Bose AM-30P Service-Handbuch - Seite 9

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THEORY OF OPERATION

3.2 Audio Chain (Refer to the shcematic sheets 1 through 3 enclosed)

The following discussion references the left channel electronics. Right channel operation is

identical and therefore not specified.

The differential input amplifier stage (op-amp U13, pins 1, 2 and 3) buffers and preamplifies the

CD-20 output signal. Gain is limited to 10.6dB to prevent potential overload (from the maxi-

mum 2Vrms input signal). Capacitors C102, C103, C104 and C162 are for suppression of

Radio Frequency Interference (RFI). D100 and D101 are protection diodes for input overload

or electrostatic discharge (ESD).

The output of the differential amplifier is applied to the "common eq" stage (schematic sheet 2)

comprised of op-amp U6 (pins 8, 12, 13 and 14). This is an equalizer stage that has frequency

response corrections that are common to all the satellite speakers, but does not affect the low

frequency band below 200Hz, which must remain "flat" so as not to affect the bass channel

later in the chain.

The output of the common eq stage is applied to the dynamic equalizer stage. This stage has

two paths; one is through the 37Hz bandpass filter (U6, pins 1, 2 and 3), the other is through

the digitally controlled dynamic bass volume IC (U8). The output of the volume control is

buffered by op-amp U17, and the two paths are summed together through resistors R120 and

R121. At full volume, the audio signal flows predominantly through U8 and U17, and thus the

frequency response is flat. At lower volume level, more audio flows through the bandpass filter,

causing a gradual increasing peak in the frequency response centered at 37Hz. The volume

level of U8 is dependent upon the level of the master volume control (U9) and is explained in

the next paragraph. The frequency response contour of the dynamic equalizer stage is shown

in Figure 4.

The output of the dynamic equalizer stage is applied to the master volume control IC (U9). Like

U8 and U12, it internally consists of a 12 element resistor ladder network, with digitally con-

trolled analog CMOS switches that adjust signal attenuation in 1dB increments from 0 to -80

dB. When the strobe line (pin 10) is low, volume up/down commands are received from the

microcontroller. When U9 is set between 0 and -20dB, U8 is kept at 0dB (full volume). As U9 is

decremented below -20dB, U8 volume begins to decrement from 0dB in step with U9 (but

offset by 20dB).

The output of the master volume stage is applied to buffer amplifier U7, which has a gain of

14dB. This is followed by a ±10dB treble control stage (potentiometer R50). The frequency

response graphs of this stage are shown in Figure 6.