Tom Oberheim started out in the ealy 1970s building effects boxes like a ring modulator or a phaser. Just like Dave Smith, he also built a digital sequencer for analog synthesizers. Those were predominantly the Minimoog or the ARP 2600 at the time. Oberheim realized, that when you had the sequenzer play the (monophonic) Minimoog, you had nothing left to play with. So he designed what he called a Synthesizer Expander Module (SEM), which was a synthesizer with basic functions, but without a keyboard. That made it compact and (kind of) affordable. So you could have the sequenzer run a bassline into the SEM and you had your Minimoog spare to noodle away with to that sequence.
Despite its very basic architecture, the sound of the SEM is so noble that someone called it the Stradivari of Synthesizers. That maybe mostly adressed to its special 2-Pole multimode Filter which has a very contrasting sound to the 4-Pole lowpass Filter of the Minimoog most ears are used to.
After the introduction of the SEM in 1974, Oberheim had the idea of combining several of them in a box together with a digital scanning keyboard, invented by E-MU. This led to the Oberheim 2-Voice, 4-Voice and even 8-Voice synthesizers. The latter combining no less then 8 individual SEM modules, stacked in two rows. While being very painful to program (you have to dial in the same settings in all of the individual modules - and not to forget, correctly tune their up to 16 oscillators), the sound of these early polyphonic synthesizers is nothing but devine.
Unfortunately a vintage 4- or 8 Voice is almost unobtainable. So as a lover of that sound, I set to record my single SEM module several times to build those magic carpets of the most noble sound in synthesis.