The QSC/USL JSD-60, 80, and 100 cinema audio processors have a built-in emergency backup digital to analog decoder.  This is intended for use if there is a catastrophic failure of the JSD unit.  It gives a "down and dirty" sound output sending a mixed mono signal to the left and right amplifier outputs.  This bypass mode is activated when the main power switch is turned OFF.  This bypass is not powered by the main power supply but rather by an external 12VDC 1.25A AC adapter that was supplied with the unit.

We have observed several instances where either an incorrect AC adapter is used or the supplied AC adapter is failing.  This may manifest itself as a loud "AC" hum over the sound.  This can be diagnosed by unplugging the AC adapter on the back of the unit. If the hum goes away the cause of the problem is the AC adapter.  If the problem persists other possible causes must be considered.

If the AC adapter fails it is a relatively easy unit to source.  It is important, however, to use a good grade unit as lesser units may have a rough, poorly rectified output that would cause hum or noise in the system.