Oberheim OB-X (1979–1981)

Thirty-two programs of pure analog thunder, tuned by a microprocessor and played like a muscle car.

Overview

The big selling point of the OB-X was printed right on the front panel: thirty-two programs, stored and recalled with a group button and a program button. The owner's manual lays out the routine plainly - press AUTO to tune all oscillators, choose a GROUP from A through D, pick a PROGRAM from 1 to 8, and play. The unit shipped from the factory pre-programmed with thirty-two sounds, which helped players who had never owned a programmable polyphonic before. Oberheim's own literature describes the instrument as a combination of its established polyphonic circuitry with microprocessor control, and calls the result "a myriad of polyphonic sounds with unheard of ease." That's marketing copy, and it reads like it. The architecture underneath was real, though.

Each voice has two oscillators and two full ADSR envelope generators. There's also a noise generator and a five-octave keyboard. The manual's feature list names polyphonic portamento and polyphonic sample-and-hold, and that separates the OB-X from a lot of its contemporaries. The glide and the sample-and-hold happen per voice, independently, rather than being smeared across the whole instrument. Each voice glides from note to note on its own. So chords don't arrive as a single blob of pitch-bending. They arrive as individual voices doing individual things.

The control surface is split in two. The MANUAL section - volume, auto-tune, hold, chord - is not programmable, and the manual is blunt about it. The CONTROL section is fully programmable, and includes portamento, which the manual notes also works in UNISON mode. Switching UNISON on collapses all voices onto a single key depression, and the keyboard follows a low-note rule, so the lowest note played always wins. That feels different from a conventional mono mode, and it's where the stacked-oscillator thickness of the OB-X is most obvious.

Tuning is automatic. The AUTO button sets the microprocessor to tune all oscillators, and the output amplifier is muted during the process, so the tuning routine stays out of your speakers. The manual carries a warning: all pitch-affecting controls are disabled during auto-tune except the pitch bend lever. Lean on the bend lever while the machine is tuning and the oscillators will land out of tune. The MASTER TUNE control has a dead zone near the top center. Park it there and the instrument sits at standard A-440, provided auto-tune has been run.

The rear panel is built for the stage. Five foot control inputs are provided: volume, filter, vibrato, sustain, and program advance. The sustain input sets both envelope generators to their longest release times when engaged. The program advance switch steps the programmer forward one location at a time - from A6 to A7, and from D8 back around to A1. The volume pedal lets a player preset a maximum on the front panel and ride the level underneath it. The filter pedal sweeps the cutoff of all filters together, from slightly below the panel setting up to significantly higher. All three pedal inputs expect a 50 K Ohm linear potentiometer with the wiper on the signal lead and the counter-clockwise terminal grounded. The foot switch inputs just want a switch that shorts signal to ground when pressed.

Four input/outputs expose the control voltage and gate of Voice 1 to the outside world. That lets an OB-X talk to a modular system or a CV sequencer without modification, which mattered more later than it did in 1979. The cassette interface is built in, and the manual treats it as an ordinary feature. An audio cassette recorder with reasonable frequency response and a pair of mini-plug cables is all it takes to dump the program memory externally. If the playback process hits errors, the PLAY light flashes.

The programmer does more than the front panel lets on. EDIT mode lets the physical controls operate incrementally from a stored setting. Entering edit changes nothing until you move a knob, and then the sound shifts to reflect the new position. If a control is already pinned at its maximum or minimum and you want to go further, rotate it all the way the opposite direction and then set it where you want. Programs can be moved between memory locations, and new programs can be built from existing ones by entering edit, making changes, and writing the result. Writing requires holding WRITE until the light comes on, then selecting a group and a program while still holding it. Inside the chassis, on the processor board, sits a PROTECT switch that blocks writes to program memory entirely. Reaching it means removing the top two screws on each end bell and raising the cover, so it's a set-and-forget switch rather than something you toggle mid-session.

The modulation panel sits just left of the keyboard. The modulation lever adds vibrato to both oscillators and only works in one direction - pulling it the other way does nothing. The pitch bend lever's range is set by a NARROW/BROAD switch: one whole step in NARROW, a full octave in BROAD. An OSC 2 ONLY switch restricts the bend lever to the second oscillator of each voice, which the manual flags as particularly interesting on programs where oscillator 2 is synced. A three-position TRANSPOSE switch moves the whole keyboard up an octave, to normal, or down an octave, extending the effective range to six octaves. The HOLD button sustains a note or chord indefinitely when pressed in combination with the keys and then released. Pressing HOLD a second time cancels it, provided no keys are being held.

Period coverage of the OB-X is thin in the surviving record, but what's there is specific. Modern Recording (October 1979, p.36) described the OB-X as a fully programmable four-, six-, or eight-voice polyphonic synthesizer from Oberheim Electronics, microprocessor-controlled for maximum versatility. So the OB-X was a family of configurations, not one instrument. The manual's service notes confirm it, referring separately to six- and eight-voice units when describing cable reconnection to the second mother board. A six-voice OB-X and an eight-voice OB-X are the same design with different amounts of voice circuitry, heat, and cost inside the box.

Modern Recording (May 1981, p.38) announced the OB-Xa as an updated, evolutionary development of the original OB-X that retained all of the earlier unit's character. Read alongside the 1979 piece, the two notices bracket the OB-X's run almost exactly: introduced as a fully programmable microprocessor-controlled poly in 1979, superseded by an evolutionary revision in 1981. The OB-Xa became the model most people picture when they say "Oberheim poly," which has done the original no favors in the market.

What the manual reveals about living with an OB-X is mostly calibration and drift, and it doesn't sugarcoat any of it. The service documentation describes voice card replacement with a warning that AC power must be off during removal and installation. It also offers a clever field diagnostic: with UNISON off, step through the voices using the keyboard until the defective voice is gated on, then touch the tempco resistors on each card while holding that voice. The card that produces a pitch change when touched is the bad one. That procedure was written by people who expected owners and techs to open the thing up. Portamento calibration is described just as honestly: with the trimmer set for maximum glide, a one-octave change takes anywhere from 1 to 2.5 seconds, and the variation between voices can be as much as a 2-to-1 ratio between the shortest and longest. The portamento was never perfectly consistent voice-to-voice, and the factory documentation says so outright.

Power supply calibration involves two adjustable voltages, +19 V and -19 V, each trimmed to within 200 mV, with every other rail handled by fixed three-terminal regulators. The documentation also notes that pin E4 should read a minimum of 2.3 V with the power off. That's the backup battery holding the program memory. It's the number to check on a non-working OB-X, because a dead backup battery is a cheap fix and a dead power supply is not. Mother board calibration is a two-trimmer distortion adjustment, done with the front panel volume at maximum and no keys depressed, targeting 0.00 V ± 20 mV on each channel's final op-amp output. Techs without a meter handy have an audio alternative: plug in headphones or an amp, hold the AUTO TUNE switch, and adjust for minimum loudness of the "thump."

One trap is documented plainly and catches people constantly. If a mono amplifier is plugged into either stereo output, certain voices will likely be quiet, because of the factory setting of the pan pots. The fix is the mono output jack, which exists for exactly this reason. Owners who don't read the manual conclude their OB-X has dead voices and start shopping for voice cards they don't need.

Shoppers searching "Oberheim OB-X" on the used market should expect to sort through a wall of OB-X8 results before finding an original, and should read every listing title carefully.

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