I know SG look into it but don't know how far he got.
Well guys...I didn't get far cause I didn't need one, but if you look at Reply #8 in Timbo's ROTOSONIC thread:
http://el34world.com/Forum/index.php?topic=17572.0 I mentioned the use of PWM (pulse width modulation) as it applies to wire feeders in welding.
PWM is used to provide a variable speed control so you can dial in your wire feed speed to match the voltage of the welding power source and application in MIG welding.
There is usually one pot on the front of these feeders that sets the wire feed speed by supplying a PC board with a control signal that gets converted into a control voltage that spins the drive motor at your desired speed.
So lets say the drive motor is 24VDC...it would receive a control voltage from 0VDC - 24VDC with 0VDC = a dead stop and 24VDC = full speed.
Brad, I might be able to harvest the parts you need to try this and ship them to you if you'd like to take a run at it.
I just don't have time/inspiration to hook it up myself right now. I choose the schematic that I posted originally because of the 120VAC input requirement,,,for simplicity.
My only big concern would be if the RPM range would be usable???
It appears as though the Leslie had a slow=40RPM and a high =390RPM,,,so I'm not sure we could hit top speed.
We could do 0-?
The drive motors we use are over-kill for this application, but there are some light duty units that might work.
MY part of the dream would be to add a foot pedal to the equation that would contain just a pot, so you could have speed control from min -max. with your foot.
I also have access to some used/obsolete foot pedals here because of their use in TIG welding applications. They have switches that engage the circuit before the pot comes in to control the circuit. That's another possibility...have the switch activate the leslie, and the pedal travel work the speed.
This all would take someone more motivated than I to pull together, but sure would be cool. Maybe someday.
Looks like there's plenty of stuff on YouTube to check out, and smarter guys have probably solved this with way less effort.
It just crossed my mind this way because I'm around THIS stuff.