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SONY
STR-VX
Series
Circuit
Operation
Impedance Detector
The circuit feeds a signal to the relay
drive
circuit to open the relay switch
in order
to protect the speakers if ex-
cess
current
flows in the output
stage.
When
the
base-emitter
voltage
of Q718
is raised
to 0.6V,
Q727
turns
on.
Then,
the collector
voltage of Q727
goes low.
Because
the collector
of
Q727 is connected
to the base of Q722,
Q722 turns off, disengaging
the relay
switch as shown in Figure 11.
Current Limiter
When
a very low impedance
speaker
is
connected
or extremely
high output
cur-
Fig.
12.
12
rent
flows
in the
output
stage,
the cur-
rent limiter operates
to protect
the am-
plifier.
If the base-emitter
voltage of
Q716 is raised
to 0.6V
because
of an
excessive
voltage
drop
across
R730,
Q716 turns
on,
and
the
voltage
between
points A and
B will become
about
0.6V
as shown in Figure 12.
This cuts off
the
driver
transistors
(Q710,
Q712)
and
output transistor
Q714.
Likewise,
if
the base-emitter
voltage of Q717 is
raised
to 0.6V
via R728
from
R731,
Q717 turns
on.
Then
Q711,
Q713 and
Q715
switch off.
D705
and D706
are
connected
to prevent
current
from
flowing from the output to the input
of the driver stage via Q716 and R727,
or Q717
and R728.
O B+ (40V)
O B— (-40V)
Current
Limiter.
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