An ion-optical device comprising: a plurality of electrodes (2); a first AC voltage supply (6); and a transformer (4) having: a toroidal core (8); a primary winding (10) connected to the AC voltage supply (6) and passing through the aperture within the toroidal core (8); and at least one secondary winding (13,15) wound around the toroidal core 8 and electrically connected to multiple ones of said plurality of electrodes.