Bose CD-20 Service Manual page 11

THEORY OF OPERATION
4.0 Audio Electronics (continued)
Each of the volume control chips has two independently mutable outputs. The #1 outputs are
used for the speakers (as described above). The #2 output of the Zone 1 volume chip is used
for CD-20's Headphone output (the #2 output on the Zone 2 chip is not used). Anyone plugging
in headphones would therefore hear the same source that was previously being played by the
Zone 1 speakers. The microprocessor monitors the HP_SENSE line from the headphone jack
to tell if headphones are plugged in. When they are, the micro mutes the #1 outputs from the
Zone 1 volume chip (to the speakers), and unmutes the #2 outputs (to the headphones). The
opposite occurs when headphones are unplugged. The headphone signals are buffered by
U104 in order to drive the required 32 Ω loads.
Audio from the CD player typically contains a great deal of motor noise. This noise occurs
because of ground conductor voltage drops on the ribbon cable connecting to the mechanism.
To eliminate this motor noise, op-amp U103 has been configured as a differential amplifier. Any
motor noise signals on the left and right audio lines are also on the ground reference line, pin 2
of the CD ribbon cable (pin 2 of connector J110). Therefore, U103 eliminates this common-
mode component without affecting the actual audio (which is purely a differential-mode compo-
nent). At the same time, U103 provides the necessary gain for the CD signal.
5.0 Tuner Electronics (Schematic Diagram Sheet 4)
The FM antenna signal comes in through F connector J301 and enters the FM front-end
module (U301). U301 contains a tuned RF amplifier, the FM local oscillator, and the first mixer.
The 10.7 MHz IF output signal appears on pin 4 and is passed through a 10.7 MHz ceramic
filter (CF300). The filter's output is amplified by the first IF gain stage. This stage consists of
Q307, Q308 and their associated components. The signal is then passed through a second
ceramic filter (CF301), a second gain stage (Q309, Q310, etc.) and a third ceramic filter
(CF302). These filter stages reject unwanted FM stations and noise.
The output of the final ceramic filter is fed into the main detector IC (U300). This device con-
tains a second mixer, the FM detector, FM stereo MPX decoder, stop-level detection, as well
as most of the AM circuitry. U300 further amplifies the IF signal and then performs FM detec-
tion using a double tuned quadrature detector formed by T303 and T304. T304 is adjusted for
FM center frequency by adjusting it for 0 VDC between the AFC terminal (pin 4, U300) and the
VREG terminal (pin 28, U300). T303 is adjusted for minimum distortion. (These two adjust-
ments are interactive, so a few repetitions may be required.) The recovered audio appears on
pin 8 of U300.
The recovered audio is filtered by C326 and the associated components and fed back into
U300 on pin 9. U300 performs the FM stereo MPX decoding and outputs the decoded L and R
channels (when FM is selected) on pins 14 and 15. The separation is controlled by the resis-
tance from pin 12 to ground. The PLL decoder is clocked by 456 kHz resonator X300. The PLL
loop filter components are connected to pin 11. The FM stop level is set by the variable resis-
tance R349 on pin 30 and is nominally set to 33 dBf.
The signals are buffered and amplified by op-amp U303 and its associated components.
Components R301, C302, R305, and C303 perform FM de-emphasis. MPX filters T301 and
T300 remove any unwanted out-of-band signals.
11