Power Supply Circuit - Pioneer SA-9500II Service Manual

2. Overload Detector Circuit
Shorting of the power amplifier load or a load
impedance below the specified value causes a
command to be sent to the relay drive circuit.
This is illustrated in Fig. 9.
Fig. 9. Basic circuitry for overload detector
With the output stage in class B operation, when
Qa is operating in the positive half cycle, Qb
becomes cut off and the signal current flows as
indicated by the solid arrows in Fig. 9. Point D
potential at this time is the point A potential
divided by Rt and R3. Also, point C potential is
the point A potential divided by RE1 and RL
(load). Point D is connected .to Qr base and point
C to Qr emitter through R2 and REz. When RL is
extremely small, the point C potential becomes
considerably lower than point D. This potential
difference forward biases Qt. Qt tums ON and
current flows in D..
Qb operates in the negative half cycle and ea
becomes cut off. The signal flows is indicated by
the broken line arrows in the center of Fig. g.
Qr is biased by the potential difference between
point C and point E. If RL is extremely small, the
point C potential becomes considerably higher
than that of point E. Q, turns ON and current
flows in D..
If large current flows in Qa and Qb, Q, becomes
ON due to the RE1 and RE2 voltage drops, and
current flows in Dr. C, prevents faulty operation
due to external noise.
3; Center Point Potential Detector Circuit
If a DC potential is produced at the junction point
of the power amplifier, a command is sent to the
relay drive circuit. Fig. 10 shows this operating
principle.
Q, and Qa compose a differential amplifier. When
the same input is applied to both input terminals
(Q. and Qo bases), no output is present. However,
if there is a difference between the terminal inputs,
the difference is amplified and becomes the output
between the two collectors. During normal opera-
tion, an AC signal only is present at the junction
point. As C2 reactance is sufficiently low, the same
signal is applied to Q. and Qa bases, resulting in an
absence of output at the collector sides.
When a DC potential is produced at the junction
point, it becomes the input of Q, only. If the
voltage is negative, Q, collector current declines.
and at Qa the collector current increases and the
potential drops, causing current to flow through
D o .
If the DC voltage is positive, Q, collector current
increases and the potential drops, while at Qa the
collector current decreases and the potential rises.
Current therefore flows through Ds.
Power
ornpi ifier
F i g . 1 0 . B a s i c c i r c u i t r y f o r c e n t e r p o i n t
potential detector
5 . 5 P O W E R S U P P L Y C I R C U I T
In the power stage, power is supplied independ-
ently to both left and right channels, using 2
separate power transformers. Separate bridge recti-
fiers (one for each channel) and 1-2,000pF (x 2)
capacitors to provide the plus and minus voltages.
Power for other sections is supplied via a bridge
rectifier connected in series to separate windings
(not those used for the power amplifier) feeding
plus and minus voltage regulator circuit.