NOISE REDUCTION CIRCUIT

PURPOSE:To improve remarkably the reduction in phase margin caused by a high-band suppressing operation, by using a high-band suppressing circuit having a resonance point and an antiresonance point. CONSTITUTION:An operational amplifier 47 acts as a constant gain amplifier at a compressing operation and a voltage dividing circuit 73 forms a feedback circuit. Further, a voltage dividing ratio of the voltage dividing circuit 73 determines an input level at a compression input terminal 41 with respect to a monitor output. The high-band suppressing circuit 62 is provided with a voltage division circuit 74 and the voltage dividing ratio of the circuit 74 sets an expansion output level corresponding to an operating reference level of a block 66 at a compressing circuit. Further, an output from the block 66 is fed back to an inverting input terminal of the amplifier 47 via a voltage division circuit 75 at the time of expansion and the high-band suppressing circuit 32 and the block 66 form a feedback path of the amplifier 47, then the expansion and the restoration of the high-band suppression are attained as a whole.