Pioneer PX-7 Service Manual page 98

13.
8.
2
Sound
Mixing
Circuit
The
PSG
analog
signal
outputs A,
B,
and
C
are
mixed and
distributed
(A
to
center,
B
to
left
channel,
and
C
to right channel)
at a
constant
power
ratio to obtain a
two
channel output
(ASCL
and
ASCR)
with
balanced sound
pressure
level.
Since the
PSG
analog output
stage
more
or
less
constitutes a current
source,
the outputs
are
converted to
voltage levels
by
external
shunt
resistances
R93
thru
R95
before being applied
to
a
mixing
amplifier consisting
of the
Q10
and
Qll
amplifiers.
The
mixing
ratios are set
so that the
A
output
assures center localization
at
a ratio
of
1/VTin
respect to
the
B
and
C
outputs
(this
setting
involves
the
resistances
R79
thru
R82
for constant
power
ratio). In the
same
way, the PPI (IC4)
PC
port
bit 7
output
(SOUND)
is
mixed and
localized
centrally via
C56, R77, and R78, and
the sound
input
(SUNDIN)
from
the cartridge
slot
is
mixed
and
localized via
R15, R16, R96, C64, C63, R91,
and R92.
Thus
the audio sound outputs
ASCL
and
ASCR
are
formed, and passed to the analog
ass'y
via
pins
4 and 2 of
J100.
The
ASCL
and
ASCR
signals
applied
to
the
analog
ass'y
are passed
via
a low-pass
filter
and
VR108
where
the
mixing
level
is
adjusted before
the
signals
are applied to the mixing
amplifier
(
see
Fig.
13-41).
The
signals
mixed
with input
signals
from
AUDIO
IN
in
the
mixing
amplifier
are
subsequently amplified by
a
power
amplifier stage
before being passed to
built-in
speakers or head-
phones.
The
levels
at
each point
are as indicated
in
the
level
diagram
in
page
77.
SB
+ 5V