A plug-in socket for coupling to a corresponding standard plug and for connection of an electrical terminal device to a primary voltage. The plug-in socket improves the use of mains units with voltage converters. The plug-in socket comprises a socket housing, a first electrical contact arranged in the socket housing for coupling to a corresponding standard plug electrical contact, a voltage converter integrated in the socket housing and converting a primary voltage into at least one secondary voltage for the electrical terminal device, a second electrical contact arranged on the socket housing for electrical contacting of the secondary voltage, wherein the voltage converter is integrated in the socket housing so that the external dimensions of the socket housing are substantially predetermined by the dimensions of the corresponding standard plug.