Catalyzes the reduction of 3-ketodihydrosphingosine (KDS) to dihydrosphingosine (DHS). NADP(+) + sphinganine = 3-oxosphinganine + H(+) + NADPH Lipid metabolism; sphingolipid metabolism. Belongs to the short-chain dehydrogenases/reductases (SDR) family. The gene for this protein is duplicated in strains AX3 and AX4. These strains contain a duplication of a segment of 750 kb of chromosome 2 compared to the corresponding sequence in strain AX2.