Re: Audio AVC
This thread has been very educational for those of us who haven't dived into the front ends of a radio nearly as reply as we've done on the output or other RF sections. Thanks, to all.
For what it's worth, I present here the Crowbar Method. It incorporates the best of a quest for maximum sensitivity along with absolute overload protection. It began when I saw a post here for a sideband RF preamp from partsandkits, an item that I've added to both ubitx projects here. My first stage of AGC begins with that board.
See here, the underside of the Zombie Apocalypse Survival HF Go-Kit -
Under the mainboard, the P&K he preamp is mounted convenient to the RX signal path.
It's a pretty simple matter to add a 2N7000 or similar FET to short out the incoming RF signal (via a small ceramic cap, for a little isolation) and to use rectified audio from a LM386 to provide clamping drive. The FET starts clamping pretty well with about .8v so experimentation with feed resistors and a touch of drain bypass was needed.
It so happens that I had already installed a kit board ND6T-style AGC which was working OK on its own, but I still wanted a preamp for use with spotty woodland-based aerials during the upcoming apocalypse of the walking dead. So, that's why the Crowbar here is what I called a first stage while the regular AGC is now a second stage. The bonus is that, being as I'm using the i2c-2nd-Nano S-meter mod, I do not require the ND6T board's S-Meter output for that job.
I decided to use that output to replace the 386 clamping source and, somewhat surprisingly, there is no fighting or oscillation between these two "stages" of RF level control (and the dynamic range is stupid wide!). The preamp 1st stage has about no delay while the 2nd is on a medium-fast timing.
Schematic available if wanted.