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7 . C I R C U I T D E S C R I P T I O N S
Equalizer Amplifier
The 54,-610 features a 3-stage direct-coupled
equalizer amplifier for greater reduction of noise
and distortion.
Besides the use of an ultra low-noise transistor
(25C26O2) in the first stage, the adoption of low
impedance input resistance and equalizer circuit
has contributed to an S/N ratio of at least 82dB
(at 2.5mV input, IHF-A).
Power Amplifier
This all stage direct-coupled pure complemen-
taly SEPP circuit features a current mirror load
differential amplifier in the first stage, and incor-
porates the tone control circuit in the NFB loop.
Although the incorporation of the tone control
circuits in the power amplifier stage reduces the
number of elements that the signal has to pass
through, and thereby further reduces noise, dis-
tortion, and cost, the power stage does require a
higher gain and a higher degtee of stability. In the
SA-610, this high gain and high stability
are
achieved by a cunent
mirror
load differential
amplifier in the first stage and by the use of a
constant current circuit for the load of the pre-
driver stage. Furthermore, the tendency for the
pre-driver stage capacitance impedance load to
cause deterioration
in the high end frequency
response is suppressed by inserting a capacitor (C)
between the emitter of the pre-driver stage (Q2)
and the base of the constant current circuit (Q1) as
shown in Fig. 7-1.
At frequencies where the
reactance of this capacitor may be ignored, the
push-pull action of Ql and Q2 serves to counteract
the high end frequency response deterioration.
The power amplifier stage is a complementary
2-stage' Darlington
connection,
resulting in an
output power rating of 45W + 45W (8Q, 20Hz-
2OkHz\, harmonic distortion of less than O.O37o
(20H2
20kHz at rated output),
and output
power bandwidth of 5Hz - 50kHz (0.O3% THD).
Certainly a superb performance for an amplifier
of this class is obtained.
/. Constant
current
/
c i r c u i t
)to
o,,u.,..r"n"
\Pre-driver
staga
siA-61 cl
Protection Circuit
Besides protecting the speakers if a DC voltage
should happen to appear at the power amplifier
stage outputs, this circuit also mutes the signal
path when the power switch is turned on and off
(See Fig. 7-2).
If for any reason a DC voltage (in excess of
about tbV) should happen to appear in the output
of the power amplifier stage, it is detected imme-
diately by either Q23 or q24. qn
is turned on by
a positive voltage, and Q24 by a negative voltage. In
either case, Q25 is also turned off, thereby opening
the relay contact to disconnect the power amplifier
stage from the speakers. ,
The muting action when the power switch is
turned on is achieved by delaying the rise of the
Q25 base potential by means of the R95/C77 time
constant circuit. When the power switch is turned
on, C77 is charged up via R95, thereby increasing
the voltage across both ends of this capacitor.
When this voltage exceeds the zener voltage of the
D15 zener diode, Q25 is biased in the forward
direction, and is thereby tumed on to close the
relay contact.
When the power switch is turned off., C77
discharges rapidly via D18, resulting in Q25 being
turned off, thereby opening the relay contact.
L C H S P O U T
R CH SP OUT
Fig. 7-2 Frotection circuit
!f'
r l
I
f,
I
i F
Fig. 7-1 Pre-driver stage
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