Bose ACOUSTIMASS 3 Service Manual page 12

12. DC Offset Protection
If any one of the three amplifier channels should fail, it is likely that a large DC voltage will occur
at the output of that channel. If this occurs, the DC offset detect circuit will cause the AM-3P
amplifier power supply to shut down. It will remain shut-down until the unit is unplugged from
the power source. This provides a safe indication to the customer that the unit requires service.
Detailed operation is as follows: The amplifier outputs are summed by resistors R149,249,318,
and 418, and (low pass) filtered by capacitor C29. For a sustained DC offset, C29 will charge
either negative or positive. A positive offset will trigger Q7 and a negative offset will trigger Q8.
This in turn causes Q6 to conduct and latches Q5 permanently on. Q5 then shorts the emitter
of Q4 to ground (off). Q4 is the transistor that controls turn-on of 2P1T relay K1 (on the Amp
PCB) that is used to connect +/- 20V supply rails to the power amplifiers. Thus, the amplifiers
are de-energized.
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