Bose BUILT-INvisible TA-1 Service Manual page 17

Detailed Theory of Operation (see schematic diagram SD256431)
Digital Signal Processor (DSP) Board, Sheet 1 of 4 (continued)
Analog output buffers consists of U103, U104 [B-D2], and associated components. Gain is set
by the ratio of R132 to R131 [D2]; to about 6x. Note that the ratio of R189 to R188 [D2] must
match. Gain of the output buffers = (codec_out - codec_reference) - amp_ground_sense).
Sheet 2 of 4 (microcontroller and associated components)
The bass and treble control signal enters the DSP board at J9 pins 1 and 2 [D8]. This signal is
generated on the Input/Output board and connected to a pair of microcontroller ADC inputs at
U202 [C5] pins 19 and 20. All of these connections are EMI filtered.
Thermistor R207 [C8] is mounted on the topside of the DSP PCB. When the DSP PCB tempera-
ture rises to about 70 degrees C, the microcontroller starts turning down the output volume by
programming the codec, effectively controlling the maximum temperature inside the amplifier
enclosure and preventing damage.
R211 [D7] connects the signal "CMOUT" to one of the microcontroller's ADC ports. CMOUT is
the codec's 2.3V reference voltage. This voltage is monitored as one of the codec's "vital signs",
if out-of-bounds, the microcontroller reboots the codec.
R208 [C8] connects the 3.3V power supply to one of the microcontroller's ADC ports. The
microcontroller has the ability to turn the 3.3V regulator on and off, thus having the ability to
check that the 3.3V supply is within limits. The microcontroller needs to be able to turn the 3.3V
off at power down to avoid draining the +20V supply faster than the -20V supply. If the upper rails
sag faster than the lower rails, the bass amp will unmute causing a "thump" to be heard (about 5
seconds after power down).
R44 throught R48 [I/O schematic sheet, E2] sum together all the satellite outputs and feeds them
out through J4-5 [I/O sheet, E4] to DSP J9-5 [A7] to one of the microcontroller's ADC ports at
U202 pin 25. The microcontroller watches for grossly excessive DC offset at any of the speaker
outputs. If DC is detected the microcontroller will shut down the AC power.
U201 [B6] is the non-volatile memory. Read/write access is via the 2-wire serial I
U200 [D6] generates a 250msec reset pulse for the microcontroller whenever the +5V supply
drops below 4.75V.
D200 [A5] forms a simple level translator. The CLIP signal is generated on the amp PCB when-
ever one of the satellite power amplifiers starts to clip. It's an open-collector output that pulls
down to -12V. D200 level shifts this signal from -12V through +12V, to 0V through 5V. This level-
shifted signal goes to both the microcontroller and DSP2 (via a 5V to 3.3V buffer). As it turns out,
the microcontroller ignores this signal, but DSP2 turns down the system volume until the CLIP
signal disappears, then lets the volume drift back up to the original setpoint.
Q204 and Q201 [B3] form a simple S/PDIF detector. Q204 sets the bias for Q201 such that it is
just barely off. An S/PDIF signal arriving at Q201 will cause C216 to be discharged, causing the
voltage at C216 to drop from 5V to something closer to ground. This voltage is connected to one
of the microcontroller's ADC ports and is sampled periodically by the microcontroller.
THEORY OF OPERATION
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C buss.