The OPTO compressor revisited simple and works well

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:

Additional notes here:


link to ExpressPCB layout:

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.


4 thoughts on “The OPTO compressor revisited simple and works well

  1. Hey Ray,

    Do you know if the H11F1M is a must have? I have numerous H11AA1 opto’s available through work. Think things could be tweaked to have that work instead?

  2. thanks for posting this up! currently trying to understand opamps/fet’s etc. I wondered, could I slow the attack by adding a capacitor to ground just before R5? or by changing C4? (bit of a guess based on very little actual knowledge lol)

    • To change the attack add a resistor in series with D3 and the connection to C7 – this slows down how fast C7 can charge up and therefore respond. If its bigger than a few K…move R8 on the diode side of the new resistor such that D3 and R8 only connect to C7 through the new Resistor.

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