Exact envelopes of complexes

It is known that every complex of left R-modules has an exact cover. We con sider the question of finding R for which every complex of left R-modules h as an exact envelope. We first give a simple proof that for every R every c omplex of left R-modules has a special exact preenvelope and then use this to show that the left perfect rings are those R for which every complex has an exact envelope. So, for example, this result holds for all artinian rin gs.