Bose FREESPACE BUSINESS Service Manual page 12

This is achieved with the use of a .12 Ω resistor (R313 for right channel) in series with each
amplifier output. Under normal conditions, peak current to the 8 Ω speakers should never
exceed 3.5 amperes. If current flow exceeds 4.5 amperes, then Q309 will conduct and trigger
monostable multivibrator circuit U401 (pins 1,2 and 3). Pin 1 will go high (for about three
seconds) and will trigger mute transistors Q409 and Q408 through diode D406. Turn-on of
Q408 causes all three amplifiers to mute. Referring to the bass power amplifier, actual muting
occurs by turning on transistors Q331 and Q340, which cause both driver transistors (Q332 and
Q341) to turn off. When the driver transistors are off ,there can be no current flow through the
output transistors.
11. DC Offset Protection
If any one of the three amplifier channels should fail, it is likely that a large DC voltage will occur
at the output of that channel. If this occurs, the DC offset detect circuit will cause the
FreeSpace™ main power supply to shut down. It will remain shut-down until the unit is
disconnected from the power source. This provides a safe indication to the customer that the
unit requires service.
Detailed operation is as follows: Outputs of each amplifier are summed by resistors R407,R408
and R409, and low pass filtered by capacitor C401. For a sustained DC offset ,capacitor C401
will charge either negative or positive. A positive offset will trigger transistor Q404. A negative
offset will trigger transistor Q405. This in turn causes transistor Q403 to conduct, and latches
Q404 permanently on. Transistor Q402 then shorts the base of Q401 to ground. Q401 is the
transistor that controls turn-on of relay K1 that is used to energize power transformer T2.
Therefore, when Q401 is disabled, main power is shut down.
12. Compressor Detect
The output of each amplifier is half-wave detected through diodes D409,D410 and D411
respectively. This voltage is divided down by resistors R422 and R423, and is presented at the
emitter of transistor Q410. Power supply voltage is similarly divided down by R425 and R426,
and is presented at the base of Q410. Diodes D419 and D420 simulate the saturation voltage
drop that occurs in the power amp stage. Therefore, when audio output of any amplifier comes
within two volts of the instantaneous power supply voltage, Q410 will conduct, which signals
the compressor (limiter) to act. Current from Q410 charges hold capacitor C405 and release
capacitor C422. Voltage at C405 is buffered by transistor U205 (pins 1,2 and 3) and decreased
down to a logarithmic voltage by resistor R438 and transistor U205 (pins 12,13 and 14). This
voltage is then presented to the base of compressor transistors U205 (pins 5,6 and 7) and U205
(pins 8,9 and 10) which are described in the compressor section.
The DC control voltage present across capacitor C405 is derived through the use of an
improved hold and release characteristic. This results in lower compressor distortion. In
conventional limiters this DC control voltage is derived by a simple series diode and parallel RC
network to ground. Unfortunately, for low frequency overload, this results in substantial ripple
voltage present on the DC gain control voltage. This results in the all too common problem of
mid-range voices or instruments being badly "modulated" by low frequency overload. This
problem is solved by having two separate capacitors, one that "holds" the smooth DC voltage
just long enough to prevent low frequency ripple (approximately 40ms) and the other (C422)
that quickly releases the hold cap (through diode D412) at the proper time to maintain overall
100ms release time.
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