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An active RC filter with an m-derived low-pass response is described. With a cutoff of 45 cps and a sharp attenuation peak at 60 cps, it is particularly suitable for eliminating power-frequency interference from EKG, EEG, or other biological potential amplifiers. It is small in size and requires only one 12AX7 or its transistor equivalent for the active elements. Typical design data and performance curves are given.