Abstract: An improved well logging system of the type utilizing a single conductor cable to provide power from the surface equipment to a down hole subsurface unit and for simultaneously supplying current mode telemetry signals from one or more radiation detectors in the subsurface unit to the surface equipment. The surface equipment includes a power supply for the subsurface unit comprising a constant current source having a high output impedance connected to a series pass voltage regulator having a low output impedance. A switching mode voltage regulator is employed within the subsurface unit to compensate for variations in the voltage supplied to the subsurface unit which result from variations in cable resistance and other factors.