Abstract: The present invention provides a preamplifier, by which it is possible to prevent influence of noise components when there are noise components in power line. There is provided a transformer having a primary winding with a center tap and a secondary winding on output side, and the primary winding of the transformer is connected between plates of two vacuum tubes for performing differential amplification, and positive voltage is applied on the center tap of the primary winding. A switch may be arranged in such manner that input signal and output signal can be separately set to balanced state or unbalanced state.