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TA-2000F
Stage/Control
REC OUT
Muting circuit
Q911, Q912
Function
sociated bias circuit, while Q909
(Q910)
is forward
biased
by
R911.
Asa result, Q909 (Q910)
is ON, shorting the output cir-
cuit to ground, and
effectively
muting the output signals.
As Q908 is gradually turned ON
due to the slowly-increasing base
current
flow,
Q908
conducts
and
cuts off Q909
(Q910), re-
moving
the muting.
Q907
is
employed
to
discharge
C902
quickly
when
power
switch
is
turned
off, preparing it for the
next muting operation.
This circuit is employed to mute
the REC
OUT
signals by means
of relay REL-1,
preventing the
loud
''pop"' or click noises just
after turning the power
switch
to ON.
The
base of Q911
is
connected
to the collector
cir-
cuit of Q912, while the base of
Q912
is connected
to an
RC
network
(R903, C901) having a
long time constant.
When
you
first turn ON the power switch,
Q912
remains
off due
to the
long time constant of the associ-
ated
bias
circuit.
This
keeps
open
the muting
relay, discon-
necting
the
buffer
amplifier's
(Q01)
output
from
the
REC
OUT
signal
path.
As
Q912
is gradually turned
ON
due
to
the
slowly-increasing
base
cur-
rent
flow,
Q912
conducts
and
turns
on
Q911,
energizing
the
muting relay to deliver the buf-
fer
amplifier's
output
to
the
REC
OUT
terminal.
Power Supply Section
Two independent regulated pow-
Stage/Control
Voltage regulator
Q901, Q902, Q903
(Q904, Q905,
Q906)
Function
er supplies are employed to ob-
tain
stable
operation.
One
is
a low-voltage
supply
delivering
37 volts dc, and the other is a
high
voltage
supply
delivering
150
volts dc.
As both power
supplies have the same
configu-
ration,
only
the
low-voltage
regulator
circuit
is
described
here.
The high-voltage supply is
identical
except
for
reference
numbers.
De output from bridge rectifier
D901
and
D902
(D903
and
D904) is filtered by C18 (C17)
and
applied
to series regulator
Q901
and
Q902
(Q904
and
Q905).
Q903 (Q906) compares a sample
of
the
output
voltage
picked
off across power
supply
adjust
control
R926
(R920),
with
reference
voltage
supplied
by
zener diode D905 (D906).
A change in the output voltage
is detected at the base of Q903
(Q906)
and
therefore
alters its
collector
voltage.
Since
the
collector
of Q903
is directly
coupled
to the base
of Q902
(Q905),
the
change
in output
voltage
alters
the
conduction
of Q901
and
Q902
(Q904 and
Q905) by the amount necessary
to maintain
the output voltage
constant.
An
increase
in output
voltage
causes an increase in the imped-
ance
(decrease
in
conduction)
of Q901
and
Q902
(Q904 and
Q905),
and vice-versa.
The dc
output
voltage
supplied
to the
preamplifier section is therefore
extremely stable.
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