Abstract: A hearing aid circuitry comprising a high pass filter and a low pass filter, for filtering the sound wave signals to permit adjustment of the gain of the high end of the frequency spectrum relative to its low end. The low pass filter is formed of a differential amplifier which performs a subtraction of the sound wave signal and the output signal of the high pass filter. The outputs of both filters are individually added together in proportional quantities by a single control. The use of the differential amplifier, or alternatively, a simple R-C filter of 6 dB fall off, minimizes frequency effects in the output signal in the frequency range where the high pass filter and low pass filter overlap.