Simple Microphone Preamp with AGC

[alphabeta]

OK, this may be a bit off topic, but I hope someone can help me here.

I am looking for a simple preamp circuit with AGC to boost the output from an electret microphone to something like line level (or even a bit higher).

It's going to be part of a sound-to-light type unit, so audio quality isn't a key criteria. I need AGC because the circuit will had to adapt to different ambient sound levels without intervention.

Oh and if it works on a single 5V supply that would be even better!

Any suggestions/links would be appreciated.

[lineup]

This project idea below
for Audio AGC uses 1 OP-amp and 2 transistors, 1 JFET + 1 bipolar.
This is a common way using one JFET transistor.

In this circuit JFET Q2 acts as variable RESISTOR
and R2 120k and this JFET acts as a voltage divider.
This input signal is amplified by OP-amp.

The output of OP-amp is fed to Q1
which change resistance of JFET to adjust level of output.

The resulting audio output is from potentiometer P1.


As the input needs 40 mV, you would have to put a mic amp before this circuit.
There are many mic preamp around, a search here at diyAudio
would give you many options.

For practical components selection, I am sure there are many nice people here
that are willing to assist you
if you want to build this project.

Effective AGC Amplifier Can Be Built At A Nominal Cost

[alphabeta]

Thanks. I had also found that circuit which does look the business. I just don't have a suitable FET in the parts box at the moment, and I was hoping to avoid having to buy any new bits!

[lineup]

You better buy yourself some NPN and PNP JFETs
they are quite useful in many ways

In this case most any P- JFET substitues for SJ176 will do well, I think

[sreten]

maplin used to sell a chip that does exactly what you want,

think it was intended for the mike in a ham / cb radio.

/sreten.