I'd like to use this AGC circuit to mix some CCTV mic's together. They have a built in powered pre-amp next to the condensor mic module, which is powered with 12v dc and the output signal is 1v pp.

Does it matter whether the P1 pot is linear or log?

I want to take the output from P1 for setting the master volume level, then split that into 3 or 4 mono feeds, each with seperate pots for matching what I feed each signal into.

I can then use P1 as a master volume control to set the maximum output levels for the other audio feeds split off from there.

Each seperate audio feed after P1 will also have it's own pot, to match the sensitivity of each device I feed the signal into.

I'd also like to power this AGC amplifier using the same low noise 12v dc mains power supply that I will be using for the CCTV mics.

 

The thread you hijacked is 6 years old - http://forum.allaboutcircuits.com/showthread.php?t=1767

Better to start with a new thread.

 

The old thread also had many errors.
The output resistance of an opamp that has a 10k load is not 10k ohms. It is extremely low (0.04 ohms or less) at low audio frequencies.

 

What is the purpose of the diode being placed across the transistor emitter and base for this circuit?

http://archive.electronicdesign.com/files/29/6272/figure_01.gif

"An extra diode (1N914 or 1N4148 etc) across the transistor base & emitter (cathode to base) would both improve the effectiveness of the level control & remove the bit of DC offset. "

Is this creating some sort of clamping circuit?

I know it's an old circuit, but I have got most of the parts ordered now. Let's see how my prototype works. I've added a few more components to it around the FET as well, as suggested by others that have looked at this circuit.

 

The base-emitter of the transistor is a diode that quickly charges C3 with a polarity that causes the transistor to turn off when the signal is continuous which is wrong. With an added diode connected the reverse polarity to the base-emitter diode then C3 charges through the base-emitter diode then equally discharges into the added diode so C3 does not get charged by the signal and the transistor follows what the signal is doing.

 

Thanks for that AudioGuru. I understand why the extra diode is required now, and will report back once I have built the circuit. Parts are in the post on their way!

Here's a neat parts supplier that I got my Pots and control knobs from:

http://www.futurlec.com/PotRot.shtml