I have implemented this circuit which is supposed to do AGC and filter out the signal between 20 Hz-20 kHz, but it seems have some problems. The bandpass filter works well, but the agc circuit doesn't have a different gain for different input voltage amplitude. Theoretically, the gain of this circuit should be 1+R3/Rjet. Could someone help me with this design?
I'm not saying that there aren't other problems but....
So, if you fix this, the op-amp will be able to create a negative voltage output that can be rectified by D1, smoothed by C4 and, be able to properly control the n-channel JFET as a "variable" resistor. As you currently have it, there can be no control and no AGC.
I would also suggest that C5 is shorted out.
One more issue to watch out for; where you have R3 (the op-amp feedback resistor) connected could cause problems. You should probably have it directly from the op-amp out to the inverting input (as opposed to after the filter).
