Simple AGC with opamp

It is possible, but not with this circuit. From time to time it shows up again, but it only distorts, boosts the bass and attenuates the treble. Don't waste your time with this crap. It is usable as a Fuzz-Box only. You can instead use a FET as a variable resistor and drive the gate with the rectified output voltage. It is hardly more complicated and works, but not for 72dB of input range. You can also combine a CDS-Photoresistor and a LED for the same purpose. If you want high quality, use a VCA and some RMS detector. Google for "THAT4301".

thank you

As Ban said, its crap and 100uV to 1/2V is one hell of a dynamic range, but try CdS cells from

martin

Hey, don't knock the log amp! This is exactly what merco is looking for, albeit with some refinement. Maybe google could help:

To Merco - for that kind of input range you're better off with a log amp, rather than AGC. Its transfer function is something like

Vo = A * ln(Vi)

Cheers! Rich

The TDA7052 is a low cost, 8pin dip, 1Watt audio amp, with 80dB of DC controlled gain. Great fun to play with for all manner of LF jobs. Also look at the LM4865 and TEA7530. john

I wont discuss the merits of the circuit, but in general you can get such circuits to run on a single supply by just omiting the negative supply and conecting it to 0v, but you then have to generate a reference wich is half the supply to use in place of 0v in some parts of the circuit, in this case just for the base of the transistor (a simple resitor divider would do here), but you also need to use ac coupling on the input and outputs as they will no longer be at 0v.

Colin =^.^=

Actually it's -A * ln(Vi). It won't work anyway, the compression is asymmetrical. Try taking the log of a negative number. Even adding an offset to keep the signal positive will distort badly. Graph input/output on a log transfer curve and you'll see.

One that might work, if you can take the distortion, is the htan function of a diff pair, posted by either Kevin (where's he wandered off to?) or one of the Johns some time ago. I can't recall who right now. At least in Spice it looked pretty neat.

- YD.