The differences between the DM501 and the DM501A are like the differences between lightning and lightning bugs (to paraphrase Mark Twain). The DM501 is an object lesson in the definitions of "accuracy" and "precision". The DM501 is precise to 4 1/2 digits, but not accurate to 4½ digits. The combination of a D-to-A converter built from op amps and discretes combined with a noisy high-frequency isolated power supply make for barely achievable 3½ digit accuracy over all ranges. Every TM500 DVM after the 501 (including the DM501A, the DM502, and so on) used integrated A-to-D converters and a 60-hertz isolated power supply. If you have a choice between a DM501 and any other TM500-series DVM, choose the latter.