PURPOSE: To reduce a noise included in a servo output signal when an input signal level is low, and to hardly change the (f)-value of the input and output of the servo under a condition that the deterioration of a sound quality due to the noise is not generated.

CONSTITUTION: A filter circuit is interposed between one terminal (a) to which the input signal to be processed is applied, and a variable gain amplifier 3 which controls a gain for amplifying the input signal. The gain of the variable gain amplifier 3 is controlled based on the detected value of the signal level after the input signal passes through the filter circuit 10, and the servo circuit can be allowed to have a frequency characteristic.