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The use of filters whose cutoff characteristics are controllable by electronic means is often desirable in problems dealing with audio signals. Based on the recent work on fixed RC active filters by J. G. Linvill, variable active low-pass and high-pass filters have been developed using transistor negative-impedance converters. The design theory of such filters is summarized, and measured characteristics and other experimental results are presented. An application, in which the cutoff characteristics are controlled by the incoming audio signal for use in formant tracking, is described, and experimental results are given.