Bose BUILT-INvisible TA-1 Service Manual page 16

Detailed Theory of Operation (see schematic diagram SD256431)
Digital Signal Processor (DSP) Board, Sheet 1 of 4 (continued)
The S/PDIF input enters the DSP board at J9 pin 10 [B8] and is routed to the codec by means of
C151, R1, R2, D1, C1, C7 and R5 [B5]. C151 AC couples the digital S/PDIF signal, and R1
terminates the input with 75 Ohms. R2 and D1 clamp the input (a typical S/PDIF input is about
500 mVpp into a 75 Ohm load). C1 AC couples the signal at D1 (which is ground referenced) to
the codec input (which will be biased at about 2.3V when the S/PDIF receiver is working). R5
and C7 are an EMI filter designed to prevent RF from leaking outside the shield.
The codec's power supply consists of U1 [A5] and associated components. R10 and R11 are
power resistors designed to drop the voltage at the voltage regulator's input, for reducing the
power dissipation of the voltage regulator itself.
D2, D3, and R4 [A5/6] are used to draw a little current from the +12V regulated supply when the
power supplies start to sag severely, as they will at low AC line and heavy amplifier load. The
+12V regulator is fed from the +40V unregulated supply and has considerably more headroom
than the +5V regulator fed from the +20V supply. Also fed from the +20V supply is the 3.3V
switching regulator, which tends to draw more current as its input voltage drops (it delivers a
fixed amount of power, at a fixed conversion efficiency). So, as the +20V supply starts to sag,
the current load on it actually starts to increase, which tends to make it sag more.
The +5 volts supplied by the regulator is sent to two different components; the codec U100 [C4]
and the microcontroller U202 [sheet 2, C5] and associated components. Since the regulator is
located outside the EMI shield and the +5V supply line might be carrying EMI (Electro Magnetic
Interference), ferrite beads L1 and L2 [A4] were placed in series with each of the supply lines.
L1 and R3 [A4] serve to isolate the two different power supply pins on the codec (the "analog"
+5VA from the "digital" +5VD). C5 and C6 are bypass capacitors placed directly under the
codec.
Codec glue logic
U101 [D5] is a 74LCX244, a 3.3V IC that can tolerate 5 volts on its inputs. It's used to convert
5V to 3.3V logic signals.
U102 [D3] is a 74ACT244, a 5V part that understands the logic levels used by the 3.3V IC. It's
used to convert the 3.3V logic signals to 5V logic signals.
U106 [A7] is a flip-flop, used to convert a falling edge (from the codec) to a narrow pulse (to the
DSP). This removes a potential timing ambiguity.
R192 [D6] (and others) is a 75 Ohm series resistor used to series-terminate some of the high
speed logic signals, to preserve edge fidelity at the receiving end.
R6 and R7 [C3] comprise a low performance 5V to 3.3V shifter. Using such large series resis-
tances in the presence of even moderate amounts of stray and input gate capacitance slows
down the signal's edge, but this particular signal is very low bandwidth.
THEORY OF OPERATION
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