Oberheim Two Voice (1975–1979)
Two SEMs, one keyboard, and a sequencer that runs on a clock. Tom Oberheim built the Two Voice to show that polyphony didn't have to mean a room full of modules.
Overview
The Two Voice takes two of Oberheim's Synthesizer Expander Modules and hard-wires them into one compact keyboard instrument. Tom Oberheim first manufactured the SEM in 1974, a small self-contained voice box that let players run sequences through a relatively affordable sound generator instead of tying up a Minimoog or an ARP 2600. Each SEM has its own VCO, VCF, VCA, and ADSR, and the filter is the familiar Oberheim 12dB/octave multimode design. Two of those side by side, played from one set of keys, gives you a polyphonic sound that ARP and Moog gear of the period simply didn't offer.
It arrived in 1975 and ran through 1979. Oberheim Electronics, founded in 1969, was by then developing some of the first commercially available polyphonic synthesizers, and the Two Voice sits right at the front of that run. It's a self-contained keyboard instrument in a single cabinet: two complete SEM voices, a keyboard, and a built-in sequencer. There's no patch memory and no MIDI. It's a pre-MIDI, CV/Gate instrument, and the front panel is the only place your sounds live.
The interesting part is how the two voices interact with the keyboard. The manual lays out the logic plainly. Voice module one always follows the leftmost key depressed, voice module two always follows the rightmost, and a third key in between has no effect at all. Depress one key and only voice one gates. Add a key to the right while holding and voice two gates on with the new note. Add a key to the left instead and the whole assignment re-sorts itself - voice one takes the lower note, voice two gets the upper, and both gate together. It's an unusual, physical way to play, and it makes the instrument feel less like a preset machine and more like two monophonic synths sharing a brain.
The record disagrees about the keyboard itself. Multiple sources, including tomoberheim.com, describe a 37-note keyboard, while other research text says 49 notes, and at least one source describes the instrument as keyboardless. The 37-note figure has the strongest backing and is what the manufacturer's own site states, so that's the one to trust. Sequencer step count is similarly muddled in the secondary literature, but the primary accounts converge. Wikipedia describes an onboard 8-step sequencer, and Sound On Sound characterizes the mini sequencer in the original 2-Voice as a two-row, eight-step maximum design paired with a Sample & Hold generator. Eight steps, two rows, plus S&H. The manual's walkthrough of the S/H unit backs this up. It describes random voltages from the sample/hold feeding the VCOs or the filter, with a lag knob that introduces portamento into the pitch changes, and the middle position of the VCF control selector labelled "EXT" routing S/H voltage to the filter.
Listings also disagree on the output section. Some describe two outputs, one per voice, while others say mono only. That one stays unresolved. The manual describes an output mixer with three rotary pots (one level control per voice module plus a master) and two phone jacks. That confirms a mixed output path, but the exact configuration of those jacks isn't spelled out in the material available.
Key Features
Two SEM Voices, Hard-Wired
Each voice module is a complete SEM: VCO, VCF, VCA, and ADSR, with the 12dB/octave multimode filter that became Oberheim's signature. The two modules are identical in their controls. The manual is explicit that the only difference between them is how the keyboard and sequencer modules control them. That means you can set up two completely contrasting patches and switch between them with the output level controls, using the instrument as a pair of preset voices. The manual suggests exactly this. It also notes the opposite approach: combine both modules in single-voice operation with complementary envelope settings, a percussive envelope on one and a long "bowed" attack on the other, so the sounds fade from one to the other.
Two-Row, Eight-Step Mini-Sequencer with Sample & Hold
The sequencer is wired into the voice assignment. Assignment switches in the minisequencer section determine whether each voice module follows the keyboard or the sequencer. In the down position, a module gets keyboard control voltage and keyboard gate. Move the switch to the middle or upper position and both are disconnected. The envelope generators then take gate signals from the clock, and the oscillators take control voltages from either the sample/hold unit or the 8-step sequencer, depending on which position you've chosen. The clock has a spring-loaded "step" position that advances the sequence one step at a time, which makes setting up an eight-step pattern quick work. The manual's suggested trick is to set up a sequence on voice one, then play voice two from the keyboard along with it.
Keyboard Assignment Logic
The 2ND VOICE switch at the left of the keyboard governs whether voice two gates immediately with any key press or waits until a second key is depressed. The manual's "keyboard logic" section lays out the behavior in detail, including the less obvious cases. Legato playing on the second voice won't produce new pitches unless the 2ND VOICE switch is off, while legato on the first voice produces new pitches regardless. Portamento and pitch bend don't affect both keyboard voltages identically either, and exactly what they do to the second voltage depends on that switch position. The manual's closing advice is continuous legato on the lower voice and slight non-legato on the upper, the most conservative and predictable way to play it.
Digitally Scanned Keyboard
Oberheim paired multiple SEMs with digitally scanned keyboards to produce the Two-Voice and Four-Voice synthesizers, which made a compact polyphonic instrument possible in 1975. There's no dedicated polyphonic voice-allocation circuit. The keyboard scanning and the SEM pairing do the work.
No Memory, By Design
There's no patch memory and no MIDI. Every sound is a front-panel setting, and every performance is a live one. The lack of memory makes live use impractical for many owners, though the SEMs themselves draw consistent praise for their sound.
Historical Context
Oberheim is an American synthesizer manufacturer founded in 1969 by Tom Oberheim. The Two Voice is where the company moved from the standalone SEM module to a self-contained keyboard instrument. Beginning in 1975, Oberheim developed some of the first commercially available polyphonic synthesizers, and the Two Voice, paired with the Four Voice that followed the same year, put the company at the front of that category. ARP and Moog gear of the time didn't deliver that polyphony.
Tom Oberheim has repeatedly called the Two Voice his favorite design from the original era. The Four Voice built on the Two Voice's approach, combining four dual-oscillator SEM modules with their own filters and envelopes, a simple analog mixer, and a 49-key keyboard. That's eight oscillators and four-voice polyphony from four independent monophonic synths in one box. The Two Voice is the smaller, earlier version of the same idea.
At least one notable artist kept one in the studio. The Two Voice appears in Liam Howlett's setup during The Prodigy's Invaders Must Die recordings, where it contributed the Puma whistle. Modern Recording (April 1982, p.33) surveyed the broader category of programmable synthesizers of the era and noted the trade-off between flexible control paths and the limits imposed by manufacturer-chosen configurations. That fits the Two Voice's front-panel-only approach, which asks the player to accept a fixed architecture in exchange for immediacy.
Collectibility & Value
The Two Voice is a rare and collectible instrument, with asking prices floating around $9,500 - $10,500. That's a wide band, and it reflects how much condition varies on a fifty-year-old analog synthesizer with no production records to anchor the market. Individual parts surface from time to time. A Portamento/Tune control section was listed on eBay at $899.
Buyers should expect to find units unserviced, with original electronics and power switches that may need attention. The keyboard contacts and the tantalum capacitors are the known trouble spots after five decades, and neither is a surprise on a mid-1970s Oberheim. There's no original MSRP, no dimensions, no weight, no official production count, and no manufacturer spec sheet in the record, so provenance and condition matter more than paperwork. Sellers tend to list these as-described rather than offering returns, which puts the burden on the buyer to ask the right questions before committing. One owner's note on a serviced example said that if it doesn't meet the price expectation, it stays.
eBay Listings
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.