Amplifier

To restrain noise voltage output caused by a rapid change in output current in starting an output stage, a reproduced signal detected by an MR head is amplified by an amplifier and out-put volt-age and output current are outputted, A middle point of the MR head is controlled to always maintain potential of ground level. The output current is controlled by a current control portion (delaying circuit) (not included in FIG. 1) for converting an ON/OFF control signal into a signal having an integration waveform. The signal having the integration waveform is provided with a rise portion having a constant time constant, thereby there is restrained noise voltage output caused by a rapid change in output current, hunting of a power source of an amplifying portion, or inner inductance in a conventional example.