There is another builder/designer Jon who has experimented with a number of my compressor circuits. He recommends a lower value for R1 of around 22k instead of the 100k in the schematic. This will lower the max compression a little, but will help with stray capacitance roll-off of high end.  It should work fine.  It will load the guitar pickups a little with heavy compression. Also the diode peak detector is biased in a more linear range by changing R8 and R6  from 470k to 47k and C7 from 1uF to 10uF. I think these values will work a little better.

Over my choices are:  100K for R1,  47K for R6 and 10uF for C7

I have been playing with all sorts compressor ideas and the circuit below is about the simplest thing I have come up with that works well. It uses just a handful of parts and needs no critical adjustments. It has fast attack and slow decay – which I find the most useful  for guitar.

here is the schematic:

https://circuitsalad.com/wp-content/uploads/2013/03/simple-opto-comp1.gif

Additional notes here:

https://circuitsalad.com/2013/03/30/some-possible-value-mods-for-the-simple-opto-compressor/

link to ExpressPCB layout:

http://www.fileswap.com/dl/PFYBP3Gwp4/

It utilizes a number of features from my other designs and I have also integrated some suggestions from other designers (like using a darlington for the amp stage).

This design has the folowing features:

a  H11F1m OPTO FET variable resistor to control gain.

a darlington transistor as a shunt/shunt feedback amplifier. The biasing and feedback (from the collector to base) is critical to the design. The reason is that it keeps the voltage across the OPTO FET very low so as to keep it’s behavior linear. With out this feedback – there can be distortion.

LM358 beef stew op amp as a compensated diode peak detector and as a current mode amplifier the drive the H11F1m OPTO FET variable resistor.

The LED indicator is tied to the compression feedback loop and indicates compression level with variable brightness.

Demo  comming soon.

Another designer tried my mosfet compressor using a darlington pair as the gain stage  device Q1. He described some improvements in performance – so I tried this as well. I love it! Using a darlington such as a MPSA14 (or a homemade one) is a drop in replacement for the MOSFET Q1. It gives a little more gain and reduces noise in the control loop slightly (allowing more freedom in changing and or eliminating C7, c8, C9). I definitely can reccomend it.

New Schematic:

https://circuitsalad.com/wp-content/uploads/2012/11/mosfet-compressor1.gif

Got some LM358’s and they work fine. I did have  to adjust the mosfet compensation resistor (R10) from 10k to 47 k and I have tweaked a couple other values but it seems to work well.

Here is the schematic for a new compressor design, which has very small parts count but flexibility in threshold, attack and decay settings.

The design uses generic NMOS FETS such as the BS170 or 2N7000 and the only critical part is the one dual op amp which needs to tolerate voltage swing at or  below ground. I am using a LMC6482 but others will work. Another designer built my original compressor using the LM358 and after looking at the data sheet believe it will work here as well.

I am using a 2N7000 as a voltage controlled resistor and it works well but cannot tolerate a drain to source voltage of even 100mV. I solve this by using shunt feedback from the drain to gate in the amplifier stage. This creates a cancelling signal at the input node, proportional to the gain, which reduces the net voltage seen across the 2N7000 down to tens of mV(with a 2 volt input).

The 2N7000 has turn on voltage starting at about .8 volts. The circuit has a 2N7000 configured as a supply independent voltage reference which provides the bias for the voltage controlled resistor. This reference is adjustable and can be used to set a variable threshold of compression or fixed at whatever threshold desired.

This design also uses a simple op amp peak detector which would be normally used in a sample and hold circuit. A bleeder resistor is added to create a decay response as desired. Because a peak detector such as this tracks instantaneous level changes – it needs to have it’s very abrupt shifting of output level smoothed out to eliminate sharp noisy spikes as the amplitude changes rapidly. This is achieved with a simple low pass integrator on the input of the 2N7000 voltage controlled resistor. This smoothing filter sets the attack. The peak detector can use just about any diode. I am using an LED which lights up and varies in intensity with respect to the amount of compression. This provides a visual indicator of how much compression is occurring. No adjustments are required to use different diodes.

In all, you can adjust the attack, decay, threshold, compression level (from 1:1 to greater than 3:1), and output level with this design, and it only uses three mosfets and one op amp.

Check out the schematic here: Consider this obsolete….. look below for improved design

https://circuitsalad.com/wp-content/uploads/2012/10/simple-compressor.gif

Update: I am changing the design of this compressor somewhat to eliminate some noise caused by the closed loop peak detector. The updated circuit is below. It uses an open loop compensated  peak detector instead of the closed loop type. Also I separated out the compression level indicator – which really works well. It has the benefit of giving visual indication of threshold adjustments which can be set by R6 and R10.

I don’t need the final buffer either because the threshold settings and control loop gain allow the output match input level at max compression.  Many of the component values can be adjusted so expect to play around with some values. Currently, I am using : R6(100k), R9(1Meg), R10(10k), C10(.1uF). Changing R9 and C10(smaller cap and larger resisitor) lets the op amp slew a little faster – no big deal. If I have any big revelations that something is better – I will post it!

New Schematic:

https://circuitsalad.com/wp-content/uploads/2012/11/mosfet-compressor1.gif