A sheet feeder apparatus and method with throughput control. By varying the speed at which sheets are fed from a supply, the sheet feeder apparatus and method assures that the throughput capacity of a downstream operation is never exceeded. The speed of the feed assembly (300) is controlled by a controller which receives information from photosensors (600) indicative of the length of the previously fed document to create an optimal inter-document gap. Singulator (400) is comprised of modular, pre-fabricated, self-contained conveyor sub-assemblies (460) which are easily placed upon and removed from shaft (420) and held in place by end cap (439).