A quick bit of reading (including this page) appears to indicate that you have to:... A removeable plug at the 12V trigger inputRear panel view of model 7/2100A (comes with the amplifier) shall be unplugged and wired with the 12V in the correct polarity.
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Setting the trigger mode switch to 12V allows thepower-on-standby function to be controlled by a12V DC voltage connected to the 12V input connector.