Marantz 2245 - Power Amplifier; Power Protection Circuit; Speaker Protector Relay Circuit

5. Power Amplifier
The
signal
from
the
tone
amplifier
is
applied
to
the
differential
amplifier
(base
of
H751)
through
the
coupling
capacitor
C751.
The
differential
amplifier
provides
very
high
input
impedance
and
its
collector
output
(H752)
is
applied
to
the
base
of
H753
which
in
turn
applies
its
Output
to
the
next
stage;
H756
through
the
network
R766,
C762
and
R771,
and
to
the
H757
through
the
network
R776,
C763
and
R772.
The
outputs-of
H756
and
H757
are
applied
to
the
H758
and
H757
respectively.
HOO1
and
HOO2
are
power
transistors
used
in
complementary
symmetry
configulation
and
mounted
on
the
heat
sink.
To
maintain
overall
amplifier
stability
and
linearity,
degenerative
feed
back
is
utilized
throughout
the
amplifier.
This
feed
back
is
also
necessary
to
reduce
distortion
to
within
specified
limit.
The
RC
network
R775
and
C756
condition
the
feed
back
signal
for
the
audio
signals.
R759
and
C755
are
also
a
feed
back
loop
provided
to
obtain
a
stable
zero
DC
off
set
voltage
at
tne
speaker
output
terminals.
The
R762
is
a
trimming
resistor
to
adjust
the
DC
off
set
voltage.
Dynamic
bias
is
applied
to
the
base
of
driver
transistors
H758
and
H757.
This
dynamic
bias
circuit
is
comprised
of
H761,
H760
and
R763.
This
provides
a
variable
base
bias
for
driver
transistors
that
automatically
maintains
the
proper
base
voltage
with
temperature
change.
The
ternperature
sensitive
biasing
components
of
the
dynamic
circuit
are
thermally
couppled
through
a
heatsink
to
the
power
amplifier
transistors.
6. Power Protection Circuit
Protection
circuit
for
the
amplifier
is
provided
by
sensing
resistor
networks
and
two
switching
_
transistors.
When
the
output
transistors
are
over-driven,
the
current
increase
through
the
power
-
output
transistor
causes
an
increased
current
flow
through
R789
(or
R788)
and
the
potential
“across
the
R789
will
be
increased.
This
increased
voltage
potential
is
applied
to
the
base
of
H755
~
through
the
resistor
R783
and
turns
on
the
H755.
Since
the
collector
of
H755
is
directly
connected
to
the
base
of
H757,
this
means
that
the
base
of
H757
is
bypassed
to
the
ground
through
emitter-collector
path
of
H755.
Thus
the
input
signal
to
the
H757
is
restricted
to
the
value
which
maintains
the
operation
of
power
transistor
with
in
the
safety
area.
A
resistor
network
R777
and
R781
also
works
as
a
sensing
network.
When
the
center
voltage
(collector
voltage
of
power
transistors)
is
excessively
increased
to
a
positive
value
by
certain
troubles,
the
voltage
applied
to
the
base
of
H755
makes
the
H755
turn
on,
making
bypass
circuit,
and
protects
the
power
transistor.
For
the
other
half
cycle
of
driving
signal,
the
eperatig
principle
is
applied
provided.
7. Speaker Protector Relay Circuit
The
speaker
protection
circuit
consisting
of
H808,
H809,
H810,
etc
protects
the
speaker
systems
against
any
loud
“‘pop’’
sound
developed.
This
circuit
is
so
designed
that
no
sound
is
heard
for
the
first
three
or
five
seconds
after
the
power
switch
is
turned
on
by
the
time
constant
circuit
consisting
of
C807
and
R816.
This
circuit
also
protects
the
speaker
systems
against
some
troubles
due
to
DC
off
balance
between
the
speaker
system
terminals
by
instantly
operating
the
relay
and
cut
off
the
speaker
systems
from
the
circuit.
When
DC
off
balance
voltage
(positive)
is
developed
between
speaker
terminals
by
possible
defects
such
as
broken
power
transistor,
short-circuits,
or
broken
potentio
meter
R762,
as
the
base
of
H808
is
connected
to
the
speaker
terminal,
the
transistor
H808
is
turned
on
by
this
offset
voltage
developed
and
this
makes
the
transistor
H809
and
H810
turns
off,
thus
cutting
off
the
relay
and
disconnecting
the
speaker
from
the
output
circuit.
When
negative
offset
voltage
is
developed,
this
voltage
directly
turns
off
the
H809
and
H810,
thus
speaker
is
cut
off
from
the
circuit
and
protected.
Thecircuit
also
protects
the
speaker
systems
from
the
possible
damage
when
the
amplifier
is
aver-driven
by
very
bow
frequencies
such
as 7
or
Rad
cycles.