I use it with both, but the only way to run specific frequencies is with direct to generator. The Spooky2 Central is a fixed 100 Hz output, but with more power. You can place the PEMF coil over the area in need.
As for run time, I have not found a hard limit on how long you can run the PEMF coil, even on a single frequency, having ran 10 Hz on my neck for hours at a time.
However, use of the PEMF coil just like anything else, can result in a Herx reaction. I found this out after running the Arthritis sets via PEMF on my neck.
It only took one application and I was out for the count for 2 days.
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