Caution: These servicing instructions are for use by qualified personnel only. Ensure that electrical
power to the unit is disconnected before carrying out any maintenance.
Gain/Sensitivity Settings
Gain settings are changed internally by simple jumper links. Four rows of pins marked: - GAIN A,
GAIN B, GAIN C and GAIN D - are situated on the input PCB (PCB721). A jumper link sets the gain
and the settings are as follows:
Link 1 & 2 gives 32 dB gain
Link 3 & 4 gives 26 dB gain
Link 2 & 3 gives approx. 31 dB gain = 6 dBu/1.5 V sensitivity into 4 Ohm
Note: Factory setting is normally link 1 & 2 = 32dB gain.
Setting higher gain does not change the maximum available power but changes the level of
signal input to achieve maximum power. In any case, provided that the input signal is less
than 20 dBu/7.7 V, the built in limiter circuit will prevent distortion within the amplifier.
The gain should be set to match the signal from the source, e.g. mixer, controller,
equaliser etc.