Bose BUILT-INvisible TA-1 Service Manual page 20

Detailed Theory of Operation (see schematic diagram SD256421)
Power Amplifier Board, Bass Channel Power Amplifier (continued)
Short circuit protection is provided through current sense resistor R822. At greater than 16A,
Q726 conducts and triggers the one-shot op-amp circuit U707, pins 5, 6 and 7, and mutes the
amp for several seconds. The amp will cycle in and out of mute until the short circuit is removed.
Satellite Power Amplifiers
Each of the five satellite amplifiers are operated in Class-G configuration and consist of a 50W,
Class AB monolithic integrated circuit (in a multiwatt-15 package, TDA7294). The amplifier is
short circuit and thermally protected. External to this IC is a pair of TO-220 Darlington transistors
(the same as used in the discrete bass amplifier) to perform the Class-G power supply switch-
ing. The following detailed operation is described for the left surround channel, however the other
four channels would be the same.
The surround signal enters the amplifier board at J700 pin 11 [sheet 2, D7] and is applied to
capacitor C717 [C5] and couples to the non-inverting input of the TDA7294 amplifier IC. It is
configured as a non-inverting amplifier with a voltage gain of 4 (12 dB). The output stage con-
sists of a pair of MOSFET transistors and the positive FET must develop gate drive well above
the supply voltage and hence there is a bootstrap cap, C716 [C3], between pin 6 and 14.
With low amplitude signal, the amplifier runs off of the 20VDC rails through power diodes D704
[D3] and D710 [C3]. The power supply voltage at pin 13 is subtracted by 5.6V zener diode D703
[D3] and divided down by the ratio of 1 + R731/R732 [D3/4]. This bias voltage sets the threshold
at which transistor Q703 [D4] turns on. When the audio or the input to the amplifier exceeds the
voltage at the emitter of Q703 by two diode drops, D716 [C4] and Q703 conduct. In turn this
conducts small signal transistor Q704 [D4] which in turn conducts output transistor Q705 [D3].
A negative feedback loop is established that prevents Q705 from turning completely on and the
voltage at the collector of Q705 resembles the audio output wave form plus several volts of
saturation headroom. Q705 operating in the active region (instead of as an on/off switch), results
in shared power dissipation between the transistor and the power amplifier IC. The phase lag
created by the input network R753 and C745 [C4] allows the power circuit to switch on slightly
ahead of the power amplifier at high frequencies (above 8 kHz) to minimize turn-on glitch.
Power Amplifier Board, Sheet 2 of 2
Power Supplies and Connectors
AC power is sent to the amplifier board at J702 [A/B5] from the secondary of the power trans-
former [I/O schematic sheet, D1] located in the center cutout of the Input/Output board. The raw
AC is recitified by bridge rectifiers B700 and B701 [A/B4]. The filtered DC from B701 is passed
through fusible resistors R713 and R714 [B3] and output as +40V fused. These 40V rails are
used as the higher rail voltage for the Class G amplifier output devices as shown on amplifier
schematic sheet 1. The +40Vdc fused outputs are also are regulated down to +12Vdc by regula-
tors U708 and U709 [B/C2]. This +12Vdc is fed to both the DSP board through J700 pins 2 and 6
and to the Input/Output board at J701 pins 13 and 15 [A7]. The filtered DC from B700 provides
+20Vdc and is used as the lower rail voltage for the amplifier Class G output devices.
THEORY OF OPERATION
20