A system has been designed and constructed in order to detect differential ac susceptibility as a function of temperature between 4.2 and 300 K. A dc bias field in the range of 0–50 kOe is provided by a superconducting magnet. The available ac field range is 0–1 Oe. Using the reference source of a PAR lock‐in amplifier, the measurements can be carried out in a frequency range of 0.1–10 kHz. A differential amplifier circuit is used to accurately detect the susceptibility signal. As this provides both in‐phase and out‐of‐phase compensating signals, both the real and imaginary components of the susceptibility can be detected even when using a single‐phase lock‐in amplifier. The circuit diagram and principle of operation as well as the calibration procedure and data processing methods are described in detail.

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