Main Amp Unit - Pioneer SX-525 Service Manual

SX-525
4-2
MAIN
AMP
UNIT
•
MUTING CIRCUIT FOR ELIMINATION
OF STARTING PULSE NOISE
A
common
drawback
of conventional
SEPP
amplifier
circuits
is
the occurrence of pulse
noise
when
the
power
is
turned on.
The
cause
of this noise
is
the timing of the
DC
voltage
distribution
through
the junctions
in
the
circuit.
The most
important
junction
in this
respect
is
the inner side
of
the
output coupling
capacitor.
Fig. 1
shows
a simplified, typical
SEPP
output
stage..
When
the
power
is
turned
on, the
original
+B
voltage surges immediately,
and
this
voltage simultaneously
appears,
through
R,
as
point
(A).
The
time constant of
this
C/R
network cannot
be
made
as large as desired
because
it is
part of the
signal
path. Therefore,
voltage
+B
appears almost
instantly at
the base
of
Q4,
causing
a
sudden output
coupling capac-
itor
to the speaker.
The
common
drawback
just
described
about conventional
SEPP
ampli-
fier
circuits
causes the
starting pulse noise.
Fig. 2
shows
the
newly developed SX-525's
muting
circuit
used to eliminate pulse noises
(and
distortion)
both
at the
turning-on
and
turning-off of the
power.
A
rectified,
negative
10
12V
voltage
is
applied to the base of
Q8 when
the
power
is
turned
on,
keeping Q8
cut
off.
As Ca and
Rb
are
not part of the
signal
path,
their
time constant can be
designed as large as
desired.
+B
voltage builds
up
at
the base
of
Q9
only gradually because of the comparatively
long charge time of R/C. Therefore,
Q9
becomes
conductive rather slowly, energizing the
output
stage
gradually
and with balanced
DC
surge.
Consequently,
no abrupt
pulse
flows through
the coupling
capacitor to the speaker.
When
the
power
is
turned
off,
the above-
mentioned negative voltage breaks
down
almost
immediately,
leaving
only the positive
+B
voltage
through
Ra
at
the
base of
Q8.
As
this
practically
grounds
collector
of
Q8,
Ca
can
dis-
charge instantly
through Q8,
and
all
speaker
sound
is
cut
off
immediately.