Bose 3-2-1 Troubleshooting Manual page 22

THEORY OF OPERATION
2.2.2.1 Power rectification, pre-regulation and filtering
J5 [sheet 4, D6] connects the secondary winding of the power transformer to the circuit rectifiers
D7-10 [C/D 5/6], which form a full-bridge rectifier. The DC output of the rectifier charges capaci-
tor C22 [D2] to a controlled peak voltage, via FET Q4 [D4]. The controlled peak voltage is
15.75V (± 0.75V). The pre-regulator circuit formed by ZR1 [D5], Q2-3 [C4/5], R7-13, and C21,
24-5 cause Q1 to switch off once the limit voltage has been attained.
The diodes D7-10 and Q1 are required to have good thermal contact to the heat sink. This
requires a deflection of 0.5 to 1mm of the thermal material applied in the interface. Q5 and
associated components provide a soft-switching function
2.2.2.2 Control Power generation
Both a 5 volt and a 3.3 volt power rails are generated for the DSP and audio source control
circuits.
U100 [sheet 4, B5], L70 [B4], D11, and C30, C31 form a buck switching regulator for the 5 volt
supply. The nominal switching frequency is established by components R16 and C33 [B6]. A
modulating signal derived from the transformer secondary is coupled to the oscillator circuit via
C37. R18 and R19 establish the magnitude of the modulating signal. C38 filters unwanted
harmonics and line noise from the signal. Q6 [A6] allows the Inhibit pin to be pulled low from
the console, which disables the power supply, forming the DSP_SHUTDOWN function.
U101 [A3] generates the 3.3 volt supply from the 5 volt supply. C42 prevents oscillation of the
output of U502 as well as assisting to manage voltage ripple due to load fluctuations. U6000
[sheet 1, D5] monitors the 3.3V and issues a reset of the DSP if this supply ever drops below a
regulation threshold of 3.08V.
2.2.2.3 Audio Power amplification
Dual Audio Amplifier IC's, U150 [sheet 5, C/D3] and U250 [A/B3] provide the power amplification
for the external speaker arrays. The array outputs are available on J150 [B/C1]. A similar
amplifier, U350 [B/C6], provides power amplification to the bass module woofer via connector
J350 [C4]. Both inputs of U350 are wired in parallel and both amp outputs are provided to the
woofer connector J350. Each output of the amplifiers have an RF de-coupling capacitor of
.01uF, C152 and similar, shunting the high-frequency components of the signal through a series
resistor of 3.32 Ohms, R162 and similar. The series resistor creates a lead in the pass charac-
teristic at approximately 4.7MHz in order to maintain stability of the power amplifier.
The differential inputs of the power amplifiers are coupled to the output of the Audio DAC via
1uF aluminum electrolytic capacitors, C156, C157 and similar. A shunt resistor and capacitor,
R151 and C156 and similar, form the termination of a second-order low-pass filter described
later.
2.2.2.4 Power Amplifier control, monitoring
The MODE signal is a three-level signal generated by Q450 [sheet 5, A8], Q451 [B8], Q452
[B8], D450, R450, and R451. IF the /STDBYout signal is asserted low, the internal bias of the
MODE pins of the amplifiers will cause the signal to drive to ground. This causes the amplifiers
to enter the "Stand-by" low-power state. The internal bias to the amplifier signal inputs is turned
off causing the inputs to drift to ground. Stand-by should not be entered while the DAC driving
the signal inputs is out of the reset state.
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