Audio derived AGC for old BA's

[W3RSW]

Some of my older BA receivers have decent CW/SSB receiption if you turn down the RF gain, etc. but my biggest gripe is getting constant loudness in a roundtable situation whereby one signal might blast you out of the room, have distortion from overdrive, etc. and other signals might be so low you have to manually adjust the gain.  Particulary bothersome if your just listening while working on something in the shack and can't ride the rig.

So while perusing Google lookup, I found where one guy used a very simple AF AGC right out of the spkr. (600 ohm) taps of an old receiver.

He simply used a full wave voltage doubler consisting of a couple of silicon diodes, two 3.3uf caps, a 470k loading resistor across the negative voltage output and connected that to the AGC line of the receiver.  Oh, also a 2k pot across the 600 ohm output ahead of the volt. doubler to set the AGC .   Mentioned that the pot. should be several times greater than the receiver output impedance to prevent unnecessary loading.

Anyone every try this circuit?  Sounds simple but I wouldn't expect much more than 20db or so action.  But even that would be great for an old rig.  No comment about pumping, but at only 20db might not be so noticable.

[N2DTS]

I would say get the original agc circuit working right/better.
You can usualy boost agc with just a resistor change, or minor circuit value changes.

I put a pot on the back of the homebrew receivers so I can adjust the amount of agc voltage.
You might also just need to agc an additional stage, some designs are crappy and only do one stage, I did the mixer and both IF amps, I needed no agc amplification, and it works well.

Audio derived agc really works poorly, you get a burst of loudness then it can desense for a while, most of those little qrp cw rigs use audio agc, the K1, the Sierra, and while it works, it does not work well.

You could try it as an experiment, since you have the original agc to limit things somewhat.

A better faster way to go might be to take a line level out and feed it to a hifi amp with a compressor/limiter in line.
I never had a need to do that, but like the line level into the amp, it gives ONE good speaker, a line level recording output, and no restrictions in frequency response along with low distortion.

A very few radios had good built in audio, but most were quite poor, as is anything new, typical to have 1.5 watts at 10% distortion and very limited freq response.

Brett
 

[WA1GFZ]

The problem is your AGC has a slow attack time

[W3RSW]

Well, of course, in the original old receivers, the AGC is grounded in the CW position (SSB not even mentioned or not used much yet in those days) because of the BFO feed.  A decent product detector, IF derived agc, isolated BFO, etc. are all possible but can be pretty extensive mods.

Tons of literature and some experience on picking off IF, op amp AGC circuits, you name it, just wondered if anyone experimented with something as simple as this guy's voltage doubler.

[N2DTS]

Oh, you are talking about cw reception on an old receiver!
You need to do the product detector thing, along with some sort of agc.
Not easy.

As I see it, you have more than one problem, not only does the volume change, but often the bfo will get in the way, to much bfo activates the agc, if you turn the agc off, you often still have to ride the bfo gain or RF gain to keep it sounding fair, plus without gain control, stages might get into the nasty overload parts of their ranges.

Audio derived agc fed back in will work and is worth a try.
Its not going to be like a good product detector/agc setup, but should work well for what you are trying to do.

Two issues, if taken off the output, the volume control will be in the circuit!
The cap size is critical, too big and it takes too long to charge up, and loud signals can desense the receiver for long times, too small and it might motorboat or something.

Put a diode between the new circuit and the agc line so the circuit does not effect the agc action on AM.

For audio derived agc, they take off before the volume pot, amplify, rectify, and put it into the agc line.

Brett

[W1RKW]

I don't see why it would not work.  I have an HW-8 which by design does not have AGC.  When I was doing QRP CW way back when with the HW-8, the lack of AGC was annoying with the front-end overload on 75 especially at night when conditions signal strengths would wander all over. Constant adjusting the RF gain control was a PITA.  It didn't make for good operating and copy.

To fix it, I created a little AGC circuit using a simple RC network/rectifier off of the audio amp before the volume control and fed it back to the front-end FET RF amp to vary its amplification gain. Had to dick with the RC values to get the right effect or delay. I essentially, tied into the RF gain control and fed it from that point and problem solved.

[Opcom]

there is the possibility of audio AGC and I.F. AGC. I pefer to keep the two separate.

[KM1H]

To do it properly in some BA's requires extensive work. Others, such as a HRO 50/60 are a bit easier since it can be adapted to the NBFM option socket.

Id suggest a Central Electronics A or B Signal Slicer; or a Hammarlund HC-10 if you can find an affordable one. Gonset, TMC and GE also made versions but they bring big bucks.

The old ARRL SSB Manuals had many circuits.

Carl
KM1H

[W3RSW]

Pretty much aware of all the stuff discussed.  Looks like Bob, '1RKW's come the closest to trying what I'd read in the old Glowbugs forum.

I just thought a quick, almost outboard AGC might help a lot of casual roundtable listening on an older receiver in CW/"SSB" operation without being a major mod or construction project. I'll let y'all know if it works.  Think I'll try a backwards AF output xfor first to go from 3.2 ohms up to 2000 (since I don't have a Navy RAL  ), then a 10k trimpot, then the diode doubler, etc.

http://www.ibiblio.org/pub/academic/agriculture/agronomy/ham/GLOWBUGS/ATL/Vol01_100_to_149/v01_n105.glo.html

Date: Sat, 6 Sep 1997 18:10:51 -0500 (CDT)
From: Bob Roehrig <broehrig@admin.aurora.edu>
Subject: Cheap/easy/effective AGC

I was playing around with the navy RCH receiver recently obtained and.
as with all pre-product detector receivers, I get frustrated by trying
to use it in the CW mode with no AGC.

In this particular unit, there is conventionally derived rectified IF
AGC in the AM mode, but in CW, the AGC bus is shorted directly to
a negative bias supply (derived from a 50 ohm resistor in the HV
xfmr center tap to ground). In most receivers, the AGC would simply
be shorted to ground.

Not wanting to really modify this unit, I thought I'd try audio-derived
AGC, and it is outstanding. This simple circuit could be applied to
anything from an ARC-5 to a HRO.

The circuit is simply a conventional half-wave voltage doubler. I used
a pair of 3.3uf 50V caps and a pair of 1n4001 diodes. I tack soldered
a 2K pot across the 600 ohm output transformer terminals (one is grounded)
and that feeds the circuit. The output is loaded with a 470K resistor
and connected to the AGC bus. In this receiver, that is accomplished
by removing the wire to the mode switch that comes from the fixed
bias source and connecting the detector output to that switch lug.

I tuned in a fairly strong SSB station on 40 meters, then cranked up the
gain until the signal started to distort, then adjusted the 2K pot
until I started to get AGC action. Thereafter, the gain could be cranked
up all the way with no distortion and all signals in the round-table
sounded identical in level.

AS I said, this same scheme should work with a great many receivers.
The pot value should be a few times higher than the output impedance
to prevent loading. This circuit has a time constant of around 1.5
seconds decay, which works great for both CW and SSB.

        E-mail broehrig@admin.aurora.edu           73 de Bob, K9EUI
            CIS: Data / Telecom   Aurora University, Aurora, IL
                      630-844-4898  Fax 630-844-5530

[k7yoo]

I have used this for years on an R390A. Several individuals used to build the circuit, pot them, and sell them for $25 as an "SSB adapter". Feed the "line" audio into the unit and the output is fed to the RF gain/AGC line terminals on the back of the rx. To vary the action all that is required is to adjust the "line out" level. It works OK and takes all of 1 minute to hook up. If I remember correctly there were even radios made with audio derived AGC (Cubic ASTRO 102,3). It was weird but adequate.
Minimally invasive quick and dirty audio derived AGC is a great idea.
Quick and dirty beats postponed perfection any day!
clip leads rule!
Skip

[n4wc]

I do believe the Collins 618T() uses audio derived AGC.
The more modern HFS 700 and slightly older 628T2A use audio derived AGC.
I've repaired a few of the last two.

Bill Cook 

[K4TLJ]

Isn't all AGC for SSB 'audio derived'? After all there is no carrier.

[W3RSW]

-communications audio over RF is just superimposed audio...   if that's what your getting at, whether there's a carrier or not.    AF on IF... AF on RF.  digital on RF..
... whatever.

There's a big distinction in capturing rapidly changing RF at the IF before all the RC effects of audio amps get in the way.   But I'm ok with the flanging if I can just get some resemblance of constant volume..

Sometimes semantics over-complicate things.

gyr the wab.. babe. 

[K1ZJH]

Isn't all AGC for SSB 'audio derived'? After all there is no carrier.

No, it can be derived from the IF signal. The trick is in keeping the
BFO energy out of the IF strip and AGC detector.   Audio derived
AGC systems mask those problems.

AF derived AGC is more problematic in that is harder to keep the
leading edge from overshooting and "popping" the AGC on the
first syllable. Also, low frequency CW notes are prone to the same initial overshoot problems.

Pete k1zjh