Bose Companion 3 Service-Handbuch - Seite 27

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Theory of Operation

2.0

Main Amplifier Board (continued) (refer the schematic diagram 271838)

The signals coming into J308 (C7) are sent to sheet 3 of 3 (L_VOL and R_VOL). Three amplifiers in

U201 (D8-4) are the equalizers for the right channel and U125, (A8-4) are the equalizers for the left

channel. U951 (C8-3) in conjunction with U950 (C-2), the voltage controlled amplifier, form the bass

summer and dynamic bass equalizer.

The three stage equalizers for the equalizer perform the bass cutoff, mid-range cut, and high end

boost, in that order. They are 4-pole Salen-Key elliptical filters.

The bass signal is formed from the two inputs summed and low pass filtered by U951. The bass level

is detected by U125, D951, D952, Q950, Q951 and Q952 (B-6) forming an active peak detector. C955

and C957 filter the detected peaks to form the control voltage for the VCA in U950. When the bass

level is very low, the signal is boosted by U951. U950 is in the negative feedback path for U951, and

when its gain is low, U951 gain goes up, boosting the bass. U950 gain goes down when control

current generated by Q952 goes down, as controlled by peak detector circuit.

Q953, Q954 and D954 (D-2) form the peak detector on the bass outputs. Peaks near the negative rail

are conducted through D954 and Q953 to charge C969. Voltage on C969 is converted to a current by

Q954 which controls the VCA in U950. U950 is in the negative feedback loop of U951, so when U950

gain is high, less bass is output. Near clipping detected by the peak detector is converted to high

current by Q954, causing high gain in U950, reducing the gain in U951 and less bass. R980 and

C969 determine the attack time of the compressor, and R981 and C969 determine the release time.

Also on sheet 3 of 3, U201 (A-3) is the virtual ground generator. The 10 volt line is divided by two

R225 and R226, filtered by C970 and C235, and input to U201. The 5 volts generated by U201 is

used by all the signal processing circuitry as the virtual ground reference.

The right and left outputs, EQ_R and EQ_L are sent to page 2 of 3 (B3). U102 (B3) forms the true

space processing and the satellite signal compressors. TrueSpace™ processing consists of U102B

and U102D which act a low pass filters with a -3 dB point of 3 kHz. This signal is subtracted from the

opposite channel signal by U102A and U102C.

R204 and R105 determine how much TrueSpace signal is subtracted. The 127K compared to the 10K

value of the main signal inputs gives a TrueSpace signal of -18 dB relative to the main signal. To keep

the levels the same in mono conditions, the same amount of low pass filtered signal is added back to

the main channel of itself, through R106 and R206.

The satellite compressor consist of D401, D402, and Q401 (D7) which detect the right and left chan-

nel level, charging C401 when the R+, R-, L+ and L- signals get near the negative rail. The compres-

sor attack time is determined by R403, and the release time is determined by R404. When C401

charges negatively, Q402 and Q403 convert this to current which increases the gain of U401A and

U401B. U401 is in the negative feedback path of U102A and U102C, and higher gain reduces the

gain through the satellite channels. Therefore, high signal levels near clipping reduces the gain of the

satellite channels, compressing the signals.

The compressed signals are sent to the main amplifier on sheet 1 of 3 over signals Sat_Drv_L and

Sat_Drv_R. The bass signal from the bass compressor on sheet 3 of 3, BASS_DRV, is sent to two

stages of bass equalization at U3B and U3D (D7). These form the 40 - 50 Hz boost, and the 100 Hz

cut for the bass box equalization.