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POWER ON/OFF CIRCUIT
The power on/off circuit is designed to turn the amplifier on slowly
and off quickly. When AC power is initially applied to the subwoofer,
AC signals on the secondary of the power transformer are negatively
rectified through CR2 and CR7 into peak holding capacitor C31, and
divided by R19 and R20. When the resulting DC voltage at pin 2 of
U15A is lower than the -1.4v reference at pin 3, the output of U15
goes high, allowing the soft start circuitry to slowly turn on the
amplifier. When AC power is removed from the subwoofer, pin 2
very quickly goes toward 0V and flips U15A low to -15V, immediate-
ly turning the amplifier off before the power supplies collapse.
SIGNAL SENSING TURN-ON CIRCUIT
The amplifier will not turn on until an input signal is detected. The
first stage of the detection circuit amplifies the WOOFER signal by
40dB, to ensure that even the smallest input signal is detected, and
high pass filters it to eliminate DC offset errors. This amplified signal
is negatively rectified by CR9 and held by C85. When the voltage at
pin 6 of U16B goes lower than the -0.15V reference at pin 5, the
output goes high, allowing the soft start circuitry to turn the amplifier
on. After signal is removed, the 470-second time constant of C85
and R145 will discharge slowly towards 0V until it is higher than
-0.15V, causing U16B to go low to -15V, turning the amplifier off.
THERMAL PROTECTION
The thermal protection turns off the amplifier when the heatsink
reaches an excessively high temperature. This condition is detected
by the NTC (Negative Temperature Coefficient) thermistor TS2. At
higher temperatures, TS2 has a lower resistance causing a higher
voltage at pin 6 of U15B. When this voltage exceeds the reference
voltage at pin 5, the output of U15B goes low to -15V and turns the
amplifier off.