[Boatanchors] Audio derived AGC ?

[Gary Schafer]

Hi Philip,

One of the rigs I have around here I think has audio derived AGC in it. Been a
long time since I looked. You might check the schematics on the following:
Heathkit HR 1680  or WRL Galaxy 5.
The SBE33 and SBE34 transceivers had audio derived AGC in them (Solid state
units) but it only operated at high signal levels. The one receiver gain control
was really an RF gain control and the audio gain was fixed. The AGC did not
operate unless you turned the gain way up or a strong signal came on when you
had the gain set at a moderate level. It did work pretty well as there was
usually never any AGC action to distort the signal. Very clean audio this way.

There is really nothing to it though. You will probably need one audio stage of
amplification and a diode to rectify the audio. Just feed it to your AGC buss
just like you do with the RF derived AGC. You will need a little lower source
impedance with audio AGC than you need with RF derived. The reason being that
you need to do more filtering of the audio than you do at RF. This means larger
capacitors and longer charge times. A low source impedance from your audio stage
will charge the AGC filter capacitor quicker and prevent overshoot, before the
AGC takes over.

As for the BFO crystal, you might consider hooking up the BFO coil in the
receiver. Then you can adjust the CW beat to any tone you desire.

73
Gary  K4FMX


Philip Atchley wrote:

> Hi.
> Can anyone direct me to some of the old "Toob" transceivers that used Audio
> AGC rather than RF Derived, whose circuits I can download from BAMA?  I
> designed my homebrewed Longwave CW receiver with the usual RF derived AGC
> and it works quite well in that regard.  HOWEVER, the audio stage consists
> of an 8  pole switchable selectivity (yes I used Op-amps) that I want to
> include in the AGC loop.  Right now strong signals outside the audio
> bandwidth can "pump" the AGC.
>
> Yes, I now that "usually" RF derived agc is better but not in this case.
> AND, I know I can design my own AF drived agc circuit but why re-invent the
> wheel?  This will be used for CW ONLY so doesn't have to be shaped to voice
> patterns.
>
> For those interested in such matters, this receiver consists of 10 "toobs"
> and 9 IC's (audio section), solid state rectifers.  It tunes from "zero" KC
> (usable down to about 15KC or less) up to 500 KC.  It was built in the
> "hulk" of a Lafayette HA-350 hamband receiver AND the ONLY thing stock on it
> is the power transformer and the VFO assembly (2nd IF tunes from 3500-4100
> KC) All RF and IF sections have been ripped out and "redesigned.  AF
> converted to SS for heat reduction next to the VFO and to permit the AF
> filters.  It is effectively triple conversion with the product detector and
> audio filters acting as the 3rd IF.  I chose the HA-350 due to it's basic
> circuit design and availability.  I didn't want to fight making cabinets,
> tuning scales etc.  AND the HA-350's tuning dial is 0-500KC, PERFECT!
>
> 73 de Phil  KO6BB
>
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