Bose BUILT-INvisible TA-1 Service Manual page 19

Detailed Theory of Operation (see schematic diagram SD256431)
Digital Signal Processor (DSP) Board, Sheet 4 of 4 (continued)
C506 [D3] is the "soft-start" capacitor, which forces the regulator to start up slowly when power
is first applied, thus limiting the inrush current. R510 and Q500 [D3] allow the "soft-start" pin to be
pulled low, shutting off the regulator. The 3.3V supply is turned off as part of the normal power
down sequence.
Detailed Theory of Operation (see schematic diagram SD256421)
Power Amplifier Board
Note: Unless otherwise noted, refer to the schematic diagrams for the power amplifier board for
the following information. The references in brackets [ ] are the schematic sheet and grid coordi-
nates for the component.
Sheet 1 of 2
The satellite channel power amplifiers are a discrete high efficiency Class-G design. Maximum
power is 35 W per channel into 4 Ohms at less than 0.1% THD. In Class-G operation the amplifi-
ers are powered by two different power supplies depending on the amplitude of the signal input.
When the audio amplitude is low, the amplifiers run off of the lower supply rails, but during musi-
cal peaks they switch to the higher supply rails. Efficiency is typically increased from 20% to
40% and power dissipation is reduced by a factor of 2.5.
Bass Channel Power Amplifier
Audio input is applied to the amplifier PCB at pin 12 of connector J700 [amplifier schematic sheet
2, D7] and is AC coupled through C727 [D8]. The amplifier is controlled by negative feedback to
op-amp U707 [C6], which is configured as an inverting amplifier with a voltage gain of 12 (21.6
dB). With no signal applied to the input, all output power devices are biased off. For a negative
input signal, pin 1 goes high and conducts driver transistor Q713 [D6]. Collector current is pulled
through R707 and R774 [D6] until the voltage drop across R774 reaches about 1 Volt, at which
time the high gain darlington transistor Q718 [D6] begins to conduct emitter current through
power diode D711 [D5/6], which connects to the +20VDC supply.
Collector current from Q718 [D6] flows through the speaker load and the voltage at this node is
regulated by feedback to the op-amp via resistor R789 [D7].
When the audio output voltage approaches the 20VDC power supply rail, output transistor Q718
begins to saturate and conducts much more base current than the normal maximum of 5mA. At
approximately 8mA the voltage drop across 75 Ohm resistor R707 [D6] exceeds 0.6V and small
signal transistor Q714 [D6] begins to conduct. This in turn conducts Class-G Darlington transis-
tor Q715 [D6], which turns on the 40VDC power supply and reverse biases power diode D711
[D6], effectively turning off the 20VDC supply. During this period, the waveform at the collector of
Q715 resembles the audio output signal plus the saturation drop of Q718 and Q715 is operated
in the active region (not as a switch), thus sharing the power dissipation.
Crossover distortion and switching transients are not an issue due to the relatively low band-
width of the amplifier (less than 250 Hz) and the ability of the bass module to roll off high fre-
quency distortion products. Crossover distortion is less than 0.5% at 200 Hz, 1 Watt.
THEORY OF OPERATION
19