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Abstract

Feedback elimination apparatus for a sound system which includes a notch filter assembly comprising a plurality of active notch filters connected in parallel with one another. Each of the notch filters is formed of a loop amplifier circuit including an operational amplifier having a resonant feedback network so arranged that in the normal mode an auxiliary signal peaked at the resonant frequency is applied out-of-phase to the system, thereby to establish a notch in the frequency characteristic curve of the system. An improved loop circuit is used to produce a notch which is sharp and well defined, with auxiliary circuitry for aligning the notch with a feedback signal. In the preferred embodiment means are provided for temporarily switching the notch filter into an oscillating mode in which it oscillates at notch frequency to produce an audible reference signal. A manual control in the notch filter smoothly varies the frequency of the resonant network so that the reference signal is swept over the audio range to establish zero beat with a "first" acoustic feedback signal. Thus when the notch filter is restored to its normal mode the notch is in alignment with the feedback signal, the depth of the notch being subsequently increased as necessary to the point where the feedback signal is cancelled. The system gain is then incrementally advanced to produce additional feedback signals and the procedure is repeated for each of the notch filters until each acoustic feedback signal has been disposed of.