At first glance, a couple of things jump out on the OB-6. The Blade Runner-style horizontal blue lines, the long row of program buttons and the stacked filter and loudness envelopes are clearly pulled from the original OB series keyboards. However, you may notice a strong resemblance to Dave Smith's other new analogue polysynth, the Prophet-6. It was no accident; the OB-6 was built on the foundation of the Prophet-6. This allowed the designers to worry less about nuts and bolts considerations like MIDI programmability, sequencing, presets and effects and concentrate on the core pieces needed to achieve the characteristic Oberheim sound.
Given the DSI foundation, the OB-6 has a very similar spec sheet to the Prophet-6. At 31.8 inches long, it's slightly shorter than the Prophet-6, but they both share the same four-octave keyboard, pitch bend and mod wheel controls. This octave range has been controversial since these keyboards were introduced, with many wishing for the additional fifth octave found on the originals. Moving to the back of the unit you'll find the same pair of stereo jacks and a headphone jack for output, USB and 5-pin slots for MIDI and footswitch/expression pedal inputs for sequencer start/stop, sustain, filter cutoff and volume. The only difference here is the behaviour of the filter cutoff input.
Since I've recently reviewed the Prophet 6 Module, I'll concentrate on the ways in which the OB-6 distinguishes itself. The first area is the oscillator design. While the OB-6 has a similar voice architecture to the Prophet-6—specifically two discrete VCOs per voice plus a sub oscillator and noise source—there are concrete differences. In interviews, Dave Smith mentioned that the OB-6 oscillators were taken from Tom Oberheim's famed SEM design. Tony Kara from DSI confirmed as much, saying that, "The OB VCO did come directly from the SEM, but of course in order to make it programmable, we had to add extra stuff. Also, the triangle never existed in the SEM, so that was a lift from the P6." Both of the oscillators have continuously variable wave shape, but only oscillator two has the triangle wave. Also, the sub oscillator on the OB-6 is a square wave rather than the triangle found on the Prophet-6. All this adds up to a raw sound that's much more closely related to the OB-Xa than anything else in the modern era of analogue polysynths. If you're interested in hearing what this difference sounds like, Marc Doty has a good video on his Youtube channel comparing the Prophet-6 and OB-6 oscillators side-by-side.
The other big difference in the OB-6 is the filter architecture. Where the Prophet-6 has a four-pole low-pass filter and a two-pole high-pass filter operating independently of one another, the OB-6 sticks to the Oberheim heritage of a single two-pole state-variable filter. This filter can slide seamlessly between low-pass, band-pass and high-pass modes with the Filter Mode knob or you can substitute notch for band-pass at the press of a button. The OB-6's filter mode can be modulated with the LFO, keyboard aftertouch or the X-Mod section, which allows you to modulate with the filter envelope or the frequency of oscillator two. This filter is a direct lift from filter two of Dave Smith's powerhouse monophonic Pro-2 synth, which itself was borrowed from the Oberheim SEM—a sign that perhaps hinted at this future collaboration. There's no doubt that this is the biggest source contributing to the OB-6's distinct sound. In notch mode, there are sweet spots in the filter where you can find the marriage of solid bass fundamentals and high-frequency sizzle historically associated with the Oberheim sound. The various modulation options for both filter frequency and filter mode allow you to make sounds that are impossible to achieve on the Prophet-6, but there are arguments for the other side as well—the Prophet-6 filter can self-oscillate, for instance, and some people prefer the flexibility of two independent filters.
At $2999 ($200 more than the Prophet-6), one could argue that the OB-6 needs to bring a lot to the table to justify the cost. However, when you realise that the OB-Xa was significantly more pricey 40 years ago and the Oberheim Two Voice Pro costs more for four less voices, the OB-6 is a no brainer if you're after the Oberheim sound in a reliable modern package. The foundation that the DSI team has built for the Prophet-6 translated extremely well to the OB-6 and with the Oberheim oscillators and filters, the result is a truly beautiful sounding instrument. Outside of the somewhat limited modulation possibilities that I mentioned in the Prophet-6 review, the only complaint I have is that there is no desktop module version available for the OB-6 at the time of this review. I know many folks who would prefer to grab a less expensive version for their MIDI-capable studios.