Rod, KQ6F requested a schematic of the feed forward hum reduction circuit mentioned above.
Here's WA1QIX's PDM generator schematic in case someone else needs it too:
http://www.classeradio.com/pwm_generator_rev_f.pdfThe FF input is at the bottom center.
It notes a 47K resistor between the 135 VDC PS and input, but for a higher voltage supply, take extreme HV sampling precautions to prevent problems.
T