Updated Squezal PWM Compressor Schematic, Layout and Demo

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:


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)


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



7 thoughts on “Updated Squezal PWM Compressor Schematic, Layout and Demo

  1. Hi, I saw this design (and newest revision 1.2.2) being discussed on DIYstompboxes…

    I’m not an EE but have a few questions:
    -Pot1 controls input gain correct? So higher input gain=more compression
    -An attack control could be added by a variable resistor before/after D2?
    -Changing the value of R16 affects release?
    -A pot for R19 would be like a ratio/max compression control?

    • You don’t want to make R19 adjustable – make it 1K or less but it sets compression curve behavior not really the ratio. Pot 1 sets the ratio and or the threshold. I find that it has more gain than I need for compression levels I use, but since everyone’s input level may be more or less – the gain is set high. You may find increasing R14 (lowering gain) is preferable for feed forward – leave it as is (high) for feedback mode. Your max compression is about 23dB so making the the gain higher just gets you there faster – not more compression – which is really like changing the threshold. R15 sets your attack – 100 ohms or higher. R16 set decay – the useable range is probably 47k -470k.

      To do the switched mode take a single pole double throw switch – then connect the input side of C10 to the center pin of the switch, then connect the shown connection of (R3/C3) to one side of the switch and the the other side of the switch to the R9/C6 node – which will give you the feedback connection.

  2. Also, the “side chain input” note isn’t entirely clear to me…it would be interesting to have a toggle between feedforward and feedback modes, but I wouldn’t know how to implement it

  3. Hi Ray,

    I am starting to build the Squeal circuit and would like to be able to make a PCB. However, the link to the expresspcb layout seems to be broken. Would it be possible for you to upload the file again or email it to me? That would be much appreciated.



      • Hi Ray,

        Thank you for sending the files. I have gone into my university’s electronics lab to print the circuit board, but they are unable to do them without the gerber files. The technician said he would need the bottom layer, board outline and drill file to print. He also said the drill sizes he had were 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.4mm, 1.5mm and >3mm. Are you able to upload the gerber files? If so, will the drill holes on the schematic work with the mentioned drill sizes? I’m rather new to all this so apologies for the hassle.



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