Magnetic zero-flux current transformers are widely utilized to measure AC and DC electrical currents. To improve the AC characteristics between 10 Hz and 100 kHz, a method is proposed to evaluate adjustments in the circuitry; increase of the amplification in the AC feed-back circuit and introduction of an external coaxial read-out shunt resistor. The method is generally applicable and includes a coaxial current path within the entire reference system and traceability to national standards. The AC working range is extended from about 1 kHz to about 3 kHz. Outside the nominal AC working range, the scale factor variation is reduced from 45 % to 3.5% and the phase angle error offset changes from ca 5.8 μs to below 0.3 μs.