PWM Compressor updates

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.

 

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The PWM SQUEASAL

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: