A two-stage transformer, built as a current comparator with a shielded magnetic core, is used to make a novel ac bridge for calibration of ac shunts for large currents and other ac current-to-voltage converters. A fine adjustable reference transresistance is made by current transformation and a non-reactive precision resistor connected to the secondary. By using an auxiliary transformer and resistor, all four two-stage transformer secondary terminals are brought to ground potential and consequently both magnetizing and capacitive errors are kept below one part per million.