Introduced in December 1980, the Oberheim OB-Xa was a programmable polyphonic analogue synthesiser developed as the successor to the Oberheim OB-X. Although it retained the basic concept of two voltage-controlled oscillators per voice and the direct front-panel programming approach of its predecessor, the Oberheim OB-Xa represented a substantial redesign internally and added important performance features including programmable keyboard splits, layered sounds and selectable 2-pole and 4-pole filtering. Available in four, six and eight-voice configurations, the Oberheim OB-Xa became one of the defining large-format American analogue polyphonic synthesisers of the early 1980s.
The Oberheim OB-Xa replaced much of the discrete voice circuitry used in the Oberheim OB-X with Curtis integrated circuits. Its oscillators, filters and envelope circuitry were built around the Curtis chip family, reducing component count while improving manufacturing consistency, reliability and serviceability. The change also allowed Oberheim to expand the instrument’s capabilities while maintaining an entirely analogue audio signal path under microprocessor control.
Each voice of the Oberheim OB-Xa contained two voltage-controlled oscillators. Oscillator 1 provided a four-octave range selected in octave intervals, while Oscillator 2 covered a five-octave range in semitone increments and could be detuned relative to Oscillator 1. Sawtooth and pulse waveforms were available, with pulse width continuously adjustable. Oscillator synchronisation allowed Oscillator 2 to lock to a harmonic of Oscillator 1, enabling substantial timbral changes when the frequency of the synchronised oscillator was altered.
The filter envelope could also be routed to the frequency of Oscillator 2. The amount of this modulation was controlled from the filter section, allowing the pitch of Oscillator 2 to follow the shape of the filter envelope. This replaced the oscillator cross-modulation facility of the earlier Oberheim OB-X and became part of the characteristic voice architecture of the Oberheim OB-Xa.
The filter section was one of the most important changes introduced with the Oberheim OB-Xa. The synthesiser used Curtis CEM3320 filter circuitry and allowed the player to select between 2-pole and 4-pole low-pass filtering. The 2-pole mode provided a 12dB/octave slope, while the 4-pole mode provided a steeper 24dB/octave response. Filter frequency, resonance, envelope modulation and keyboard tracking were programmable, while Oscillator 1, Oscillator 2 and the noise generator could be individually routed into the filter.
Oscillator 2 could be introduced at either full level or approximately 5dB below full level. This provided a simple means of changing the balance between the two oscillators without requiring a conventional continuously variable oscillator mixer.
Each voice incorporated separate ADSR envelope generators for the filter and amplifier. The filter envelope controlled the development of the filter cutoff frequency and could simultaneously modulate Oscillator 2 when the F-ENV function was selected. A second ADSR envelope controlled the voltage-controlled amplifier and therefore the amplitude of each voice.
The Oberheim OB-Xa’s modulation section provided an LFO with an approximate frequency range of 0.1Hz to 20Hz. Sine, square and sample-and-hold modulation sources were available, and modulation could be routed to the frequencies of Oscillator 1 and Oscillator 2 or to the filter. Pulse-width modulation could also be applied to either oscillator.
A separate performance modulation section was positioned to the left of the keyboard. The modulation lever controlled the amount of LFO modulation applied during performance, while the pitch-bend lever provided continuously variable pitch control. The pitch-bend system could operate over a narrow or wide range and could be switched to affect only Oscillator 2, particularly useful when oscillator synchronisation was being employed.
One of the major advances of the Oberheim OB-Xa was its ability to operate as a bi-timbral synthesiser. In its normal Full mode, the available voices operated together as a single polyphonic instrument. Split and Double modes divided the voice architecture into independent Upper and Lower sections.
With an eight-voice Oberheim OB-Xa, Split mode divided the instrument into two four-voice synthesisers. Different programs could be selected for the Upper and Lower sections, while the split point could be positioned anywhere across the five-octave keyboard. The two sections could also be independently transposed.
Double mode allowed the Upper and Lower programs to be played simultaneously across the keyboard. On an eight-voice instrument this produced four-note polyphony, with each key triggering one Upper voice and one Lower voice. The Upper and Lower programs remained independently programmable, while their relative levels could be adjusted using the Balance control.
The modulation panel could also be assigned independently to the Upper or Lower section while operating in Split or Double mode, allowing performance modulation to affect one part without necessarily affecting the other.
The ability to store complete Split and Double configurations was particularly significant. Later Oberheim OB-Xa/120 instruments provided eight Split programs and eight Double programs in addition to the expanded library of conventional patch memories.
The original Oberheim OB-Xa was introduced with 32 program memories. During the instrument’s production life Oberheim repeatedly revised its software and hardware, ultimately expanding program capacity to 120 patches. The later instrument was described by Oberheim as the Oberheim OB-Xa/120.
The Oberheim OB-Xa/120 incorporated 120 patch programs together with eight Split programs and eight Double programs. The instrument was supplied with 104 programmed sounds, eight Split programs and eight Double programs, leaving locations available for user-created programs.
Programs were selected through four Group buttons and eight Program buttons. The four Group switches could be used individually or in combinations, providing the 15 groups required for the 120 program locations.
Existing programs could be edited directly from the front panel. Rather than forcing the player to match a knob physically to its stored value before editing, moving a control added to or subtracted from the stored parameter value. This allowed rapid alteration of recalled sounds while leaving the stored program intact until a deliberate Write operation was performed.
Manual mode bypassed the stored program settings and made the sound correspond directly to the physical positions of the front-panel controls. Completely new sounds could therefore be constructed using the conventional analogue synthesiser approach before being written into program memory.
During the development of the Oberheim OB-Xa, Oberheim also added square-wave modulation through the performance modulation system, improved the output circuitry, introduced an improved automatic tuning system, revised the cassette interface and expanded the available portamento modes.
The Oberheim OB-Xa incorporated true polyphonic portamento, allowing individual voices to glide between notes. Later instruments introduced Linear and Quantised portamento modes. Linear mode allowed the voices to glide continuously between pitches, while Quantised mode produced the transition in semitone steps.
Later versions also included a Portamento Offset function. With the offset active, individual voices moved at slightly different rates, deliberately preventing every voice from reaching its destination simultaneously.
Unison mode assigned all available voices to a single played note. The keyboard then operated with low-note priority, giving the Oberheim OB-Xa a monophonic performance mode using its complete collection of voices simultaneously.
Hold could sustain notes or complete chords indefinitely. The Hold and Chord functions could also be combined so that a programmed chord could subsequently be transposed from individual keys. Oberheim also provided transposition controls capable of shifting the entire keyboard one octave above or below its normal range.
The Oberheim OB-Xa included stereo and mono audio outputs. Individual internal pan controls were provided for each voice, allowing the voices to be positioned independently within the stereo image.
The Upper and Lower voice groups could therefore be distributed across the stereo outputs while the front-panel Balance control adjusted their relative levels. The mono output combined the signals without being affected by the individual voice pan positions, although Upper and Lower balance remained effective.
This per-voice stereo arrangement was a continuation of an approach Oberheim had used on its earlier multi-voice synthesisers, where individual synthesiser voices could be positioned across a stereo field.
As with other large voltage-controlled analogue polyphonic synthesisers of the period, maintaining oscillator calibration across multiple voices was an important practical consideration. The Oberheim OB-Xa incorporated a computer-controlled Auto-Tune system that calibrated the oscillators.
Later versions improved this process further by automatically removing a voice from the allocation system if it could not be correctly tuned. This allowed the instrument to continue operating with the remaining voices rather than repeatedly assigning notes to an incorrectly tuned voice.
Individual voice-disable controls were also available internally for servicing, calibration or operation when a voice card developed a fault.
The Oberheim OB-Xa incorporated a cassette interface for external storage of program data. Sounds could be recorded onto an ordinary audio cassette and subsequently transferred back into the synthesiser.
The system provided Record, Play and Check operations. Check allowed a recorded data set to be verified without overwriting the programs already held in memory, an important safeguard when cassette tape was being used as the primary external backup medium.
The cassette system also retained a degree of compatibility with earlier Oberheim instruments. The Oberheim OB-Xa/120 could read program data recorded by earlier Oberheim OB-Xa/32 and Oberheim OB-X instruments, although differences between their architectures meant that some parameters were translated rather than reproduced identically.
The Oberheim OB-Xa was designed before MIDI became an industry standard, but Oberheim provided its own computer interface for integration with external digital equipment. The rear-panel parallel computer interface connected the Oberheim OB-Xa directly to the Oberheim DSX Digital Polyphonic Sequencer.
The Oberheim DSX could record notes and program changes from the Oberheim OB-Xa and reproduce them automatically. When combined with the Oberheim DMX Digital Drum Machine, the synthesiser and sequencer became part of the integrated system marketed by Oberheim as The System.
The Oberheim System represented an important transitional stage in electronic music production. A synthesiser, digital sequencer and programmable drum machine could operate together as a coordinated production system before MIDI provided a manufacturer-independent method of connecting electronic instruments.
The Oberheim OB-Xa occupies an important position in the development of the programmable analogue polysynth. Introduced in December 1980, it followed the Oberheim OB-X and marked the point at which Oberheim combined its established polyphonic analogue architecture with considerably more sophisticated performance programming.
It was the first Oberheim synthesiser to use the black front panel with blue horizontal pinstripes that subsequently became one of the company’s most recognisable visual designs. More importantly, its internal architecture marked Oberheim’s transition from the largely discrete voice circuitry of the Oberheim OB-X to extensive use of Curtis integrated circuits.
The ability to divide the Oberheim OB-Xa into independent Upper and Lower synthesisers was particularly important. Programmable Split and Double configurations allowed one instrument to perform two different sounds simultaneously, either across separate keyboard zones or layered together. At a time when many programmable polyphonic synthesisers still operated primarily as single-timbre instruments, this represented a substantial expansion of what could be achieved from one keyboard.
The selectable 12dB/octave and 24dB/octave filters also broadened the architecture beyond that of the Oberheim OB-X. The Oberheim OB-Xa could retain the gentler 2-pole response associated with earlier Oberheim designs while also providing the steeper 4-pole filtering increasingly common in contemporary analogue synthesisers.
Program memory evolved considerably during the instrument’s short production life. The progression from 32 memories to the 120-program Oberheim OB-Xa/120, together with stored Split and Double setups, demonstrates how rapidly microprocessor control was transforming analogue synthesisers during the early 1980s.
The connection between the Oberheim OB-Xa, Oberheim DSX and Oberheim DMX was equally significant. The resulting Oberheim System provided automated sequencing, program changes and drum synchronisation before MIDI existed. It represented an early attempt to build a complete electronically integrated composition and production environment from separate programmable instruments.
The Oberheim OB-Xa also became widely used during the early 1980s, with documented users including Prince, New Order, Gary Numan, the Thompson Twins, Van Halen and other major recording and touring artists. Its large programmable polyphonic architecture proved particularly suited to the increasingly synthesiser-driven arrangements of rock, pop, new wave and funk production during the period.
The Oberheim OB-Xa has also become closely associated with Van Halen’s “Jump”, partly because the instrument appeared prominently in connection with the song and became identified with its synthesiser part. Historical accounts differ over whether the original studio recording was actually made with an Oberheim OB-X or an Oberheim OB-Xa, so the precise instrument used on the recording remains disputed.
The importance of the Oberheim OB-Xa within the company’s history is reflected in the modern Oberheim OB-X8, which specifically recreates the Curtis-based Oberheim OB-Xa filter architecture, envelope behaviour and original factory programs alongside those of the Oberheim OB-X and Oberheim OB-8.
The Oberheim OB-Xa therefore represents more than another early programmable polysynth. It combined the architecture of the original Oberheim OB series with bi-timbral performance, greatly expanded patch storage, selectable filter responses and integration with an external digital sequencing system. These features place the Oberheim OB-Xa squarely within the transition from the first generation of programmable analogue synthesisers towards the interconnected electronic production systems that dominated the remainder of the 1980s.
Features
- Four, six or eight-voice programmable analogue polyphony
- Two voltage-controlled oscillators per voice with sawtooth and pulse waveforms
- Selectable 2-pole 12dB/octave and 4-pole 24dB/octave low-pass filtering
- Separate filter and amplifier ADSR envelope generators for every voice
- Programmable Split and Double modes for bi-timbral performance
- Up to 120 patch programs with eight Split and eight Double programs on later Oberheim OB-Xa/120 instruments
- Polyphonic portamento with Linear, Quantised and later Offset operation
- Unison, Hold, Chord and keyboard transposition functions
- Stereo and mono outputs with individual internal voice panning
- Oberheim computer interface for direct integration with the DSX Digital Polyphonic Sequencer