ページを印刷する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
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