PURPOSE: To obtain an output signal in which unbalanced noise is suppressed considerably at each stage, by providing a mid-point terminal to the secondary winding of a transformer and grounding the mid-point terminal through the grounding terminal of a balanced input type amplifier.

CONSTITUTION: The majority of unbalanced noises entering the secondary windind side through an interwinding electrostatic capacitor is grouned directly through the mid-point terminal 25, so unbalanced noises appearing between terminals 23 and 14 and the ground are reduced greatly. Further, a few remaining unbalanced noises have every small amplitude, so they are removed through the balanced input type amplifier 3. The ground impedance of the balanced input type amplifier 3 viewed from the terminals 23 and 24 is not influenced at all because of the grounding of said mid-point terminal 25, so the amplifier operates normally to output no unbalanced input noise to an output terminal 51. Said grounding of the mid-point terminal 25 causes a triffle increases in the amount of unbalance input noises converted into balanced noises through the transformer 2 itself, but noises outputted to the output terminal 51 are reduced.