Pioneer SX-1250 Service Manual page 16

Tone Defeat Circuit
By setting the TONE switch to OFF, the above
described tone control circuit becomes bypassed.
Filter Circuits ( Fig. 16)
These are NFB type active filters with -12dB/
octave slopes. The circuit combines a 2-stage CR
element and emitter follower. Normal function of
the emitter follower is provided in the passband
and by 100% NFB, gain becomes essentially "1".
NFB is cancelled in the area of the cutoff fre-
quency, composing a peak in the frequency re-
sponse. This leads to improved and sharper CR
filter response. In the actual circuit, CR elements
for low and high filters both employ the same
emitter follower and each can be selected ON-OFF.
Power Amplifier
Fig. 17 shows the power amplifier section. The
input stage is composed of Q1 & Q2 differential
amplifier, with the input signal applied to e1. eg
& Q4 differential amplifier comprise the second
stage, with Q3 output composd at Q4 by Qb &
Q6 cunent mirror. Connecting collector and base
of Q5 produces the equivalent of a diode. Q3
collector cunent produces a forward voltage be-
tween Qb base and emitter, biasing Q6. Rl and R2
in this circuit are equal, while Qb arrd Q6 are the
same type transistors. Consequently, when voltages
between bases and emitters of Qb & Q6 are equal,
cunent flows in these transistors also become
equal. This results in equal current flows in Q3 and
Q6. Mutually opposite phase signals drive Q3 and
Q4, resulting in mutually opposite phase collector
cunents. Q4 and Q6 therefore operate as push-pull.
+ f c c .
Fig. 16. Basic circuitry of filters
Power stage bias voltage is supplied by the voltage
drop across D1, D2 and VR2. VR2 is a semi-fixed
resistor for power stage bias adjustment, while D1
and D2 are Varistors for temperature compen-
sation.
A 3-stage Darlington connection is employed in the
power stage (Q7-Q14),
with output transistors
Q11-Q14 in a complementary symmetrical parallel
+ Y c C ,
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Fig. 17. Schematic diagram for power amplifier