Sunday, November 13, 2016

AGC for the BITX

Many BITXers have longed for a simple AGC circuit. So here it is, it just fits all around the volume control. Here is the circuit :

The Audio-In from the original volume control comes into the circuit on the left and the Audio Out is sent to the volume control's hot end. The 2N7000 and the drain resistor together form a volume control. The tapped down audio is amplified by the transistor (use any NPN transistor) and rectified into a DC voltage by the diode. As the incoming audio level increases, the bias on the 2N7000 goes up and the drain-source resistance drops, bringing the volume down. Below is my ugly version of this AGC. I built it in a few minutes. For the last few hours, it has been fun using it.

8 comments:

  1. ¡Thanks Peter. Very good thing! Regards. LU1AR

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  3. the AGC action can be improved by replacing the 100k resistor connected to the anode of the diode with another diode. Cathode of new diode connects to the anode of the existing one

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  5. I need a circuit that uses only npn transistors, as I have no FETs other than a few IRF510's, and those I got to possibly make an amp, and replace the PA if needed.

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  6. "The Audio-In from the original volume control comes into the circuit on the left and the Audio Out is sent to the volume control's hot end. "

    The original wiring control I presume is the potentiometer? If so what color wire pin on the pot are we talking about? Would appreciate help and advice to mitigate any damage wiring up.

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  7. I, too, would appreciate a clarification. I tried this two ways: one intercepting the yellow wire to the pot (using it as input, with output to the pot terminal). The second was intercepting the orange wire (to the pot wiper terminal) and using the pot wiper terminal as input, the orange wire as output. I liked the results of the second option best, but would appreciate clarification of the original design intent. Thanks!

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  8. With my hookup as noted in the previous post, I found this circuit doesn't have enough dynamic range. I built the circuit at this link, and it works very well. I measured 38 dB of attenuation. I used a 600:600 ohm transformer to feed it the audio signal, and I tweaked R1 and R2 for my headphones. Works great. https://hackaday.com/2014/06/29/automatic-audio-leveling-circuit-makes-scanning-more-fun/#more-125567

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