Graham, I guess I'ld like to know for a give secondary and toriod size what the resonance frequency is going to be and some idea of the Q and input impedence if driven as a magnifier. Oh yes and I'ld like to know exactly how insane I am to be playing with these things :) Doesn't have to be too accurate +/- 0.000001% will do ;) Good luck, Alan Sharp (UK)