PURPOSE: To attain ease of setting and adjustment of the operating level of an associated circuit by rectifying a noise component and forming a DC voltage so as to drive and control the associated circuit.

CONSTITUTION: Only the noise component in an output signal of a signal processing circuit 2 is applied to a transistor (TR)26 of a noise detection circuit 40. After the noise component is converted into a DC voltage by being rectified by a rectifier circuit comprising diodes 34, 35, the result is applied to a display circuit 9' and a muting circuit 11' of the next stage. The degree of change of a noise characteristic is smaller in comparison with that of a signal meter output characteristic A. Thus, the setting/adjustment of the operating level of the associated circuit is performed easily by utilizing the noise characteristic C.