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Figure 12-Power supply snippet
M1 is an N-channel MOSFET that turns on to energize the coils of the speaker relays
found on the amplifier PCB. M1 turns on when its gate to source voltage, which is also
the voltage across C1, gets a bit above M1’s threshold voltage, about 2.5 volts. R1
charges C1 rather slowly, over the course of a few seconds, to make this happen. This
slow charging provides the time delay that allows the amplifier modules to stabilize
before they relay energizes. This is the action that prevents pops and clicks on turn-on
and turn-off.
D1 makes the on-off timing of the time delay repeatable and largely independent of quick
on/offs of the power supply. Without D1, the voltage on C1 would eventually rise to 10
volts. With D1, the voltage on C1 only rises to about 0.6, no matter how long
the amplifier is on. That makes the time to reset the capacitor independent of how long
the amplifier was turned on.
The circuits described in the next section only allow the voltage on C1 to rise when both
DC voltage is good and AC voltage is present from the transformer.
