Abstract: A planar wave transducer assembly comprising a rigid structure formed by a pair of foot pads, each with an upstanding leg and a span bar extending across the top of the legs. A piezoelectric transducer element is attached to the span bar, preferably in a channel extending between the legs, to convert wave motion in the plane defined by the flat bases of the foot pads into an electrical signal. The transducer assembly herein disclosed has been found to be remarkably effective as a pick up when applied to the soundboard of a piano, achieving excellent signal isolation as well as enhancing the acoustic sound produced by the instrument.

Abstract: A notch filter system for eliminating a specific "stop" frequency from an AC electrical input signal, such as 60 Hz "hum" from an audio program signal. An L/C tuned circuit that is tuned to the stop frequency is used to develop a modified input signal that is in phase with the input signal only at the stop frequency. The two signals are amplitude-balanced and fed to the respective inputs of a balanced differential amplifier which is responsive to both input signal phase and amplitude differences so as to cancel the signals at the stop frequency where they have the same phase and amplitude but not at frequencies above and below the stop frequency where they differ in phase and amplitude.

Abstract: A system which corrects for adverse characteristics such as reactance, inertia and resonances of a power amplifier driven load such as a speaker or multiple speaker system. Program voltage is applied to a reference load which has electrical characteristics that simulate characteristics of the driven load, and the response of the reference load to the program is used to develop a correction voltage signal for the driven load. The program and the correction voltage signal are simultaneously applied to the power amplifier to simultaneously reproduce the program and correct for the adverse characteristics of the load.

Abstract: According to the present invention circuit means and method are provided which correct for variations in an amplifier load such as a speaker by sensing the load current, developing a feedback signal related to the load current, comparing this feedback signal with the amplifier program, and instantaneously adjusting the gain of the amplifier to compensate for load current deviations from the program applied to the amplifier. By this means the performance of the speaker or other load that is driven by the amplifier is positively controlled and forced to track accurately with the program applied to the amplifier despite such opposing factors as reactance or inertia of the load, whereby the acoustic or other load response is a substantially accurate reproduction of the program.

Abstract: A rigid end plug adapter which serves both as a strap or end button and electrical connector for transducer pickups is provided in the end block of a musical instrument such as a guitar or violin-type instrument.