Review: Dave Smith Instruments OB-6

A flawless blend of the Prophet-5 functionality and SEM sound
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Every time I wonder if Dave Smith will run out of cool synthesizer ideas, he releases another mind-blowing keyboard. With a product line that currently includes nine polysynths at various price points and with a variety of architectures, what’s left? Collaborating with Tom Oberheim on the OB-6, naturally.

With a front panel reminiscent of a vintage OB-8, the Dave Smith Instruments OB-6 is a six-voice analog synth that combines modern functionality with the classic sound of the Oberheim SEM. The result is a six-voice monster that has more in common with Oberheim’s pioneering SEMbased gear than his later OB-Xa and OB-8 keyboards. This is a wise decision, too, as those later Oberheim synths relied on CEM3320 Curtis chips for their lowpass filters. While the classic Curtis chips are certainly nice and warm, they’re not as flexible as Oberheim’s original state-variable design. Combined with Smith’s legendary modulation and oscillator expertise, the OB-6 is a keyboard that sounds like nothing else in the history of polysynths.

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Like the Dave Smith Instruments Prophet-6, the OB-6 sports a four-octave velocity-sensitive keyboard with Aftertouch and a front panel festooned with dedicated controls. As with many vintage analog synths, you select patches using buttons that serve double-duty for accessing the instrument’s global parameters such as MIDI, tuning, velocity sensitivity, and so forth.

Fig. 1: In addition to stereo outputs on 1/4" jacks, the OB-6 accepts two footswitches and two expression pedals. You’ll also find MIDI In, Out and Thru on DIN connectors along with a USB port. The back panel offers the essentials—a pair of unbalanced 1/4" outputs, a 1/4" headphone jack, DIN and USB MIDI ports, and jacks for two footswitches, as well as volume and filter expression pedals (see Figure 1). There is even an internal power supply (no wall warts here), making the entire package feel pro-level. And like other Dave Smith Instruments’ products, the all-metal chassis with wood end-caps exudes roadworthiness.

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Sound sources

Each of the two oscillators includes slightly different features. As with the Prophet-6, their waveforms are continuously variable, with oscillator 1 sweeping from sawtooth to pulse shape, while oscillator 2 can go from triangle to sawtooth to pulse. This is one of those features that should be included on every analog synth, because it offers a lot more sonic versatility than static waveforms.

Oscillator 1 includes hard sync, whereas oscillator 2 offers a low-frequency mode and switchable keyboard tracking, giving it plenty of flexibility for modulation purposes, especially with its variable waveforms. As for other tone sources, you can access a square-wave sub-oscillator and white noise generator in the oscillator mixing section.

One of the coolest features of the OB-6 (and Prophet-6, for that matter) is its unison mode with detune knob. In addition to the classic stacked-monophonic mode, DSI’s approach to unison also includes chord memory functionality and the ability to specify the number of voices stacked on a single key. Used in conjunction with unison, the detune knob allows you to spread the tuning of the voices to create massive textures. With unison off, the detune knob serves to re-create the idiosyncratic tuning inconsistencies of vintage gear, making each voice slightly out of tune. Frankly, I love this sound, as adding small amounts of detuning lends an organic quality to obviously electronic textures.

Filters and Modulation

When it comes to the filters, we’re talking vintage Tom Oberheim. Based on the unique state-variable 2-pole filter from the original SEM (and its subsequent multi-voice configurations), the OB-6 filter’s ability to smoothly sweep between lowpass, notch and highpass modes—or serve as a bandpass filter—gives the instrument a wider range of sounds than either of the current DSI Prophets. Granted, those synths include a fatter, 4-pole lowpass option that some players prefer.

The OB-6 also includes a feature that SEM users have requested for decades: the ability to modulate the filter state from an LFO or envelope. With the exception of the DSI Pro 2, this sound was previously the domain of softsynths. In the OB-6’s analog implementation, the results are breathtaking, delivering shimmering, resonant sweeps that most other analog synths can’t touch.

With a single LFO and two ADSR envelopes, the OB-6’s modulation amenities seem straightforward. The LFO offers five waveforms—sine, up saw, down saw, square, and random—with simultaneous routing options for both the pitch and pulse-width of each VCO, as well as VCA, cutoff and, yes, filter state, which is nifty for phaser-like effects.

Although the two envelopes are hardwired to filter and amp, just like most classic analog synths, if you dig a little deeper into the X-Mod section you’ll quickly find that there are many more routing options for the filter envelope, too. These focus on VCO 1 parameters such as pitch, waveshape, and pulse-width, as well as cutoff, filter state, and switching the filter between normal and bandpass modes. Moreover, this section includes VCO 2 as a modulation source, which is impressive because it can be used as either a second LFO or audio-rate modulator for distinctive textures that you won’t find on any other analog polysynth (unless it’s from DSI, naturally).

The end result is quite similar to the legendary Prophet-5 Poly-Mod section, with the addition of Tom Oberheim’s unique filter options. Make no mistake, the apparent simplicity of the X-Mod options conceals a treasure trove of sonic possibilities. In addition, there is a dedicated section for customizing the synth’s Aftertouch destinations— and depth—on a per-patch basis, which helps the OB-6 really breathe in live performances.

Analog and Digital Effects

The OB-6’s integrated 24-bit digital effects are the same as those found on the Prophet-06 and include reverb, delay (including a lovely re-creation of old-school analog BBD models), chorus, flanger, as well as faithful emulations of the original Oberheim phase shifter and ring modulator. There is also a stereo analog-distortion effect available, which is great for adding warmth or creating industrial mayhem.

As with the Prophet-06, the effects include a proper circuit bypass that preserves the analog signal path when the digital effects are switched off. For purists, this is another thoughtful addition.

Arpeggiator and Sequencer

The OB-6’s arpeggiator offers up, down, up/ down, and random patterns, along with an assignable mode that remembers the order of notes as they are played. As you would expect, the arpeggiator can sync to tempo via MIDI.

The polyphonic step-sequencer on the OB-6 is a simple record-and-play affair offering 64 events, including ties and rests, similar to that found on the original Roland SH-101 and the newer Arturia MicroBrute. This is great for ’80s-style synth-pop and dance music and each pattern is saved as part of your preset. Furthermore, the factory presets include intelligent musical patterns that show off each patch. And with 500 user slots available, there is ample room for artists to duplicate presets and add their own unique patterns.

Back to the Future

For keyboard-synth fans, the OB-6 is a dream come true. It combines the unique elements of the Prophet-5 and Oberheim SEM, then adds a truckload of contemporary features that allow for more complexity than either of its predecessors.

If you’ve been holding out for the right analog polysynth, this may be the one for you. The OB-6 provides the perfect combination of classic and modern designs, with filters that will blow your mind.

Snap Judgment

Pros Completely analog signal path with real VCOs. State-variable filter, based on the Oberheim SEM. Modulation destinations include filter mode. VCO 2 offers LFO. X-Mod for complex routings of VCO 2 and filter envelope. Integrated effects that sound fantastic.

Cons Some global features require multiple trips to the manual.

Bottom Line

A flawless blend of the Prophet-5 functionality and SEM sound.

$2,999 street