Abstract: A bottom sheet separator feeder is described for separating and forwarding sheets seriatim from the bottom of a stack of sheets to be fed. It comprises a tray for supporting a stack of sheets to be fed, a vacuum belt feeder extending through at least a front end of the tray for acquiring and advancing the bottom sheet of the stack, and an air knife arranged to inject air into the front end of the stack. A common blower creates a negative sheet acquisition pressure at the vacuum feed belts and a positive air pressure at the air knife. An air knife relief valve can be opened or closed to vary the air pressure at the air knife relative to the sheet acquisition pressure at the vacuum feed belts. The feeder may be shared by two trays of which one of the trays can be raised and lowered into and out of position within the other tray, the valve being open when the one of said trays is raised and closed when the tray is lowered.