I have enjoyed my PW modulated compressor and decided to make a feedback(the original was feed forward) version with improved attack and shorter decay response. The result is excellent – I love this version. I use a Zetex current monitor IC as a full wave rectifier and this lets me really speed up the attack. So the response is super fast, very low noise, very low distortion, and very large dynamic range.

Schematic

Link to design files:

https://www.adrive.com/public/PNQ67U/squeazal_design%20files.zip

I have completed the PWM compressor design and created a layout to fit a 1590B enclosure. The design has been simplified some and has excellent performance both in feed forward or feedback mode. I removed the full wave rectifier circuit. A simple compensated peak detector works fine. I also implemented a analog switch instead of the PFET.   It provides a little more duty cycle range and significantly more input  headroom.

The original blog post for reference:

https://circuitsalad.com/2013/08/04/coming-soon-the-pwm-squeasal

The schematic is highly annotated – the original PFET switch can be used in place of the analog switch if desired.

Schematic Here: updated(9/12/2013)

Compressor Demos:

Link to Design Files: (Done in epresspcb single sided thru-hole)

https://www.adrive.com/public/PNQ67U/squeazal_design%20files.zip

To scale PDF image of layout for toner transfer: (you will still need to look at the expresspcb layout to see part designators)

I did some experiments and found the PFET switch has a limitation of about 1.6 volts total swing it can tolerate without clipping (using a 470 ohm load) you can get about 2 volts max with a higher  load of 4.7K or so. This is perfectly fine for most guitar applications.For more headroom you can use a an analog switch such as the TS12A4515. A number of manufacturers make this device. It is a normally closed analog switch. Of course others will work also. Using this as a switch basically makes the supply rails the headroom limit. I am going to play with the circuit some more and come up with a board layout and will post my final design in a few weeks.

I spent the last week working on a simple feed forward compressor using the TL5001 pulse width modulation IC to create variable pulse with switching circuit where signal output approximately linearly proportional to duty cycle of a 500KHz switching signal. The benefits of this include: very low noise, low distortion and lots of attack/decay flexibilty. I had some problems with noise early on but now the design shows great promise. It can work either in feed forward mode or feedback mode and has tons of possibilities for modification and refinement. Hopefully someone else can play with it and come up with some improvements. It is super quiet because no gain is changed of any amp stage – just the signal level itself. Other switching elements are possible, but the PFET works very well.

Schematic:

Here is an alternate version which I may like a little better where the compression ratio is controlled by simply bypassing the switch:

Video Demo:

PCB layout coming soon:

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://circuitsaladdotcom.files.wordpress.com/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.