Mixing Stage - Pioneer MA-62 KL Service Manual

Mixing
Stage
The mixing
circuit of
this
unit
is
a
very
ortho-
dox and highly
stable
series resistance
circuit
using
isolation
resistors. Resistors
RMr
thru
RM6
in
the
diagram
are
such isolation
resistors.
The
signals
to
be
mixed
are
fed
to
terminals A
thru F,
and perfect
mixing is obtained
at
point
X in the
diagram.
If low
resistance values
are
used
as
RM's,
changes
in
the
signal levels
from
each source may occur due to the
different
output
impedances
of
the
sources
connected to
A thru
F. To prevent
this,
resistances
of
33OkSl
are
used
as
RM's
in
this
unit-differences
due to
output
impedance
are
negligible.
To
prevent
interference between
the
various
signals,
the
impedance at
point X
(the mixing point)
must
be
kept quite
low in
relation to the RM
values.
To fulfill
this condition, parallel feed-type
NFB
is applied
from
the
emitter of
Qq
to
the
base
of
Q: in
the mixing
ampli{ier. This
not only
helps
to
reduce
distortion and
noise
(the effect
of
NFB),
but
also
keeps
the input
impedance
of
Q3
verj
low,
maintaining
stable
operation.
cc
o
F
(J
uJ
J
LU
a
F
f
o-
F
l
o
L
)
O
cr
F
z
o
C)
J
Lri
ui
I
MIXING CIRCUIT (L CHANNEL)
RNF
OUTPUT
RM,
-RM6: ISOLATION
RESISTORS
(330k,f2)
RNF:
NFB
RESISTOR
(560ks-,)
L&R
RM,
RM,
RMe
To Mixing Circuit
(R
Channel)
Fig.2
10