Handbook for Sound Engineers, 5th Edition

A slight redraw of Fig. 29-63C gives Fig. 29-63D, a more conventional portrayal of the classic Sallen-Key high-pass filter arrangement. As the Sallen-Key filter evolves, it turns out that an equal value filter (where the two capacitors are equal and the two resistors are equal) results in a less than adequate response shape. An expedient method of tailoring and smartening up response to become Butterworth-like (working on the assumption that a couple more resistors are cheaper than a special two-value ganged potentiometer) is to alter the damping by introducing gain into the gyrator buffer amplifier (providing also a healthier mode of operation for the amplifier—followers are bad news), see Fig. 29-63E. A side effect of this technique of damping ...