26. Audio Module (fig. 20)
Note. A solid state audio module is used in place of V608 in certain receivers. The theory in this paragraph applies to receivers using a solid state audio module.
The audio module is a transistorized plug-in unit designed to directly replace second af amplifier tube V608. The maximum audio output of the audio module is 1 watt with less than 7.5 percent distortion when operated with a 28-volt dc supply voltage.
- When an audio module is used in place of V608, V607 acts as a cathode follower. The output signal taken across cathode resistor R628 is coupled by capacitor C627 to the base of transistor Q601 of the audio module.
- Emitter follower Q601 matches the
impedance between V607 and amplifier
Q602. The audio input signal is coupled
from the cathode of cathode follower V607,
through pin 3 of the socket to the base of
Q601. The +28 volts is applied to the collector
of Q601 from pin 5 of the module
socket. Resistors R643 and R644 make up
a divider which provides the proper base
bias for Q601. Resistor R645 is the load
resistor for Q601. The output of Q601 is
applied to the base of Q602. The audio output
of Q602 is transformer coupled by T604
to the bases of the push-pull power output
stage which consists of transistors Q603
and Q604. Capacitor C638 reduces the
higher frequency response of T604.
- The +28 volts is applied from pin 6 of
the audio module socket through the primary
of T605 to the collectors of Q603 and
Q604. Resistors R647 and R648 establish
the proper base bias. Resistors R650 and
R651 are in the emitter circuits to compensate
for temperature variations. Resistor
R649 provides a negative feedback
current to Q602 for improved amplifier
gain stability and frequency response. The
output signal from the collectors of Q603
and Q604 is transformer coupled through
transformer T605 and applied directly
through pin connections 4 and 8 of the audio
module base to the primary winding of
transformer T603.
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- 2021 Errata:
- This solid state audio module is known to cause oscillations in some early model R-392 receivers. BEWARE.