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1
.
MAIN
AUDIO PATH
SECTION
6:
Circuit
Description
Please
refer to the block
diagram and schematic
in
Appendix
E.
The
circuitry
is
divisible into
several
major
blocks:
1)
main
audio path
2)
noise
reduction
circuit
3)
stereo synthesizer
4)
auto-polarity
circuit
5)
mono/stereo
recognition
circuit
6)
control
logic
7)
remote
control
8)
power
supply
These
will
be described
in
order.
1)
Input
Buffer
[The
left
channel
will
be
discussed; the right
channel
is
identical.]
The
signal
enters the
273A
in
balanced form. C101,
C102
shunt
RF
from
the input
leads
to
the chassis.
These
capacitors are not effective
at
VHF
and
higher
frequencies; therefore,
ferrite
beads have been placed around the
input
and
output
leads
to
suppress such high frequency
RF.
It
should be
noted
that
this
degree
of
RF-proofing
is
moderate
but
adequate
for
a vast
majority
of installations.
However,
installation
next
to
a
high-power
transmitter
may
still
cause problems. Additional
RF
suppression, careful
examination
of
the
grounding
scheme,
and other
considerations
familiar
to the
broadcast engineer
may
have
to
be used
in
conjunction with the 275A's
built-in
RF
suppression.
The
filtered signal
is
applied
to IC9b, a very low-noise
opamp
configured
as a
differential
amplifier
with
a
gain
of 0.5.
When
both non-inverting
and
inverting
inputs are driven
by
a
source
impedance which
is
small with respect
to
100K
(such
as
600
ohms
or
less),
the amplifier
is
essentially insensitive to signal
components
that
appear
equally on
the
non-inverting
and
inverting inputs (such as
hum), and
responds with
full
gain to the difference
between
the non-inverting
and inverting
inputs.
Thus
it
serves
as
an "active transformer".
Ordinarily, best results
are
obtained
for
unbalanced
signals
if
the non-inverting
(+)
input
is
grounded
and
the
inverting
(-)
input
is
driven.
IC9b
will
overload
if
its
differential input
exceeds approximately +26dBu.
2}
Crossfade
Circuit
The
output
of
the
left
input buffer
is
applied directly to 1C51,
which
consists of a
photoconductive
cell
and
an
LED.
The
output
of the right input
buffer
is
passed
through
a
voltage-controlled
polarity-flipping circuit
(IClOa)
whose
gain
is
+1
if
Q101
is
OFF
(gate
at
-15V)
and
-1
if
Q101
is
ON
(gate
at
ground).
The
output
of
IClOa
is
passed
to
IC52,
another photoconductive cell/LED.
6-1
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