Abstract: In an audio amplifier for amplifying signals from an electric guitar or the like having a conventional tone control circuit for selectively emphasizing input signals in the bass, middle and treble frequency ranges of the instrument, a multifilter circuit for emphasizing input signals with frequencies in selected, relatively narrow, frequency bands in the upper portion of the frequency spectrum to minimize aural fatigue. The multifilter circuit comprises a plurality of parallel connected, two-pole, resonant filter sections with different, relatively narrow, resonant frequency bands separated from one another by an amount on the order of 1/4 octave and forms a signal path that supplements the signal path provided by the conventional tone control circuit. The signals produced on the output of the tone control circuit are mixed with the multifilter output signals.

Abstract: A distortion sound effects circuit producing an output signal corresponding to an input signal but containing one of four selected types of distortion. A compressor which includes a voltage-controlled amplifier first compresses the audio input signal within preselected limits, and the distortion operations are performed on the compressed audio signal to advantageously render the subsequent distortion operations insensitive to gross amplitude fluctuations of the input signal. The compressor includes means for full-wave rectifying the compressed audio signal. The full-wave rectified signal is provided as a distorted output signal containing hard-even distortion of the type commonly achieved by severely, asymmetrically clipping a signal. A distortion circuit includes single voltage controlled square-low amplifier to which the compressed audio signal is applied performs the other three types of distortion.