PURPOSE: To attain the correction of auditory sense corresponding to a sound pressure by using a photocoupler in the gain variable circuit of a graphic equalizer and providing an output level detecting circuit for a volume, a limiter circuit for the output amplitude and an operational amplifier that drives the photocoupler.

CONSTITUTION: A graphic equalizer 63 is equipped with an operational amplifier OP1 as the variable gain amplifier of an input IN and a serial resonance circuit RES that limits the feedback resistor Rf, the voltage-dividing resistor RD and the gain variable sound area of the operational amplifier. The photocoupler is used as a gain variable circuit PC. At such a case, the RD is a photodetecting element. An operational amplifier OP2 is used for the driving of a light emitting element D1, making change a driving current (i) corresponding to an input control voltage VC. Meanwhile, a level detector 64 makes the control voltage VC of the photocoupler PC by amplifying, detecting positively and amplitude-limiting the input IN. Thereby, the influence of the resonance circuit RES is reduced along with the level rising of the signal IN, and a frequency characteristic becomes flat at all areas. Since the RD is also small when the level of the signal IN is low, the negative feed-back quantity of the operational amplifier OP1 centering the frequency of the circuit RES is reduced, raising the gain at the area.