Disclosed is a basic module of a magnetic core of a wound electrical transformer. The basic module includes first and second windings placed atop one another and made of first and second materials, respectively. The first material is a crystal having a saturation magnetization≥1.5 T and magnetic losses less than 20 W/kg in sine waves having a frequency of 400 Hz, for maximum induction of 1 T, and the second material is a material having an apparent saturation magnetostriction less than or equal to 5 ppm and magnetic losses less than 20 W/kg in sine waves having a frequency of 400 Hz, for maximum induction of 1 T. The cross-sections of the first winding and cross-sections of the second winding satisfy (S1/(S1+S3); S2/(S2+S4)) of the first material, having a high saturation magnetization, compared to the cross-section of both materials together, is 2%-50%.