Abstract: In the ISDN telecommunication system, the connection between a digital local exchange and the network termination ensues via a public two-wire line at whose ends what are referred to as U-transformers reside. When the network termination leads to a terminal subscriber, then this is called a U.sub.k0 interface whereat the line length between the transformers can amount to up to 8 km. The bit error rate of the transmitted pulses must be <10.sup.-7, whereby the pulses have direct currents for the remote feed of the terminal subscribers that can amount to up to 80 mA superimposed on them. For such U.sub.K0 transformers, a toroidal tape core of a soft-magnetic, amorphous, magnetostriction-free alloy tape is employed. The manufactured toroidal tape core is tempered in a magnetic field, this thermal treatment ensuing in a protective gas atmosphere.