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)

Click to access squeazal-single-sided.pdf