Zenith J584W - Page 30

FIGURE 15 - CHASSIS 3WJR52 AUDIO PREAMP
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CHASSIS 3WJR52-AUDIO CIRCUITRY
Figure 15 shows the schematic for the Pre-Amplifier and
related controls. Explanations will refer to the right channel,
but will also apply to the left channel. Audio from the band 
switch will be applied to point "RP", C401 and to the base
of Pre-Amplifier Q401 which is an emitter follower stage.
From the emitter of Q401 the signal will go to the tape output
jack as well as to the high side of Loudness Control R408,
then to the familiar Bass (R41 1) and Treble (R415) controls.
Following these controls is the Balance Control (R416). Con 
nected from each end of R416 is a series circuit consisting of
a capacitor C408 and one section of Hi Cut Switch (SW5).
When this switch is in the "OFF" position, as shown, frequency
response will be normal. On the other hand, when the Hi Cut
Switch (SW2) is in the "ON" position, higher frequencies will
be shunted to ground through SW2 and C408.
Following the controls, the signal is applied, via C414, to
Pre-Driver Q402 and Driver Q403 (See Figure 16). A brief
look at the complementary symmetry output circuit will
reveal direct coupling between the collector of the driver
transistor (Q403) and the base of the output transistor nearest
ground (Q405). Direct coupling will decrease distortion and
provide consistent output measurements. The oiitfillt
design is such that there is a center point between the emitters
of Q404, and Q405. This point has a DC voltage which, under
no-signal conditions, is equal to approximately one-half of
the voltage on the collector of the transistor Q404 nearest
the high side of the supply voltage. Connected to this center
point are:
1. A feedback loop (via resistors R423 and R428) to the
emitter of the pre-driver (Q402).
2. A feedback loop, via resistor R421, to the base of the
driver transistor.
3. An output coupling capacitor C422 to the speaker.
Between the base of each output transistor there is a series
connected resistor-diode circuit (R426, CR401). Bias voltage
is developed across this resistor-diode combination and con 
trols idling current in the output stage. Under no signal con 
ditions voltage drops across the resistor-diode are equal to
base-emitter voltages of the output transistors, plus the vol 
tage drop across emitter resistor R427. Due to the very low
value of the emitter resistor, and the voltage drop across it,
voltage on the emitter can be considered to be same as the
center point voltage. While many refer to this as a class "B"
circuit, it should be called class "AB" due to the idling cur 
rent. Bias diode CR401 has non-linear voltage/current relation 
ships resulting in the following characteristics:
1. Voltage across the diode changes mere ?'? w| y than
current flowing through it.
2. Current is approximately proportional to supply voltage.
3. Voltage across the diode will decrease with rising temper 
ature.