Bose BUILT-INvisible TA-1 Service Manual page 7

THEORY OF OPERATION
Input/Output (I/O) PCB (continued)
Power supplies, +8V and +5V (continued)
Until Q6 is turned on, the rest of the amplifier (the Power Amplifier and DSP PCBs) is off.
The AC mains is applied to T5 [C1], rectified by D22 (full wave bridge), and filtered by C31.
The +5V supply is created by U12 [C1] in a standard 3 terminal regulator application; so too is
+8V, created by U14. These supplies only deliver very low power outputs, capable of no more
than two to three watts total.
Power supply, main transformer control
The amplifier's main power supply is created from the secondaries of a large toroidal transformer
that is turned on and off by the triac Q6. This is the transformer mounted in the center of the I/O
PCB. The secondary leads of this transformer connect via J702 to the Power Amplifier PCB,
where various power supply voltages are created, including the +/-12V supplies returned to the
I/O PCB to power the op amps and signal selectors. The +/-12V leave the amplifier PCB at J701
pins 13 and 15 and enter the I/O PCB at J22 pins 13 and 15.
A logic signal (AMP_POWER) from the microcontroller U5 [C4] at pin 11, goes high to turn on
the main power supply. AMP_POWER high causes base current in Q7 [C1], which in turn
causes the LED in Q5 to turn on. The opto-coupled LED turns on the internal triac in Q5 which,
finally, turns on the gate of triac Q6 and energizes the transformer.
Pulse Width Modulated tone control low pass filters, U15
The I/O PCB interprets user commands to change the bass and/or treble levels by delivering an
analog voltage between 0V (cut), +2.5V (flat), and +5V (boost) to the DSP PCB. The micro-
controller U5 develops this analog voltage by controlling two of its output pins in pulse width
modulation mode. The inherent switching activity in PWM outputs requires a low pass filter, in
nd
this case a 2 order filter consisting of passive components and the dual op amp U15 [C2].
Clamp diodes D36 and D38 on the op amp outputs keep the filter output within bounds for its
destination, the DSP microcontroller U202 on the DSP PCB. The signals TREBLE and BASS
are passed to the DSP microcontroller U202 via J4-1 and J4-2 [E4].
TV Sensor input
J11 [A1], a three pin connector, provides ground and +8V (via resetable fuse F2 [A1]) for use by
a small TV sensor that must be attached to the TV monitor in the home theater. The TV sensor
circuitry returns a DC signal via J11-1 which, when high, indicates that the TV set is on. This will
cause the amplifier to turn on (if it is not already on). The TV sensor input is monitored by
microcontroller U5 [C4] at pin 8.
IR input
J10 [A1] acts as the gateway for incoming demodulated IR signals used by remote controls.
These signals are ultimately monitored by the microcontroller U5 [C4] and are sent to the DSP
PCB at J4 [E4]. One form of IR delivery is the electrical signal (logic level) from an externally
mounted IR receiver chip. This would typically be located in the home theater room, where it
would pick up IR radiation from a user remote control, decode it, and send it by wire to J10-3.
This signal is clamped and sent to AND gate U16 [B1], which acts as an OR selector for low
levels coming from this IR input and from the internal IR sensor. The second form of IR delivery
is via an electrical signal sent to J10-1 and –2. When this signal is asserted, the internal IR
emitter D3 [A1] is turned on and emits IR within the amplifier. This optically couples to IR sensor
U11 [A1] which toggles low and high, decoding the IR signal. This signal passes to U16's second
input, where it is OR'd with the IR source discussed above. Finally, the IR signal is sent to the
microcontroller U5 [C4] as the signal IR_IN on U5 pin 7.
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