JBL LSR28P, Техническое и сервисное руководство

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LOW FREQUENCY OUTPUT AMPLIFIER WITH MUTED POWER UP
AND THERMAL SHUTDOWN
The low frequency amplifier uses discrete components configured in a push-pull drive
architecture operating in the class AB region and have a measurable gain of 15.25 dB.
The voltage rails for the power amplifier are positive and negative 42 volts as rectified by
bridge BR1 located in the Power Supply Module. The output transistors are driven by the
preceding stages and thermally biased by Q5.On initial power up charge builds up at C27 and
turns Q17 off after a short pause of about 4 seconds. The comparator U1B then toggles taking its
output high and biasing on the constant current source at Q19 and hence enabling the low
frequency power amplifier. On power down C27 looses charge allowing Q17 to turn on and
discharge C29 such that comparator U1B then toggles taking its output low and holding off the
constant current source at Q19 and hence disabling the low frequency power amplifier to prevent
any noise occurring.
Excessive operating temperatures cause thermal switch (SW’) to short activating buffer
amplifier to mute the input differential amplifier and, thus, shutting down this module.
. .95 Vpp @ 100 Hz
Output =14 Vpp @ J1 ( Red Wire)