Abstract: In an ISDN digital communications system, the transmission between the network termination and the terminal equipment ensues via what is referred to as the S.sub.o interface, on the basis of interface transformers. Since the power supply of the terminal equipment likewise partly ensues via these transformers, a current asymmetry in the lines results in a pre-magnetization of the transformers. Thus, the ISDN demands made of the transformers must also be satisfied given a DC pre-magnetization. Compact transformers having a simple winding format that satisfy the ISDN demands are set forth, the transformers utilizing a magnetic core having a fine-crystalline iron-based alloy with an iron part of more than 60 atomic %, the structure thereof being more than 50% fine-crystalline grains having a grain size of less than 100 nm and having a remanence ratio of less than 0.2 and a permeability in the range from 20,000 to 50,000, and an inductance of less than 100 pF.