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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 reference voltage of -1.4 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, immediately
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 insure 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.
POWER ON / OFF
REFERENCE
CR15
R18
COMPARATOR
U15A
ON / OFF
CR5
AC POWER
RECTIFIER
CR2,CR7
R12, C31
R19, R20
SIGNAL AMPLIFIER
U16A
R57, R75
C28, C84
SIGNAL
RECTIFIER
AND HOLD
CR9, R145
C85
REFERENCE
R103, R84
COMP
U16B
ON / OFF
CR3
SIGNAL SENSING TURN-ON
THERMAL PROTECTION
TEMP
TS2, R147
COMPARATOR
U15B
R22, R23
ON / OFF
CR6