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880 Series Audio Distribution Amplifiers Instruction Manual
Chapter 12: ATG-880 Audio Tone Generator Module
Preliminary—Contents are proprietary and confidential. Do not photocopy or distribute.
Control Loop
The control loop converts the signal to a DC voltage proportional to its level by
means of the full-wave rectifier and integrator consisting of IC4B and IC4C.
During the positive half cycle, pin 7 of IC4B goes low, turning on diode D4 and
thus, holding pin 7 to zero volts. As pin 6 of IC4B and pin 9 of IC4C are also at
virtual ground level, the cathode of diode D5 is also zero volts. During the
negative half cycles, pin 7 of IC4B goes high, turning on diode D5 and off diode
D4, thus causing negative half cycles of equal but inverted amplitude to appear
at the cathode of diode D5. This signal is combined with the original in a two to
one ratio by summing amplifier/ integrator IC4C. The resultant DC voltage at
pin 8 of IC4C is proportional to the peak voltage of the oscillator.
The negative DC voltage from Zener diode IC7 is also added at the summing
point of IC4C at pin 9 through the 20K LEVEL trimpot to adjust the signal
amplitude.
The positive DC voltage at pin 8 of IC4C biases the anode of diode D1 through
a 4.7k resistor and the cathode of D2 through voltage inverter IC1A and a 4.7k
resistor. The bias current controls the dynamic resistance of diodes D1 and D2,
thereby controlling the net resistance from pin 13 of IC4D to ground and thus,
maintaining the gain of IC4D at 3.
