40m Direct Conversion Receiver - modification for low signal levels

Pleasing to see the level of interest this project has generated. There has been an on-air problem which does not show up in bench testing and the explanation and correction for that follows.

Issue:
The AGC does not compensate for very weak signals. This is only noticed in very low noise  environments and requires manual adjustment of the volume control to hear these weak signals.

Explanation:
The generation of the negative rail for the audio stages is injecting ripple onto the plus and minus 12V rails. This ripple is inaudible but the decoupling is not sufficient to prevents the AGC from believing it is a valid audio level. Weak received signals reaching the AGC which are lower than the ripple are not benefiting from the AGC increasing the audio gain to compensate.

The issue was overlooked on the bench when weak signal testing because the volume control had been manually adjusted.

Correction:
The decoupling of the 12volt rails by the 68ohm and 22uF capacitor is insufficient. To reduce the voltage drop that would occur if R was increased a capacitor multiplier using a transistor is used. The 68 ohm resistor is changed to several k and this RC connection is made to the base of a transistor.


See sites like  https://www.electronics-notes.com/articles/analogue_circuits/transistor/capacitance-multiplier-circuit.php for an explanation of how this works.

Note that a npn transistor is used for the plus 12v rail and a pnp transistor for the negative 12v rail.
http://electronics-diy.com/electronic_schematic.php?id=486 shows how the emitter and collector connections are reversed for the pnp transistor on the negative rail.

Outcome:
The AGC action now includes very weak signals.