A power supply for generating a high output voltage for supplying an X-ray generator system with at least one X-ray source (17), especially for computer tomography (CT) applications is disclosed, wherein the high output voltage comprises at least two different high output voltage levels (U1; U1±U2) which are fast switchable so that spectral CT measurements can be conducted with one conventional X-ray tube (17). Furthermore, an X-ray tube generator system comprising such a power supply and at least one X-ray tube (17), as well as a computer tomography (CT) apparatus comprising such a power supply is disclosed.