An adjustable shelf-type equalizer with separate control of frequency and limiting attenuation or amplification

This device provides for control of the high-frequency content of an audio program, allowing the operator to choose independently: 1) the frequency above which equalization is to occur, and 2) the maximum correction in signal strength which results in the range of frequencies under correction. Three operating controls are used. One control determines whether the high-frequency signals shall be increased, left unchanged, or decreased in strength relative to the low-frequency signals. A second control is calibrated in terms of the frequency above which correction shall occur. The third control adjusts the asymptote which represents the maximum correction which shall occur for frequencies considerably higher than that chosen by the frequency control described above. The circuit can be described briefly as follows: for high-frequency attenuation, the operation employs a negative feedback amplifier whose output may have high-frequency loss inserted by an RC network following the amplifier; for high-frequency boost, the RC network is inserted in the amplifier feedback path so that the amplifier output (which now becomes the system output) has a rising high-frequency response which corresponds to the attenuation of the RC network. All three controls are continuously adjustable within their ranges of operation.