Bose AM-30P Manual de servicio - Página 15

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THEORY OF OPERATION

When the audio output voltage approaches the 17VDC power supply rail, output transistor

Q382 begins to saturate and conducts much more base current than the normal maximum of

5mA. At approximately 8mA the voltage drop across 75Ω resistor R390 exceeds 0.6V and

small signal transistor Q383 begins to conduct. This in turn conducts Class-G Darlington

transistor Q381, which turns on the 34VDC power supply and reverse biases power diode

D376, effectively turning off the 17VDC supply. During this period, the wave form at the collec-

tor of Q381 resembles the audio output signal plus the saturation drop of Q382 (See Figure 5)

and Q381 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 250Hz) and the ability of the Acoustimass

bass module to roll-

off high frequency distortion products. Crossover distortion is less than 0.5% at 200Hz, 1 Watt.

Muting of the amplifier (via remote mute or at system turn-on or turn-off) occurs when the

voltage at the mute input line (J2 pin 3) drops below 4.4 volts. Under this condition, Q377

conducts through Q378, and in turn this conducts both positive and negative half cycle mute

transistors Q379 and Q380. These transistors saturate and cutoff output transistors Q382 and

Q387.

3.9 Satellite Power Amplifiers

Each of the four 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 ampli-

fier 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 switching. The following detailed operation is described for the left and surround chan-

nel only.

The surround signal is applied to capacitor C175 and couples to the non-inverting input of the

TDA7294 amplifier chip. It is configured as a non-inverting amplifier with a voltage gain of 4

(12dB). The output stage consists 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 between

pin 6 and 14.

With low amplitude signal, the amplifier runs off of the ±17VDC rails through power diodes

D177 and D180. The power supply voltage at pin 13 is subtracted by 5.6V zener diode D176

and divided down by the ratio of 1 + R182/R181. This bias voltage sets the threshold at which

transistor Q175 turns on. When the audio at the input to the amplifier exceeds the voltage at

the emitter of Q175 by two diode drops, D175 and Q175 conduct. In turn this conducts small

signal transistor Q179, which in turn conducts output transistor Q176. A negative feedback

loop is established that prevents Q176 from turning completely on, and the voltage at the

collector of Q176 resembles the audio output wave form plus several volts of saturation head-

room. Q176, 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 R175 and C177 allows the power circuitry to switch on slightly ahead of the

power amplifier at high frequencies (above 8kHz) to minimize turn-on glitch.