In the exact words the manufacturer (Keling) said: "You can use the power supply from 24-72VDC"

This appears to be at odds to what you've said at the beginning of your post where you stated 42V max. Are you mixing up max recommended motor V with max driver V. (The

*motors* are Keling - not the drivers).

Certainly if the drivers are these

http://www.probotix.com/stepper_motor_drivers/ProboStep (which are the only unipolar drives I can see at Probotix) then the max V is 44V.

From your link the (unipolar) motor inductance is 2.2mh, therefore your "ideal" voltage for "max" speed & torque is 32 * sqrt 2.2 = 47.4636704 Vdc. BUT this is over the 44V limit of your drives. You said your supplier recommended limiting to 42V - a tad below the limit of 44V.

So 42Vdc gives a transformer secondary of 42/1.414 = 29.7029703VAC.

Generally (in the UK anyway) transformer secondarys come in 12, 15, 18, 22, 25, 30... VAC so your nearest fit is 25VAC which will give you 35.35Vdc. This as has been stated earlier is not the "ideal" for your motors but it's the best you can do with your drivers.

Unless of course I've cocked up somewhere

Ian