A high-performance detector for soft x-ray appearance potential spectroscopy is described. It basically consists of an alkali halide photon-to-electron converting layer evaporated onto a self-supporting amorphous carbon foil. This combination absorbs the low energy photons thereby increasing the signal-to-noise ratio. Further improvement of the signal-to-noise ratio is achieved by an additional x-ray filter. The choice of filter and converter material depends on the characteristic radiation of interest. The electrons emitted from the converter are detected by means of a microchannel plate. With this detector a signal-to-noise ratio of 150 was obtained at the diamond appearance potential spectroscopic peak for 5 mC charge per point.