Bose BUILT-INvisible TA-1 Service Manual page 10

Digital Signal Processor (DSP) PCB (continued)
Power Supplies
There are several sets of internal power supply voltage rails. These voltages originate with the
main power transformer located in the center of the Input/Output (I/O) PCB. There is also a small
keep-alive transformer T5 [I/O PCB schematic, C1] located on the I/O board itself that allows the
amplifier to be turned on by SmartSpeaker commands.
The power amplifiers are supplied with both +40 and +20 VDC unregulated rails, as determined
by the Class G circuitry. These rails are developed from the transformer secondaries diode
bridges B700 and B701 [amplifier schematic sheet 2, A/B 3/4] and their associated capacitor
banks.
The digital circuitry on the DSP PCB requires two different regulated voltages, +5V and +3.3V.
The +5V regulator U1 [sheet 1, A5] is a standard 3 terminal linear regulator in a surface mount
package and is located on the DSP PCB. It's powered from the +20V rail via fusing resistor
R704 [amplifier schematic sheet 2, C6] on the amplifier PCB, and supplies about 120mA to the
DSP PCB, most of which is consumed by the microcontroller and codec.
The +3.3V regulator U500 [sheet 4, D4] is a switch-mode regulator and must be located under
the shield to avoid EMI problems (harmonics of the 100 kHz switching frequency will stray into
the AM broadcast band if left unshielded). This regulator is also powered from the +20V rail and
supplies about 350mA to the DSPs.
Operation of the 5V linear regulator is very simple. Current is supplied to the regulator via sur-
face-mount power resistors R10 and R11 [sheet1 A5], which drop the voltage supplied to the
regulator to avoid excessive power dissipation. The regulator needs a minimum of about 7V at its
input terminal to stay in regulation and it should be able to do this even as the +20V rail sags to
+10V or so.
Operation of the +3.3V switch-mode regulator is more complicated. In its simplest terms, the
unregulated input is "chopped" at about 100 kHz, and the resulting pulse train is low-pass filtered
to extract the DC. This DC output is compared to a +3.3V reference, and the duty cycle of the
switching waveform is adjusted accordingly. The high-side switch (the switch that connects
+20V to the output filter) is integrated into the controller, while the low-side switch (the switch that
connects ground to the output filter) is an external diode D500. When the controller is running,
you should be able to observe the switching waveform at L500.
Microcontroller
The microcontroller U202 on the DSP PCB is used for the following housekeeping and
control functions:
•
to interpret SmartSpeaker commands from the head-end.
•
to interpret special test mode commands from the functional tester.
•
to store and retrieve data from nonvolatile memory (U201).
•
to boot and reprogram the codec (U100) as required by the operating mode of the system.
- to monitor for bass and treble PWM voltage level from U15 on the I/O board.
- DC offset on the amplifier outputs.
- PCB over-temperature.
- presence/absence of a digital S/PDIF signal.
THEORY OF OPERATION
10