AC to DC converter with selectable voltages?

[Audiophilenoob]

I need a AC to DC converter for testing transformers for my ribbons... but I can't seem to find one with selectable voltages...

I need a power supply that will let me run voltages from 0.01mv to 4v while caring a high amperage... say something like 30 amps at 4 volts minimum with the amperage increasing as you go down to 0.01mv

I guess this is a 120w PSU minimum... a 800w one would do my purposes better though

if there's some kind of circuit I can build to do this with a normal transformer let me know

thanks