A circuit for achieving the moving or changing formant effect in an electronic musical instrument such as an electric organ. The circuit employs a band-pass amplifier with parallel T resistivecapacitive feedback circuits in which an FET (Field Effect Transistor) is employed as a variable resistance to cut in, by degrees, one of the T feedback circuits. The effective resistance of the FET and thus the band-pass of the amplifier are varied by a capacitive and resistive timing circuit that is controlled via a keying circuit from one or more keys or other player-operated controls, such as the manual keys of an electric organ.