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HITACHI HCA-7500 MKII
DESCRIPTION OF THE NEW CIRCUITS
1. DC servo circuit
DC servo involves the detection of ultra-low frequen-
cies only, including the DC componerts which are
generated
in
the
output
of
the
amplifier,
the
amplification of those frequencies, the provision of
negative feedback to the negative feedback input of
the amplifier, and the maintenance of the DC voltage
which is generated in the output of the amplifier at a
stable value of zero potential. It also involves the cut-
off of the undesirable
ultra-low
frequency
com-
ponents.
Fig. 1 shows the DC servo circuit.
The DC and ultra-low-frequency components are fed
into the ® input of the servo amplifier from the out-
put of the amplifier via the low-pass filter composed
of R2 and C1. These components are amplified by
negative
feedback
capacitor
C2
in
the
servo
amplifier, and negative feedback is provided to the
amplifier's © input via R4. The inputs of the servo
amplifier are © and @ differential inputs, and with
DC operation the © input is kept at ground potential
by R3. For this reason, the output of the amplifier to
which the @ input is connected via R3 is also kept at
ground potential.
The area enclosed by the dot-dash-dot line is an ad-
justment circuit. When variations are produced in the
first-stage differential amplifier input ® and © offset
voltage
of
the
servo
amplifier,
they
are
also
generated in the output of the amplifier and so this
circuit serves to absorb the variations in the offset
voltage. ZD1
and ZD2 are 6 V zener diodes with
outstanding temperature characteristics, and these
are used in the regulated power supply.
By using this DC servo
circuit, the model
HCA-
750OMKI
is able to feature
a direct connection
system which does not employ any input or output
Equarizer
or
Tone
amplifier
Ent zerrer-oder
Tonverstarker
Amplificateur
de
tonalité
ow
correcteur
Input
Eingang
Output
Entrée
o Ausgang
Sortie
aoe
@oee
2eg
Servo
amplifier!
Servoverstarker
Amplificateur
asservi
DC
Servo
circuit
Gleichstrom-Servoschal
tung
Circuit
dasservissement
DC
N
oO
n
<=
Lew
me
wwos
en
wane
aaa
eae
we
Fig. 1
Abb. 1
capacitors
in
the
equalizer
amplifier
and
tone
amplifier.
. Phono selector muting circuit
The equalizer amplifier in this unit is designed to
allow the gain to be changed when an MC or MM
cartridge is used by setting the PHONO
selector to
the appropriate position. In addition, the use of the
DC servo circuit permits the employment of a direct
connection
system
with
all
input
and
output
capacitors removed.
This means
that the popping
sound
heard when
the PHONO
selector
is set, is
quite loud. Therefore, in order to prevent this popp-
ing sound, a muting circuit is provided which is ac-
tivated immediately before the gain is selected. Fig. 2
shows the muting circuit.
The input of the muting circuit is connected to the
PHONO
selector. Normally it is grounded and only
during that instant when selection takes place does it
open. Normally, bias is applied to the base of Q1 via
R5 and R6, Q1 turns ON and Q2 turns OFF. But when
the input of the muting circuit is open, bias is applied
to the base of Q3 by R4, O3 turns
ON and
C1
discharges through R6. When this happens, Q1 turns
OFF and Q2 turns ON, the relay is energized, the out-
put of the equalizer amplifier is terminated to ground
and
a
muting
effect
is
produced.
Once
C1
discharges, C1 is charged through R5 and only dur-
ing this time is the muting effect available.
The
muting time is set to between about two and three
seconds.
The muting effect is designed to be activated when
the PHONO
selector is switched between ''MM 22
kQ", "MC 2.5 mV" and "MC 0.1 mV". It may very
seldom be activated between other positions due to
chattering in the sliding elements
of the PHONO
selector but this does not indicate a malfunction.
Equarizer
amplifier
Entzerrerverstarker
Amplificateur
correcteur
Input
Output
Eingang
Ausgang
Entrée
Sortie
Re lay
Relais
Relai
+B
Input
of
the muting
circuit
(to
Phono
selector)
Eingang
der Gerduschsperre
-————-©
(zumPlattenspieler
wahischal ter)
Entrée
du
circuit
de
sourdine
(vers
sélecteur
phono)
Fig. 2
Abb. 2
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