Theory Of Operation - Bose FreeSpace 4400 Service Manual

THEORY OF OPERATION

Power Supply / Amplifier PCB Overview
CAUTION: There are dangerous voltages present on most of the power supply and amplifier
circuitry. Under normal conditions, it can take 5 to 10 minutes after power is removed for these
voltages to discharge to a safe level. Please use extreme caution and resist the temptation to
probe during this period.
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The FreeSpace 4400 Amplifier is a complex system that employs high-voltage direct-coupled
Class-D switching power amplifiers, four separate microcontrollers (uCs), and numerous DSP,
interfaces, memory, logic, analog and communication devices. The accompanying software
includes error flagging and logging features that will usually help you isolate the cause of any
failures.
Power Supply
The power supply section consists of four sections. First is the main high-voltage DC supply
formed by the toroidal 50/60 Hz transformer, bridge rectifier (BR1) and filter capacitors (C1-C4).
This is a conventional unregulated split supply. Switch SW1 is used to change transformer
secondary taps. The raw DC output is about + 130VDC when SW1 is set for 70 volt output, and
+ 170VDC when SW1 is set for 100 volt output. At full power output and nominal line voltage,
these sag to about + 110V and + 150V respectively. Fuses F1-F4 provide thermal protection in
the event of a catastrophic power amplifier failure.
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The next power supply section, formed around TOPswitch regulator U2 and transformer TR2,
is referred to as the "Standby Supply" because it is operational whenever power is applied to the
unit and the rear panel power switch is on. Its purpose is to supply approximately 9 volts (locally
regulated to 5 volts) to several microprocessors and other circuitry. These microprocessors
respond to turn-on stimuli, i.e. power-up via the 4400's real-time clock, computer control or user
intervention via standby switch or wall plate.
The TOPswitch IC performs all the functions of a complete 130 kHz SMPS flyback-mode con-
troller and switch, i.e. slow-start, current limit, overvoltage lockout, fswitch dither, low current
skip-cycle, etc.
The other, large TOPswitch section formed around U1 and TR1 response to "wakeup" com-
mands from the main host processor, which operates from the standby supply. It provides + 22
volts (locally regulated to + 15V) and +10 volts (locally regulated to + 5V and + 3.3V) which
powers multiple analog, DSP, communications and indicator sections throughout the unit. The
fourth output from U1 and TR1 (labeled -160) is more accurately described as "20 volts above
the minus rail". This voltage is locally regulated to 12 volts, and provides the gate-drive current
for the main output FETs.
Also included on the power supply schematic sheet are two other ancillary circuits. U3A is a
resettable one-shot whose output is normally high with AC power applied. If there is an AC line
dropout exceeding approximately 2 cycles, a logic 0 from U3A is used to immediately shut down
the amplifiers and DSP in time to save all current configuration data and prevent any spurious
thumps etc. from appearing at the speaker outputs.
Q1 and Q2 form a simple buffer that switches the cooling fan from low to high speed if either of
the Fan-U or Fan-L signals are at logic 0. This occurs when the power amplifier thermal sensors
report a heatsink temperature of over 160oF.
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