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Fan Speed Regulation
Driver
U9A
Trip Switch
U9B
Fan Drive
Amp
Q20, Q19
Temp
TS1, R25
Cooling air for each channel is provided by a DC fan. The fan is configured to track the heatsink temperature, and
increases in speed as the amplifier runs hotter. The heatsink temperature, Temp, is determined by the voltage divider
TS1 and R25. When the amplifier is turned on the fan will stay off until the Temp voltage exceeds the reference voltage
on pin 6 of the Trip Switch U9B and drives the output high to turn on the Fan Drive Amplifier, Q20 and Q19. The Driver
U9A is configured as a linear amplifier and controls the output of the Fan Drive Amp to vary the fan speed.
Output Short Protection
Shut Down
Switch
Q5, Q4
Comparator
U4A
Short
Indicator
CR1
Comparator
U3B
Output
Signal
Drive
Signal
Short Latch
U4C, CR14
Clip Detector
U3A
Clip Detector
U4A
The Short detector monitors the Drive Signal and Output Signal levels and shuts down that channel when a shorted
output condition is detected. Recovering from the Short protection requires turning the amplifier off to reset it.
The Clip Detectors U3A and U4B monitor the Drive Signal and change state when the drive signal level is sufficient to
clip the output stage. The output of U3A goes high when the drive signal exceeds the clip threshold, and is connected
to the input of U3B. If the output signal voltage is not sufficiently positive during a clip event the output of U3B goes
low. The output of U4B goes low when the drive signal exceeds the clip threshold, and is connected to the input of U4A.
If the output signal voltage is not sufficiently negative during a clip event the output of U4A goes low.
This condition, indicating high drive voltage and low output voltage, can occur only when the output is grounded. If
the output of comparator stage U3B or U4A is low for the time sufficient for C26 to charge, then U4C latches low
activating the Shut Down Switch Q5 and Q4, and lighting the indicator CR1. The output of U4C is held low by CR14
and will reset only after the power is turned off.
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