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SECTION IV
THEORY OF OPERATION
IMPORTANT NOTE
The following
descriptions of
the
circuitry used
in
the
amplifier
are
presented here
in
order that
the
professional user
may have
a
general understanding
of
how
the
amplifier works.
They
are not
intended
as
a
guide
for service.
Service on this
product should
be
performed only
by
qualified technicians.
THERE
ARE NO USER SERVICEABLE PARTS INSIDE.
In
the
following descriptions
the
component designations for
Channel
A are used.
Component designations
for
Channel
B
are
included
in
the
schematic diagram
at the
end of this manual.
4.1
INPUT DIFFERENTIAL AMPLIFIER
The input differential amplifier
is
comprised
of
IC1
A and
B
and associated components.
Input signals are coupled
to the
amplifier after passing through one
of three input
connector
types and passing across RF suppression capacitors
C45-and
C46.
Two amplifier sections are used
as
unity gain inverters with
the
output of one summed
to the input of the
other
to
afford
common mode cancellation.
Thus signals which appear equally
on
both input terminals, such
as
common mode noise and
hum, will be
cancelled and unequal signals, such
as
legitimate input signals,
will
be
summed.
Use of the inverting mode
in
both sections
assures high speed, good common mode rejection and equal
impedance
at
both input terminals,
a
substantial advantage over
less
expensive single amplifier topologies.
4.2
MODE SWITCH
The rear panel mode switch allows the amplifier
to
operate
in
the Stereo, Dual
Mono or Bridged Mono modes.
In
the
Stereo
mode, the switch
is
out of the circuit and each channel input
goes
to its
respective output.
In
the Dual Mono mode, the output
of the Channel A differential amplifier
is
routed
to
both Channel
A and
B
gain controls.
The signal to the Channel
B
gain control
is
"impedance-switched", whereby the extremely low output
impedance of the Channel A input amplifier becomes the source for
the
potentiometer rather than
the higher impedance source from
Channel
B
input amplifier through resistor R88.
In
the
Bridged
Mono mode, output from the Channel A amplifier
is
routed through
IC2D configured
as an
inverting amplifier.
The output of IC2D
is
"impedance-switched"
to the input of the
Channel
B
voltage
amplifier.
- 22 -
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