The Brookdeal type 401A is a compact high performance lock-in amplifier capable of recovering signals up to lOOdB below the noise level. Its design includes automatic circuitry which enables it to measure such signals without unnecessary setting-up procedures and also ensures continued correct operation during long period experiments. The 401A has an input dynamic range of lOOdB (10 voltage ratio), a low-noise input with lµV full-scale sensitivity and wideband operation from 1Hz to 50kHz. Reference channel facilities include a calibrated phase shifter with 90° and 180° pushbuttons and special input circuitry to reduce the effects of ground loops. Alternative plug-in power packs are available for differing requirements.