Way back when the dinosaurs roamed the earth, maybe 1992, I tried putting a diode and electrolytic capacitor upstream of an E-unit to stop the buzzing. And it worked, but the E-Unit gets crazy hot! The coil of an e-unit is like an inductor, and resists the 60Hz, when the signal is just DC and a little ripple, just get more current, more heat.
There may be a way to keep the coil of the E-unit running on AC, but the motor running on a bridge rectifier, just keep in mind, it isn't going to be smooth DC like a battery. The trick would be at the fingers and drum of the E-unit, would have to find my K-Line book, don't have any more e-units. Most likely you will not want to have any wire of the motor running to chassis ground / outer rails, its just a weird thing with bridge rectifiers, if you study the resulting schematic with the wire ground, you would end up shorting out one of the diodes, and people have not been using bridge rectifiers with four diodes for what maybe 70 years and not notice the fourth diode is not needed, no way. Reductio ad absurdum, reduced to the absurd.
