Bose AM-30P Service Manual - Page 17

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The Class-G switching circuitry previously described for the surround channel amplifier is also

controlled by the left channel amplifier. Similarly, when the left audio input exceeds a certain

level, diode D475 conducts and again transistor Q175 is triggered on. Therefore, the power

supply voltage that powers both amplifiers will switch high in response to a request from either

amplifier. This is illustrated in Class-G block diagram Figure 7. With a 2:1 ratio in the power

supply rails, it can be shown that for typical music, each amplifier has to switch to the high

state only 4% of the time. Thus, even two amplifiers in parallel on the same power supply rail

does not result in a significant reduction in efficiency, yet a significant cost savings is achieved.

The amplifiers are muted by setting the voltage at pin 10 (mute pin) to less than 1.5VDC.

Unmute occurs at 3.5VDC or above. In the AM-30P application, mute voltage is typically zero

volts and unmute is 5 volts. The mute and standby lines are tied together through dual diode

D102. In standby mode the maximum quiescent current to the chip is 3mA maximum, and in

the unmute condition is 60mA maximum.

3.10 Compressor Operation (Refer to the schematic sheet 1)

All four satellite amplifier channels are protected by a compressor that momentarily reduces

the master volume level of the system if any one of the amplifier channel outputs clip. Detailed

operation is as follows:

Each of the satellite power amplifier ICs has a voltage comparator (1/4 of U3) connected

across its inputs. The outputs of the comparator are wired together to form a logic OR function.

Normally, under unclipped audio conditions, the power amplifier IC, through feedback, main-

tains the voltage across its input terminals to less than 1mV. Each comparator has its non-

inverting input biased at +120mV (from voltage divider R11/R12) and therefore the output is

high. When a particular amplifier clips at the output on the negative peaks by more than 480

mV, U3 comparator will trip low. This condition is detected by the microcontroller at pin 7, and

in response the microcontroller issues a volume down (-1dB) command to master volume

control IC U9. The microcontroller continues to sample pin 7 approximately once every milli-

second, and will issue successive volume down commands until pin 7 goes high (indicating

that clipping is no longer occurring). Release time of the compressor, or the rate at which the

microcontroller ramps the volume back up after no clipping, is set to approximately 8ms per

dB.

3.11 Protection Circuits (Refer to schematic sheet 1)

Several protection circuits tie into the microcontroller, and the status of these lines are continu-

ously monitored. The microcontroller has the ability to power off the system (including itself) in

the event of an emergency through the power-off line, pin 13. Normally this pin is high, but

when switched low, it conducts through R641 and R643 which latches transistors Q606 and

Q607 on, and current flow through opto-coupler U604 is interrupted, which in turn opens triac

Q605 and the primary winding of the transformer. Once the power-off command is given the

microcontroller locks up and the unit can not be triggered back on until a 10VDC turn-on pulse

is reissued from the CD-20. This only happens if the CD-20 is powered off and then back on,

which can be done through the remote control. The protection circuits that tie into the

microcontroller are described as follows:

THEORY OF OPERATION