Fundamental to Acoustical Research is the measurement of sound pressure level. The W.E. 640‐AA Microphone was used at the Electro‐Acoustic Laboratory not only as a reference standard for the calibration of other types of microphone, but also for routine laboratory measurements. Given the pressure calibration of the microphone certain instrumentation and complementary calibrations are needed for the convenient use of this microphone under the various conditions encountered during research work. The following aspects of this subject are discussed: (1) Considerations regarding the effect of preamplifier input impedance on the frequency response and noise threshold of a condenser microphone. (2) An a.c. operated, cathode‐follower type preamplifier and associated circuits. (3) A flexible extension for use with the preamplifier. (4) The use of small probe tubes. (5) Random incidence calibration and collected free field calibrations. (6) A “supercharged” cathode follower circuit and its application as a preamplifier.