An apparatus is provided for heating plastic preforms, having a plurality of resonator units, into which in each case plastic preforms can be introduced in order to heat the latter, having at least one microwave generating unit for generating microwaves, and having a feed device for feeding to at least one resonator unit the microwaves generated by the microwave generating unit, and having at least one rectifier unit for providing a rectified voltage. Further, at least one rectifier unit provides voltage for operating at least two resonator units.