Need AGC/Thump eliminator schematic

[WA6MHZ]

Trying to use Modern headphones on my 1965 Hallicrafters SR-400 became very painful at times. I would be listenning to a station WAY down in the noise with the volume turned way up, and suddenly a LOUD station would come on blasting out my eardrums. For some reason, there is no usable AGC in this radio! Also, going from Transmit to Receive I get a loud THUMP in the headphones which also hurts my ears.

So I am looking for an OUTBOARD AGC circuit I can put between the headphone jack and the headphones that will also soften that loud THUMP I hear on TX/RX.

Haven't been able to find just such a circuit on the internet. Anyone have any ideas?
Battery powered will work.
73

[AB9LZ]

I don't think you can ever really get around the thump with audio derived AGC, most modern(er) rigs start upstream and use an RF derived scheme (look at the elecraft K2), or simply do it within the DSP computer.

73 m/4

[KO6WB]

The audio DSP units may have the AGC you desire. I know the MFJ-784B does and it's a good AGC circuit in it. There is another approach that you could simply turn off the headphones during that time and then have them return to receive after the unit has switched.
There is supposed to be an AGC circuit for the SR-400 and it is obtained off the 1650Khz IF. During transmit the AGC line is grounded. The AGC amplifier V8A is the start to the circuit. If you don't have any S-meter movement then yes you have no AGC but if the S-meter is working then you should have some AGC action and maybe it just doesn't have the dynamic range to operate for low signal levels. It should work though.
One thought is to have a voltage controlled op-amp that has a ramp up on switch over. This would get rid of the thump as for audio derived AGC the newer circuit have a better reaction time and some even put a tiny delay on the audio going to the amplifier. This way the audio can hit a high level and be corrected to a proper level before the audio amplifier. That way all the audio is at the same level regardless of the signal strength. At the present I do not have a schematic diagram of such a circuit but I know it has been done. Keep looking Pat, they are out there.
73
Gary

[KO6WB]

Okay Pat, looked around for some ideas and came up with this IC which you can setup a delay giving the level detection circuits time to respond to the audio level in and send a control signal to the audio amplifier.
Look here; http://www.holtek.com.tw/pdf/consumer/8972v100.pdf.
Or maybe something a little old school will work; http://electronicdesign.com/article/...-1-bit-adc8056.
There are others that can be used.
Here's a voltage controlled op-amp circuit; http://electronicdesign.com/article/...controlled-amp.
This also; http://www.analog.com/en/all-operati...s/product.html.
And somemore ideas; http://www.edn.com/design/analog/433...ntrol-response.
There are simpler op-amp that can be controlled by voltage. I just did a quick search for them. A more in depth search will give you a single op-amp that has this capability. I know these were around in the 70's.
Good luck
73
Gary

[WA6MHZ]

Thanks,Gary! I will check those out. The S-Meter does work on the SR-400, I figured ti had something to do with using modern headphones with an old radio designed for old headphone types. Maybe they weren't as sensitive back then. I had a problem with the Heathkit SB-301 in that manner and had to put in a 600 ohm to 8 ohm transformer to get audio. Will print out those circuits and try them out.
thanks and 73!

[KO6WB]

The older headphones had very poor low frequency response. You could have the power supply filter capacitors going bad but if you were using headphones from long ago, you would not hear the gradual increase in the 60 or 120Hz hum until it was really bad. Now the older headphones did respond to a pop which was always fun.
73
Gary

[AC0OB]

I would check the circuitry and tube condition around V8A such as R46, CR28, R47, C185, and the -125 Volts bias.

Phil

[QRP-GAIJIN]

If you just want to protect your ears, a simpler solution might be a diode limiter across the headphones. Two back-to-back diodes and a series resistor should protect your ears against dangerously loud pops or signals. Very simple and requires no power.