Description Of The New Circuit; Features - Hitachi HMA-6500 Service Manual

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DESCRIPTION
OF
THE
NEW
CIRCUIT
o
Protection circuit
IC
HA12002, an exclusive
protection circuit,
is used
to
protect the
speaker
and
power amplifier.
(1)
Muting circuit
This
keeps
the relay off for
approx.
4
sec
after
the
power switch
has
been
turned on
to
reduce
shock
sound during ON/OFF operation
of the power
switch.
This
is
determined
by the
rise time
constant
of
the
voltage applied
to pin
@
of
IC90l
and
is set
by R907,
R908, and C905.
(2)
Protection
of
the speaker
(DC
voltage detection/pro-
tection
circuit)
HITACH] HMA.6500
When a DC voltage
appears
at
the neutral
point of
the
output,
a
relay turns
off
when approx.
l
3V is applied
to
pin
@
of IC90l
via R901, R902 R903 is
the
sensitivity adjustment
resistor.
Protection
of
Power MOS
FET
(ASO
protection
circuit)
When overcurrent
flows
to
thé output, the
voltage
Vcs
between
the
gzte and
source
of
FET7OI
increases,
Q70ó
and
Q707 turn ON, the
voltage
is
applied
to
pin@of
tC901
and
the relay is turned off.
(3)
FEATURES
HITACHI
have developed a
complementary
MOS FET
which
displays superb characteristics
as
a power amplif
ica-
tion
device
for
audio applications.
Since both the N-channel and
P-channel
complementary
characteristics of this
POWER MOS FET
are
top-class, thls
model features
a
simple
3-stage
circuit
conf
iguration including the poweram-
plification
stage.
This
is a design
which
makes
for
a
low distortion
across
a wide band and which
is devoted to
engineering
only
the
best
in
sound
quality.
,^
1.
Design
oriented toward
low
distoftion
across
wide
band
for the
best
in
sound
quality
The
POWER
N/OS
FET
features
a
high
input
impedance
and
it
is
a
voltage-driven
device.
These
two merits provide
a
high
power gain
and
a simply
designed
circuit
configuration. This
is
why there
is
minimai
noise
or
phase
drift
which
has
an
adverse
affect on the
sound
quality,
and
also
why
there is
no
deterioration
in the distor-
tion factor.
ln addition,
the
frequencv
response
is excellent,
the
switching
speed is fast,
stable
operation is ensured even in the high
frequen-
ciesand notch distortion
is
dramatically
reduéed.
The design makes
for a
low
distortion factor
across
a wide
band
with a
continuous
power
output of
50
watts
per channel,
min., at
8 ohms
(2O Hz-2O
kHz)
with no
more than
0.A2%
total harmonic distortion.
2.
DC
amplifier featuring
accurate
waveform
power
transmission
The capacitors have been eliminated from
the
NFB
loop and
even
the input
capacitors
have
been
put
aside
in
this
DC
amplifier
to
effect
a
great
improvement
in the
low-frequency-range
phase characteristics. The
first
stage employs
low
noise transistors
with a
high withstanding
voltage
and well-matched pairing
characteristics
for a
reduction
in
center
point drlft.
Further-
more,
there is
a
DIRECT
COUPLING/CON
DENSER
COUPLTNG
switch which
is
pro-
vided
to
safeguard
the
speakers
from
damage
caused
by
DC
component
Ìeakage
from
the
externally
connected equipment
(such as
the
control amplifier).
3.
Connection
facilities for two
pairs of
speakers
This
amplif
ier can accommodate
two
pairs
of
speakers, and
by
selecting them with
the
speak-
ers switch,
you
can
compare
the
sound
re-
produced
by
each system.
You can also listen
to
the
sound
from
one
speaker system
in
one room and from
the
other
speaker systern in another room.
4.
High-reliability protection
circuitry
ln
order
to
protect
the
POWER MOS
FET,
a
current limiter circuit
which
relles
on the
detec-
tion
of
the
area
of
safe operation and
DC
voltage detection circuit
have been
installed.
This clrcuitry
features
high reliability
which
enables
it to
cope
with
accidents.
5.
Large-sized
power
meters (peak
rneters)
The
model
comes
with
large-sized
power
meters
which
enable
the
balance between
the
left
and
right
channels and also
the output
level
to
be
checked. Their
size
and performance
are what
you would
expect
to find
in
a
POWER
MOS
FET
amplif
ier
The power
meters adopt
a
loga-
rithmic
compression
circuit
for
RMS value
dis-
plays
of
the
detected peak values
This
is why
they
can track peak
signals and enable
the
displayed
values
to
be read
out directlV
/^
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