I have tried a lower voltage. Unfortunately I don't have variable voltage (yet) but some fixed ones. 7.5 volts seems to low, there is hardly anything happening then. 9 volts is much better (perhaps a bit too strong). Anyway it feels much better now than the original 12 volts. Perhaps the sweet spot, for me, will be around 8-8.5 volts. Have to try it out with a variable regulator after I have bought one