Review: Dave Smith Instruments OB-6

The return of the son of the Oberheim SEM
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The return of the son of the Oberheim SEM
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At last January’s NAMM show, a major buzz surrounded the OB-6, the product of collaboration between pioneering synth designers Tom Oberheim and Dave Smith. Tom and Dave, friendly rivals in the 1970s at the dawn of polyphonic synthesis, had recently worked on a joint project to produce an instrument combining their considerable talents. Faced with long lines at the NAMM booth where the two men appeared together, I had only a few minutes to don headphones and check out the OB-6, but my first impression was that they had gotten it right. It definitely felt and sounded like an Oberheim, and I’ve owned six different models.

Fig. 1. The OB-6 6-voice, all-analog subtractive synthesizer is a joint project from Dave Smith and Tom Oberheim. The OB-6 is a 6-voice subtractive synthesizer with a 49-note keyboard that senses velocity and aftertouch. Each voice has two audio oscillators, a suboscillator, a variable-state filter, a white noise generator, two ADSR envelope generators, and an LFO. The entire audio signal path is completely analog. It also has two multi-effects processors, an arpeggiator, and a polyphonic step sequencer. Along with storage for a thousand programs, the OB-6 has a manual mode that lets you switch off preset memory and rely entirely on the front-panel controls to sculpt your sound in real time, just like in the old days.

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The OB-6 looks very much like other Dave Smith Instruments products, with real wood side panels and a black-coated steel exterior. At first glace, it would be easy to mistake it for another recent DSI synth, the Sequential Prophet- 6, and the similarities don’t end there. One visual cue sets it apart, however: Blue horizontal pinstripes recall the classic Oberheim OB-8 and OB-Xa. Like the Prophet-6, the OB-6’s only dedicated left-hand controllers are wheels for pitch bend and modulation (LFO depth only), and the front panel contains lots of knobs and buttons organized into functional sections. The largest of five alphanumeric LED readouts displays the current program number in three digits. The OB-6 has no LCD, which means no menus to dive into and no program names displayed.

A class-compliant USB Type-B connector and MIDI In, Out, and Thru on 5-pin DIN connectors handle MIDI data flowing to and from the OB-6. Also on the back are three 1/4-inch audio outputs for unbalanced left and right signals and stereo headphones, as well as four control jacks: one for a sustain footswitch, one for a footswitch to start and stop sequencer playback, another for an expression pedal controlling volume, and a fourth for an expression pedal controlling filter frequency. Although they’re all logical choices, I wish I could assign at least one expression pedal to control a user-defined parameter.


The oscillators, filter, and amplifier for each voice are voltage controlled, just as they were in classic Oberheims such as the TVS-1 and Matrix-12. Each voice has four sound sources labeled VCO 1, VCO 2, Noise, and Sub Octave, which generates a square wave 12 semitones below VCO 1’s pitch. You control their levels with four knobs in the Mixer section. Once you’ve run the calibration procedure a few times, the oscillators stay in tune as long as the ambient temperature doesn’t change drastically—quite impressive for any analog synth.


Because the sawtooth and pulse waves generated by VCO 1 are continuously variable, you can smoothly transition from one extreme to the other for a variety of spectral combinations. VCO 1’s Frequency knob changes pitch in half steps, with no fine-tuning capability. The Pulse Width knob changes a pulse wave’s duty cycle from zero at one extreme, through square, to a very narrow pulse at the other, but has no effect on a sawtooth. The Sync button turns on hard sync, forcing VCO 1 to synchronize its phase to VCO 2.

VCO 2 is continuously variable from triangle to sawtooth to variable-width pulse wave. You can vary its fine-tuning relative to VCO 1 using a Detune knob, which transposes pitch as much as a semitone up or down. A Low Freq button enables low-frequency mode, allowing you to use VCO 2 as an LFO when needed, and a Keyboard button lets you disable keyboard tracking—often desirable when using VCO 2 as a modulation source.

Below the Oscillators section, another Detune knob lets you add a touch of pitch randomness, so that each note you play is more or less out of tune with other notes, even subsequent notes. Detuning is especially effective when you enable unison mode, which makes the OB-6 function as a monophonic synth with as many as 12 oscillators. With unison enabled, the Detune knob detunes the oscillators relative to one another by fixed amounts and spreads them across the stereo field. One of unison’s coolest features is chord memory. If you play a chord, hold it, and then press the Unison button, playing subsequent notes will transpose that chord. If you do that and then save the program, the OB-6 will store the chord as part of the program.

The voltage-controlled filter is based on the 2-pole, 12dB-per-octave filter found in the classic Oberheim SEM. It bears most of the responsibility for the distinctive Oberheim sound. Its design is unique in that it can sweep continuously from lowpass to notch to highpass response. (Also known as bandstop or band-reject, notch filtering attenuates audio within a specified frequency band.) Pressing a button turns it into a bandpass filter. The resonance setting affects all filter modes, but unlike most lowpass filters, the OB- 6’s VCF does not self-oscillate at high resonance settings. A Track button toggles between off, half, and full settings and determines how filter frequency responds to keyboard pitch.

The VCF and VCA each have a dedicated ADSR envelope generator, and each envelope lets you choose whether velocity affects its modulation depth. The Amount knob for the filter envelope is bipolar, so that negative settings invert its effect. If velocity is enabled and the setting is negative, a lowpass filter will close during the attack stage and open during release. The VCA’s envelope, of course, is not bipolar, and its Amount knob determines its depth.


Two onboard digital effects processors offer a 48kHz sampling rate and a limited variety of common effects: reverb, delay, flange, chorus, phase shift, and ring modulation. The two latter effects include digital emulations of two classic Maestro stompboxes, the PS-1 phase shifter and RM-1 ring modulator, designed by Tom Oberheim in the early 1970s. Earlier Oberheims lacked any effects processing, so the OB-6’s multi-effects are a welcome addition.

You can access more advanced features by pressing the Globals button. For example, you can choose from 16 tunings that include quarter-tone, 19-tone equal temperament, and Wendy Carlos’s harmonic 12-tone scale. You can also choose to transmit either MIDI CCs or NRPNs when you turn the front-panel knobs, which can be set to relative, passthru, or jump mode.


Fig. 2. To help make your playing more nuanced and expressive, two footswitch and two expression pedal jacks are on the OB-6’s back panel. The OB-6’s modulation routing isn’t as flexible as on many synths, but its X-Mod (cross-modulation) capabilities contribute a lot to its distinctive sound. Instead of a mod matrix for connecting tons of sources and destinations, you get just four modulation sources in addition to the hardwired connections between the filter envelope and VCF and the loudness envelope and VCA.

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The LFO generates five waveshapes (two of them bipolar) that you can route to any combination of seven destinations, including pulse width for either or both VCOs. By selecting filter mode as the destination, your signal can rhythmically cycle between lowpass, notch, and highpass modes. Frequency is continuously variable well up into the audio range (500Hz maximum) without needing to switch it out of low-frequency mode. Selecting a random waveshape and turning the frequency up full generates white noise as a mod source. The LFO can run freely or sync to the OB-6’s internal clock.

X-Mod routes the filter envelope and VCO 2 frequency to any combination of six destinations, all of them VCO 1 and VCF parameters. X-Mod can modulate oscillator frequency, waveshape, and pulse width as well as the filter’s cutoff and mode, but it can also continuously modulate the filter from normal to bandpass filter mode—something you can’t do manually, because you enable bandpass mode by pressing a button. X-Mod is also the only way to route an envelope to any destination other than the filter and loudness.


X-Mod is especially useful with VCO 2 switched to low-frequency mode so that it serves as a second LFO. Unlike modulation from the dedicated LFO, in which all six voices share a single control signal and are therefore synchronized, low-frequency modulation from VCO 2 sends an independent signal for each voice, resulting in complex per-note modulation and more animated sounds.

Like most synths with aftertouch, the OB-6 supports channel aftertouch only, so that pressing any key causes modulation of every note. Unlike most synths, the OB-6 has a dedicated knob for controlling the depth of aftertouch modulation for each program. Below the knob are six buttons to select the aftertouch destination. In addition to more usual destinations such as LFO amount (for vibrato) and VCF frequency, you can apply pressure to modulate filter mode, loudness, or even the frequency of either VCO. What’s more, aftertouch is bipolar, so modulation can be positive or negative.

The OB-6’s internal clock controls the sequencer or arpeggiator tempo, and optionally, LFO or delay rates. The front panel’s Clock section lets you choose one of ten values, from half to 32nd note, including triplets, and furnishes a button for tap tempo. Connecting an audio signal to the Sequence Footswitch jack lets the clock sync to an external click track—very handy when you need that kind of functionality.

The onboard sequencer records individual notes or chords up to six notes, and you can easily enter rests and ties at any point. The keyboard can accompany the sequencer as it plays, too. Sequences may be any length, up to a generous 64 steps. Most factory programs include a short, simple sequence, which provides a good way to audition sounds, and you can store your own with any user program. The arpeggiator is similarly straightforward, with five common patterns you can extend up to a three-octave range. The Hold button in the Modulation section latches arpeggios or sustains notes and chords.



The OB-6 is an unlikely combination of half Oberheim and half Dave Smith synthesizer, with all the timbre-building functionality of old-school SEMbased instruments and quite a few programming conveniences found in recent-model Dave Smith Instruments releases like the Pro 2. Based on the Prophet’s Poly Mod, X-Mod is a particularly outstanding contribution. Although the OB-6 is marketed as an update of Oberheim’s SEM-based synths, it goes way beyond.

How does it sound? In a word, glorious. It captures the character of many Oberheim synths that came before it, and with 500 new factory programs, it delivers many more variations on that character than any synth ever has. To my ears, the finest Oberheims exhibit all the lushness, punchiness, and timbral versatility you’d want in an electronic instrument, and the OB-6 definitely sounds like one of the finest. The factory programs well demonstrate the serious sound design chops of their creators. As with most analog synths, realistic acoustic instrument emulations are in short supply, but if you want searing leads, solid synth basses, and floating, ethereal pads, you’ve come to the right place.

I found almost nothing to criticize about the OB-6. The only features I would change—a wider keyboard and multitimbral operation—would drive up the cost, and analog polysynths are costly enough already. If the four-octave keyboard isn’t to your liking, check out the OB-6 Desktop module, which will save you a few hundred bucks.

Back in the day, most synthesists declared a preference for the Sequential, Korg, Roland, Yamaha, Moog, or Oberheim sound. I was always an Oberheim guy, and the OB-6 punches all the right buttons in my brain. Give it a chance, and you may discover you’re an Oberheim guy or gal, too.


The Oberheim SEM

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The OB-6’s basic architecture was introduced in 1974 with the launch of the Oberheim Synthesizer Expander Module, or SEM. Originally designed to provide a companion voice for pairing a sequencer with a monosynth like the Minimoog or ARP 2600, the SEM became the core of each voice in Oberheim’s earliest polysynths—the now-collectable Two-Voice, Four-Voice, and Eight-Voice. Although Tom Oberheim has resurrected new and improved versions of the SEM and Two-Voice, his current models more closely resemble the originals than the OB-6, which has expanded timbre-building capabilities.

Authentic, classic sound. Intuitive architecture. Impressive programming versatility. Stable oscillator tuning. Syncs to audio click track. High-quality effects.

Can’t display program names. Mod wheel controls LFO depth only. Not multitimbral. Four-octave keyboard.

$2,999 street

Geary Yelton has reviewed synthesizers for Electronic Musician since 1985. He lives in Asheville, North Carolina