general purpose audio AGC/limiter (for a regen, crystal set, etc)

[WA6TKD]

Anyone recall a QST construction article (late 90s maybe) that had a super-regen circuit using a opto-coupler for the main RF regeneration feedback (rather then a coil tap)? I always hoped to build that circuit but misplaced the magazine. I always thought that had promise to get even better performance out of a regen receiver circuit.

[KX6MWS]

That sounds really neat.

Hey, instead of using a coil and a capacitor, just separate the LED and phototransistor with 40 meters of fiber optic cable.

[WA7KKP]

AGC was added to superhets . . . an easy thing to add another detector diode, and add some RF filtering and several hi-z resistors (usually in the megohm area) and feed the DC back into the grids of the RF/IF stages.

The problem with regenerative detectors is that there is no straightforward way to derive or apply AGC to them. Dittos for s meters. The usual S meter on superhets are usually in a bridge circuit that measures the tube current on a low IF stage when AGC voltage is applied.

Start studying some of the really old ARRL or W6SAI Handbooks. There's more info in there for you to pick up on.

Gary WA7KKP

[KX6MWS]

In case anyone's still interested, I did find this regenerative design that uses the audio level to adjust AGC:

http://www.ke3ij.com/AGC-80.htm

It's given me some ideas. I was going to apply the AGC purely to the audio stage; but why not apply it to the rf stage, which is where everything starts. I've finally had some time open up and I can go back to playing with this stuff again.

[KA5S]

I've been using a simple AGC circuit added to a LM386 amp which was derived from an "Ideas for Design" found in issue of EDN (Electronic Design News). It adds 8 parts to the basic LM386 circuit. This circuit limits the output to about 400 mV peak (800 p-p). This is produces a reasonably loud signal, but doesn't "blow your ears out" when using headphones.

1, p-channel J-fet, (2N3820 or similer)
1, 2N3094 NPN or similer
1, 22 K resistor
1, 15K resistor
1, 220K resistor.
1, 1 Meg resistor
1, 1 ufd cap,
1, 0.1 ufd cap
...

Similar to a passive one in CQ for June 1970. Posted that some time ago [except for schematic JPEG below]. I knew a lot less then than I do now, so it was more like a soft limiter:

w1cej audio compress.JPG

Cortland
KA5S