# Oberheim synthesizers: SEM, Four Voice, Eight Voice and Matrix-12

> Oberheim synthesizers explained: the SEM and its multimode filter, the Two, Four and Eight Voice that first played chords, the OB series and the Matrix-12.

Canonical: https://sstsynths.com/oberheim
Updated: 2026-10-31

Oberheim synthesizers began with the SEM expander module of 1974. Stacking SEMs under one keyboard produced the Two, Four and Eight Voice, the first production synthesizers that could play chords, and the Matrix-12 of 1985 crowned the line with twelve voices and fifteen filter modes.

## The SEM: an expander with a different filter

Oberheim was founded in Los Angeles in 1969. It began by designing and building effects for Chicago Musical Instruments' Maestro brand, including the PS-1A Phase Shifter and the RM-1 Ring Modulator, and was briefly a dealer for ARP; in 1973 it produced the kit that made the [ARP 2600](/arp-2600) keyboard duophonic. Its first product under its own name was the DS-2, one of the first digital sequencers.

Customers wanted to play one synthesizer while the DS-2 played a sequence on another, or to layer two sounds. The answer, in 1974, was the **Synthesizer Expander Module (SEM)**: a compact semi-modular voice with two oscillators, two envelope generators, an LFO and a two-pole filter that can work as a low-pass, high-pass, band-pass or band-reject (notch) filter. That gave it a different sound from the four-pole low-pass ladder filters of Moog and ARP, which were the norm at the time.

## Two, Four and Eight Voice: chords from stacked SEMs

In late 1975, after Norlin (the successor to Chicago Musical Instruments) cancelled several large Maestro orders, Oberheim combined SEMs with a digitally scanned keyboard and a two-channel sequencer. Two SEMs made the **Two Voice**, four the **Four Voice** — the first production synthesizers that could play chords. Both were introduced at the June 1975 NAMM Show, the first time the company exhibited products under its own name. The **Eight Voice** followed in 1977, with a single or a double keyboard.

**The Oberheim Polyphonic Synthesizers, 1975–1979**

| Feature | Details |
| --- | --- |
| Voices | 2 (Two Voice), 4 (Four Voice) or 8 (Eight Voice), one SEM per voice |
| Per voice | Two oscillators with sawtooth or variable pulse; low/band/high/notch filter with resonance; two ADR envelopes; one LFO |
| Keyboard | 49 keys on the Four Voice; no velocity or aftertouch |
| Memory | None as standard; the optional Polyphonic Synthesizer Programmer (1976) stores 16 sounds for most SEM settings |
| Connections | Control voltage and gate |
| US list price | Four Voice $4,295 ($5,690 with programmer); Eight Voice $7,790 ($10,185 with programmer) |

Without the programmer, every SEM has its own knobs, so changing the sound means setting each module by hand — and no two modules are ever exactly alike. That small random variation between voices is why players describe these instruments as unusually thick and deep, and why they remain prized despite their maintenance cost and rarity. Production ended in 1979.

## From SEM racks to the OB series

Building on the programmer, Oberheim introduced the monophonic OB-1 in 1977, which the company described as the world's first completely programmable synthesizer. After the [Prophet-5](/prophet-5) appeared in 1978, the bulky SEMs gave way to voice cards inside one cabinet with digital control: the **OB-X** (1979, four, six or eight voices), the **OB-Xa** (1980, built with Curtis chips) and the **OB-8** (1983), whose second page of panel functions doubled its programming options.

The OB-Xa introduced the Oberheim Serial Buss, a pre-MIDI interface linking it to the DMX drum machine (1980) and the DSX sequencer (1981); the three together were sold as "The System". OB-series synthesizers and the DMX became staples of 1980s new wave, synth-pop and hip hop.

## The Matrix-12 and the Matrix family

The Matrix synthesizers are named after **Matrix Modulation**, Oberheim's system for choosing modulation sources — envelopes, LFOs and other controllers — and routing them to almost any parameter of the sound. The family falls into two groups: the Xpander and the Matrix-12, with voltage-controlled oscillators and very flexible filters; and the cheaper Matrix-6 line, with digitally controlled oscillators and a standard low-pass filter. The **Matrix-12** is in effect two Xpanders with a keyboard.

**Oberheim Matrix synthesizers**

| Model | Years | Voices and oscillators | Filter | Memory | US price |
| --- | --- | --- | --- | --- | --- |
| Xpander | 1984–1988 | 6 voices, 12 VCOs (CEM3374) | Multimode, 15 variations | 100 patches, 100 multis | $3,995 |
| Matrix-12 | 1985–1988 | 12 voices, 24 VCOs (CEM3374) | Multimode, 15 variations | 100 patches, 100 multis | $6,399 |
| Matrix-6 | 1985–1988 | 6 voices, 12 DCOs (CEM3396) | Low-pass | 100 patches, 50 multis | $1,695 |
| Matrix-6R | 1986–1988 | Matrix-6 without keyboard | Low-pass | 100 patches, 50 multis | $999 |
| Matrix-1000 | 1988–1994 | 6 voices, rack-mount preset model | Low-pass | 1,000 patches (800 preset, 200 user) | $599 |

Each Matrix-12 voice has two oscillators, five LFOs, five envelopes and a filter that can take 15 forms — low-pass, high-pass, band-pass, notch and phase-shift variations, built around the CEM3372 chip. It is 12-part multitimbral with up to six keyboard zones, and its 61-note keyboard responds to velocity and aftertouch. Matrix synthesizers remain popular for their late-1980s analog sound and their patching and filter options.

## What happened to Oberheim

**Oberheim after 1985**

| Year | Event |
| --- | --- |
| 1985 | Bankruptcy; a group of lawyers buys the company and renames it Oberheim ECC; the Matrix line continues |
| 1988 | A second bankruptcy; Gibson Guitar Corporation acquires the company |
| 2000 | Gibson licenses the name to the Italian maker Viscount; development of Oberheim products later stops |
| 2009 | The founder's own company makes a new SEM |
| 2014 | A new Two Voice Pro starts shipping |
| 2016 | The OB-6, built with Sequential, revives SEM-style voice circuits |
| 2019 | Gibson returns the Oberheim trademark to the founder; the company becomes Oberheim again |
| 2022–2024 | The OB-X8 (2022, with Sequential) and the TEO-5 (2024) appear |

## Oberheim instruments on a session

- **SEM:** a single voice with CV/gate. Its multimode filter is the reason to choose it: try the high-pass, band-pass and notch modes, which a Moog or ARP ladder filter cannot offer.
- **Four and Eight Voice:** plan time to set each SEM, or ask whether the programmer is fitted. Tune the voices together after warm-up; slight differences between them are part of the sound.
- **OB series:** programmable and compact, but pre-MIDI units link only to other Oberheim Serial Buss gear.
- **Matrix-12:** deep but menu-driven. Ask the owner for a list of patch names and back up the memory before editing.

## Frequently asked questions

### What does SEM stand for on an Oberheim?

SEM stands for Synthesizer Expander Module. Introduced in 1974, it is a semi-modular analog voice with two oscillators and a two-pole multimode filter, designed to be played alongside another synthesizer or a sequencer.

### What was the first synthesizer that could play chords?

The Oberheim Two Voice and Four Voice, introduced at the June 1975 NAMM Show, were the first production synthesizers that could play chords. Each voice was a complete SEM module under one shared keyboard.

### How many voices does the Oberheim Matrix-12 have?

Twelve. The Matrix-12 (1985–1988) is in effect two six-voice Xpanders with a 61-note velocity- and aftertouch-sensitive keyboard, with 24 voltage-controlled oscillators in all.