Solid State Guitar Amp Forum | DIY Guitar Amplifiers

I have cooked up a compressor design that uses 3 2N7000 NMOS FETS and one op amp. Its very easy to build and uses a LED as a peak detector and compression level indicator, as it lights up with varying intensity proportional to compression level.

Using a MOSFET as a voltage controlled resistor requires very small signal levels or it will cause distortion. To solve this, I employed negative shunt feedback to buck the input signal at the input node. This results in signals in the range of ten mV across the voltage controlled resistor.

LINK to Schematic:

http://circuitsaladdotcom.files.wordpress.com/2012/10/simple-compressor.gif

Cool!   :tu:

Note in the attached the 100k and 0.1uF between Drain and Gate which I think is to address this distortion problem.

Yes - nice observation - there the JFET is being linearized by feeding back some of the drain voltage to the gate of the voltage controlled resisitor(VCR). This improves the drain to source voltage limitation but primarily makes the change in resistance more linearly proportional to the control voltage. In my circuit, the feedback is not to the VCR but to the input of the amplifier. It is what is called shunt feedback because it lowers the impedance of the amplifier input, thereby reducing the signal across VCR substantially.  However, in your example circuit, the voltage across the VCR is also reduced by the ratio of the feedback resistors across the op amp - so the same idea applies.

I have refined this design somewhat including the use of the more common LM358 op amp. I also changed the closed loop peak detector to an open loop compensated one with good results. I have attached the schematic. Here is a link to a youtube demo of the compressor in action.http://www.youtube.com/watch?feature=player_detailpage&v=iEcPEYB0Wao (http://www.youtube.com/watch?feature=player_detailpage&v=iEcPEYB0Wao)