After being disappointed with the results, I took a break from this experiment and then got side-tracked with a couple of other things. So now I'm back at it.

First thing I noticed is when I removed the free wheel diodes, I had a performance increase. Very good torque and smooth stepping. I think Kai was right when he said trr might be too slow. A strange thing though, even with a 50KHz+ chopping frequency, the motor has some odd sounds coming from it. Kind of like bacon frying or static on the radio and I can still hear the high pitch squeal. Depending on where the motor stops in the stepping sequence, the pitch and sound of the motor changes. As far as the sound coming from the motor nothing changed when I raised the chopping frequency to 100KHz.

I've got some faster freewheel diodes and higher wattage resistors today and am reworking the circuit. I hope the addition of suitable free wheel diodes may eliminate this awful sound.

Jon