A signal processing circuit is intended for use in a noise reduction system, which produces a target filter characteristic that would achieve optimal noise cancellation, the target filter characteristic including a resonant peak at a first frequency. The signal processing circuit comprises an analogue filter, which has an amplitude response that has a peak or trough at a center frequency, and has a phase response that switches polarity at the center frequency and tends to zero with increase or reduction in frequency away from the center frequency. The center frequency in the amplitude response is substantially equal to the first frequency. The analogue filter may be in the form of a series inductive-capacitive-resistive circuit, where the inductive component is in the form of a gyrator.