A transformer includes a magnetic core through which an insulated electrical first conductor extends to define a turn of winding. The first conductor is tubular, in that it defines a generally axial aperture. A plurality of electrically insulated conductive windings are twisted together to form a bundle, or a pair if only two. The bundle extends through the axial aperture, to define at least an additional turn of winding. The individual conductors of the bundle are stripped of insulation at locations without the aperture of the first conductor.