Bose PACKLITE A1 Manual page 49

THEORY OF OPERATION
Fan Control
The fan turns on when the signal level is high, rather than using a thermal detector. When the
input signal reaches about -10 dBV, the Q305/Q310 [B2] combination will turn on the fan.
LED Control
The green signal LED will turn on at an input level of about -26 dBV. The red LED will turn on at
about +1 dBV, when the pre-amplifier will begin to limit. U306A [B2] and U306B [A3] detect the
on-levels for the green and red LEDs, respectively. Q309 [B3] turns off the green led when the
red turns on.
The blue power LED will turn on whenever power (+18VDC) is present, and will change to red if
the OC (overcurrent) or TH (thermal) signals from the amplifier IC are present. Those signals
are OR'ed through D305 and D306 [B5]. Q308 turns on if either fault condition is present, and
turns on the red LED. Q307 [B4] is used to shut off Q306 [B4] and the green LED.
Under-voltage detection and ENABLE control
The ENABLE circuitry provides an output that will disable the amplifier under two conditions:
1. On power up for a couple seconds
2. If the +/- 18VDC supply goes below 16 VDC.
The purpose of disabling the amplifier on power up is to provide a stabilization time to avoid
large output signals on the speaker output. The purpose of monitoring the +/-18VDC level is to
provide under-voltage protection for the amplifier. Because the power converter on the amp
board is not regulated, nor is the +/-18 VDC supply, which is derived from the same transformer
primary, the 18V supply will track the other voltages in the set. Most importantly, the amp rails
at +/-40V are the highest current consumers and the most likely to cause a voltage sag. If the
load is too heavy on the amplifier, causing the rail voltage to sag, the 18 V will also sag propor-
tionally, and the amp is shut down to protect itself. This can happen at low but still operable AC
mains voltages with very high output power demands, or with actual under-voltage conditions in
an AC mains power brownout or dropout.
D310 [B7] in series with R369 and R370 [B6] provide a voltage to compare to the 0.7V NPN
turn-on voltage. As long as the +18 V supply is above 16 V, Q313 [B7] stays on, keeping Q312
off, and EN is pulled high by R366 (and limited to 5V by zener D307). If Q313 is turned off, then
Q312 turns on, bring EN low. At that point, as well as during power up, even when the +18V is
valid, there will be a few second delay before enabling as C368 charges through R366.
On power down, ENABLE is brought low relatively quickly by Q312 turning on.
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