Bose AM-30P Service-Handbuch - Seite 14

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The center and surround channels each pass through an independent digital volume control

(internally consisting of a CMOS switched passive resistor ladder network). In five channel

mode the range of the center and surround volume controls are deliberately limited, from 0 to

-4dB for the center channel, and from 0 to -12dB for the surround channel. In three channel

mode, the surround volume control is set off. In two channel mode (conventional stereo) both

the center and surround volume controls are set off.

After passing through the volume control stage, the center channel is slightly equalized in the

mid-band region by filter stage U14, pins 1, 2 and 3. The surround channel is buffered and

inverted by op-amp amplifier U14, pins 5, 6 and 7. Lastly, the center channel is negatively

summed into the left and right channel matrix amplifiers (U14, pins 12, 13 and 14 and U14 pins

8, 9 and 10 respectively). The surround channel is negatively summed into the left channel

matrix amplifier, and positively summed into the right channel matrix amplifier.

3.6 Satellite Equalizaton

After being separated into left, right, center, and surround signals, all satellite channels un-

dergo active filter equalization to band limit the response from 200Hz to 15kHz and correct for

satellite speaker frequency response irregularities. Left and right channel equalization (Figure

1) is identical. Center and surround channel equalization (Figure1 and 2) has been separately

developed based on psycho-acoustical evaluation.

The bass channel signals from the decoder are applied to differential amplifier U300 pins 12,

13 and 14. This stage is also used for gain adjustment and has a range from +6dB (boost) to

-14dB (cut). The signal is passed through three stages of equalization to band limit the

signal from 37Hz to 200Hz, and provide frequency response correction to the bass

module (Figure 3).

The bass power amplifier is a discrete high efficiency Class-G design. Maximum power is 80W

into 4Ω at less than 0.1% THD. In Class-G operation the amplifier is powered by two different

power supplies depending on the amplitude of the signal input. When the audio amplitude is

low, the amplifier runs off of the lower supply rails, but during musical peaks it switches to the

higher supply rails. Efficiency is typically increased from 20% to 40%, and power dissipation is

reduced by a factor of 2.5. Detailed operation is as follows:

Referring to sheet 3 of the schematic, audio input is applied to the Amplifier PCB at pin 8 of

connector J2 and is AC coupled through C388. The amplifier is controlled by negative feed-

back to op-amp U375, which is configured as an inverting amplifier with a voltage gain of 12

(21.6dB). With no signal applied to the input, all output power devices are biased off. For a

negative input signal, pin 6 goes high and conducts driver transistor Q384. Collector current is

pulled through R390 and R389 until the voltage drop across R389 reaches about 1 Volt, at

which time the high gain darlington transistor Q382 begins to conduct emitter current through

power diode D376, which connects to the +17VDC supply. Collector current from Q382 flows

through the speaker load and the voltage at this node is regulated by feedback to the op-amp

via resistor R398.

THEORY OF OPERATION