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The Oberheim SEM (Synthesizer Expander Module) was one of their earliest products. It was an entire synthesizer voice – two oscillators, two simple envelopes, VCA, and a very popular two-pole state variable filter design with a knob that crossfaded between low pass, notch, and high pass outputs plus a separate bandpass setting – in a cube-like case. It could be bought by itself, and was also the building block for their earlier two, four, and eight voice polyphonic synths. It was also the synth voice in the early 360 Systems guitar synths.

Several companies have emulated the SEM – in particular, it’s filter (still one of my personal favorites) – and Tom Oberheim has released a variety of new synths based on an updated SEM design. Most often today, when a modular manufacturer uses the magic letters “SEM”, they’re referring to a filter meant to emulate that in the original Oberheim synth. I worked for Tom briefly in the late 80s, and he verified that he hand-tweaked the design with just enough saturation to make it sound big and warm.

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