Pioneer SA-708 - Power Amplifier Circuit; Indicator & Protection Circuits

4.3 POWER AMPLIFIER
The
first
stage
is
a
differential
amplifier
formed
by
twin
transistor,
while
the
load
circuit
forms
a
current
mirror
circuit.
The
current
mirror
serves
to
make
this
stage
operate
in
push-pull
mode,
thereby
eliminating
the
even
numbered
harmonics,
and
doubling
the
gain.
The
pre-driver
stage
is
a
Darlington
connection,
while
the
load
circuit
forms
a
constant-current
source,
thereby
obtaining
a
high
voltage
gain.
The power stage is a Darlington connection
pure-complementary SEPP circuit, employing an
high speed transistor. The high speed transistor
is a kind of IC consisting of a number of
small
transistor
being
connected
in
parallel
via
an
emitter resistor. This provides excellent high
frequency characteristics comparable to those of
a
small-signal
transistor.
Furthermore,
because
there is no time constant in the NFB circuit in the
low-frequency region, amplification is possible
down to DC (DC inputs will be cut off, however,
by the input coupling capacitor of the power
amplifier.).
The circuit features described above provide an
extremely wide power frequency range (65W +
65W, 20Hz to 20kHz, THD 0.02%, 82).
4.4 INDICATOR CIRCUIT
The SA-708 output power and function indi-
cators feature fluorescent indicator tube (FL tube).
In this tube, thermionic emissions from the cath-
ode are accelerated into the fluorescent substance
of the segmental anodes, resulting in the emission
of light. This tube is used to indicate numerals,
letters, and other symbols.
IC (TA7T3IBP) 172
COMPRESSOR HOLD &
H DETECTOR
SIGNAL INO (6) (40 dB) DC AMP
SA-708
An outline of the FL tube drive circuit is shown
in Fig. 4-1. The output circuit signal is applied to
pin
no.
6
(4)
of
the
IC
(TA7318P-A).
The
IC
con-
tains a detector circuit, compressor (40dB), and
peak hold circuit for both left and right channels.
The dynamic range of the signal is thus contracted
by 40dB to obtain a “‘peak held’’ DC voltage.
The output power indicator segments of the FL
tube are driven by the HA12010 ICs (one for each
channel) equipped with 12 pairs of differential
amplifiers. These amplifiers are biased at increasing
levels, so each amplifier will commence to operate
separately as the input level increases. And since
these amplifiers apply the voltages to the output
power indicator segments, each successive segment
will light up in turn as the input level rises.
The function indicators are lit up as a result of
a voltage being applied to the corresponding func-
tion indicator segment according to the selected
positions of the FUNCTION switch.
4.5 PROTECTION CIRCUIT
The purpose of this circuit is to protect the
speakers and the power amplifier. The relay in the
output circuit is automatically opened in any of
the following cases;
1. During the “transient operations’? when the
power supply is turned on and off.
2. Upon detection of an overload caused by a
short circuit in the load.
8. Upon detection of a DC voltage in the output
caused by component failure or accident.
1¢ (HA 12010)
SHIFT
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Pe
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ee
SHIFT
CONSTANT
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aa
||
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OFFSET
COM~
CURRENT
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v
¥
ey
\/
\/
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PENSATOR
@)
(4)
ica
i
(9)
(0)
(i!)
(2)
(3)
(4)
eae
FLUORESCENT
INDICATOR
TUBE
A
TDN
OO
BHONO
FILAMENT
°
FUNCTION AUX
Fig. 4-1 Indicator circuit