LM386 Amp with AGC

As part of a developing receiver for SSTV, based on the RSGB Centenary Receiver project in the September Radcom , I have developed an alternative AF amplifier for the set providing AGC to give a constant output for differing inputs from the RF stage.

The amplifier is the ubiquitous LM386, used by just about everybody - hell its a very good design, well done National Semiconductor! The amplifier is set to maximum gain by the 10uF capacitor across pin 1 and 8. The input goes to the -ve side at pin 2, the positive input at pin 3 is grounded. The input is via a series 6K8 resistor and a 10uF decoupling capacitor.

The AGC circuit uses a diode bridge. On +ve and -ve cycles the output charges the two 100uF capacitors until the top two diodes start to conduct, when they do they reduce the input so providing an AGC action.

Here's the circuit:



The amplifier give a constant output tot around 0.7V rms, i.e. the forward conduction voltage of the two diode in the +/- control loop.

A bigger output can be obtained by using a potential divider at the output before driving the diode bridge, a potentiometer could be used (suggested value 1k log), to provide a volume control if directly driving a loudspeaker. The output of 0.7V is good for the line input of the USB DAC iMic connected to the iPad for SSTV decoding.
The front end is now in breadboard stage but I am waiting to find the 14230kHz crystals from somewhere...