Zenith J584W - Page 31

FIGURE 16 - CHASSIS 3WJR52 AUDIO OUTPUT
This non-linear diode voltage/current curve is similar to that of
the output transistors allowing a near linear temperature
relationship to exist between diode and transistors. Under
no-signal conditions, both the diode and transistors will show
similar effects due to any temperature change. If there is an
increase in temperature, both the diode and transistors will
show decreased diode and base-emitter voltages. The reduced
diode voltage will offset decreased base-emitter voltage in the
transistor and stabilize bias current. Since this diode has a low
AC resistance, any changes in current of the driver stage re 
sults in less effect on the output stage than would be true
with only biasing resistors.
Operating conditions for the driver transistor are determined
by base-emitter voltage and driver transistor beta. Also effecting
the operating point are values of the driver base resistor, feed 
back resistor to driver base, and two resistors in the driver's
collector circuit. All these factors will effect voltage at the
previously mentioned center point.
When a negative going signal is applied to the driver base,
current gain of the driver is reduced. At the same time, how 
ever, the charge on electrolytic C420 will maintain a near
constant current through resistor R425 connected in the base
circuit of the upper output transistor Q404. This will cause
voltage across the resistor to maintain an almost constant
value, which in turn determines base-emitter bias of transistor
Q404. Notice that current flow from this bias resistor not only
goes through the diode and collector circuit of the driver
29
transistor but also goes to the base of transistor Q404, causing
this output transistor to conduct. When transistor Q404 is
conducting, Q405 is cut-off since it is reverse biased.
When a positive going signal is applied to the base of the
driver (Q403) its collector current will increase, electrolytic
C420 will still maintain a near constant voltage across the bias
resistor of the upper transistor. Under these conditions the
upper transistor Q404 is now reverse biased to cut-off, and
the lower transistor Q405 is conducting.
Since each output transistor ’ conducts on alternating halves
of each cycle, voltage change at the center point will result
in an output signal that will duplicate input. Use of com 
plementary symmetry circuitry results in lower distortion and
better frequency response.
CHASSIS 1WJR55
Chassis 1WJR55 circuitry is similar to that of Chassis 3WJR52
(1WJR55 has a bipolar FM RF) however, new audio circuitry
will be found in Chassis 1WJR55, in the form of separate
integrated circuits in the output of each channel. This IC
circuit provides improved performance in a small package,
compared to prior low power output chassis (such as 1WGR50
introduced two years agoj. Bue to high internal gain of this IC,
more feedback can be applied, resulting in lower Total Har 
monic Distortion (THD), improved signal to noise ratio
(S/N) and better thermal tracking.