You are here

Dave Smith Instruments OB6

Polyphonic Synthesizer
Published August 2016
By Gordon Reid

The OB6’s main panel, resplendent in the fine pinstripe of its ancestor, measures 807 x 323 mm.The OB6’s main panel, resplendent in the fine pinstripe of its ancestor, measures 807 x 323 mm.

The OB6 is the product of a collaboration between two of the most respected names in synthesizer design, and it doesn’t disappoint...

Like many with an interest in polysynths, I have a soft spot for vintage Oberheims, whether from the SEM era (the 4- and 8-Voices), the black & cream era (the OBX), the pinstriped era (the OBXa, OBSX and OB8) or the hybrid era (the Matrix 6 and Matrix 12). Of these, fans tend to lust after the glorious SEM-based instruments, while performers tend to veer toward the greater practicality of the later models. Neither group is wrong, they have different priorities. Unfortunately, we haven’t had the opportunity to argue about the pros and cons of a new Oberheim analogue polysynth for nearly 30 years and, when Tom Oberheim pulled the plug on the SO4V (Son Of 4-Voice) last year, it seemed that the moment had passed. It was therefore with surprise and, for many, delight that the world learned of the forthcoming DSI OB6. How could anyone fail to be interested in a synth designed jointly by Dave Smith of Sequential Circuits and Tom Oberheim?

Dave and Tom have spoken in the past of their friendly rivalry, and I recently asked them how this collaboration had come about. Tom said, “For me, it started when the Prophet 6 came out and I had the opportunity to discuss some of its design elements with Dave. I was very impressed with the approach he took to the control aspects of the machine, and I thought the voice cards were very cool.” Dave continued, “Last summer, the idea of a new version of a classic OB polysynth came up and I realised that it would be relatively straightforward if we used the Prophet 6 hardware and software architecture and then worked together on the voice card design and the front-panel layout. So, that’s what we did. The Prophet 6 had everything we needed — very precise control voltages running at audio rates, the effects engine, and so on — so we designed the SEM-on-a-board and connected the CVs. It’s easy to see if you open the unit up,” he continued, “the motherboard in both instruments is the same, but the six voice cards in each — which are basically a complete programmable monophonic synth on each card — are completely different. It’s also important to note that they are not compatible, so you can’t swap them between the two synths.”

Each of these cards contains two VCOs, a VCF and a VCA, and the oscillators and filters are described in the blurb as ‘discrete’. I wondered whether this meant that they are genuinely built of individual components. “Yes, they’re really discrete, with transistors, op amps, and lots of VCAs for full programmability,” Dave confirmed. “There are no latter-day ICs that would work for oscillators or filters in this design.”

The Voicing

There are two oscillators on each card, and they extend the traditional SEM architecture in numerous ways. For example, VCO1 allows you to select any wave shape in a continuous sweep from ramp to pulse/PWM waves, and boasts a square-wave sub-oscillator one octave below the standard pitch. VCO2 is even further enhanced, allowing you to select any waveform in a smooth sweep from a triangle through a ramp to a pulse/PWM wave, adding another new family of timbres that the SEM can’t generate. It also boasts detune, a low-frequency mode, and the option to switch keyboard tracking on and off. If you’ve ever programmed a Prophet 5, you’ll recognise this architecture instantly. VCO1 can be sync’ed to VCO2 to obtain the usual range of harmonically rich timbres, and you can sweep its pitch to generate the classic ‘tearing’ sounds. However, since you have a wider range of waveforms as starting points, you have a greater range of sync’d timbres at your disposal. Since the frequency ranges of the oscillators and their tracking accuracies are greater than on vintage synths, DSI have provided a second detune parameter that adds a bit of random offset and drift to both oscillators’ pitches. Used carefully, this can make the OB6 sound less pristine — a bit more vintage, if you prefer to think of it like that. A fourth sound source — white noise — can be added in the mixer stage.

The filter on each card is based upon the 12dB/oct state-variable resonant but not self-oscillating filter introduced on the SEM, and offers a smooth sweep from low-pass, through notch, to high-pass responses, as well as an additional band-pass mode. It offers three keyboard tracking modes (0, 50 and 100 percent), the last of which allows you to emphasise a narrow range of harmonics in the initial timbre and have the filter track them as you play up and down the keyboard. The sounds you can obtain from this can be rather special, particularly with high resonance in band-pass mode. The filter also has a dedicated, bi-polar ADSR contour generator, with the option to control the contour amount using keyboard velocity. An audio signal amplifier then completes the analogue signal path for each voice. Again, this has a dedicated ADSR contour generator, and again the amount can be velocity-sensitive should you wish it to be.

Modulation is provided by the Prophet 6 host, and its contours and LFO are generated digitally before being converted to CVs and directed toward the oscillators, filters and amplifiers. This means that the OB6 is an analogue/digital hybrid, which can be a disincentive to some analogue purists. When I discussed this with Dave he explained, “We have over 100 CVs internally, and all can run at audio rates, so it works great. It’s also more flexible. For example, we could change the envelope shapes to better match the SEM and allow faster ramp times than the SEM is capable of for even snappier sounds.”

Since the modulation architecture is essentially that of a Prophet, it should be no surprise to find that it comes in the form of a mono-mod section called LFO and a poly-mod section called X-Mod. This means that, unlike most modern synths, the OB6 has a single LFO that affects all the voices equally. Consequently, if you want (for example) to modulate the pitch of VCO1 and the pulse width of VCO2 using the LFO, the frequency and the phase of both effects is the same across all the notes played. It offers six waveshapes (random produces either S&H or white noise, depending upon the frequency) and seven destinations: the pitches and pulse widths of VCO1 and VCO2, the filter mode and frequency, and the output gain. You can select any combination of these but the amount of modulation, which is the sum of the Initial Amount and the position of the mod wheel, is applied to all equally. The frequency range is huge — from 0.02Hz up to 500Hz — which means that, at the high end, you can create AM and FM effects, but the LFO doesn’t track the keyboard so you obtain a different sound from each key. However, you can sync the LFO to the arpeggiator/sequencer clock or to MIDI Clock for tempo-sync’ed effects, which is good.

The X-Mod section is a very different beastie, and this is where you apply modulation on a ‘per-voice’ basis. There are two bi-polar sources — the filter envelope and the output from VCO2 — and you can direct these to the pitch, pulse width and wave shape of VCO1, the filter frequency and mode, and the position in a range between the filter band-pass mode and whatever is selected in the swept mode. I had great fun sweeping between the band-pass and notch modes at audio frequencies to accentuate and then remove the same set of frequencies. This is a weird sort of AM, and some of the sounds obtained were very unusual. Many of the vintage Prophets’ signature sounds were created using poly mod, and you can create some stunning monophonic sounds using it on the OB6. But be aware that, as on a Prophet 5 and its ilk, you can’t use FM as the basis of polyphonic patches because it’s all but impossible to match the oscillator frequencies and modulation amplitudes on each SEM board closely enough to create sounds that will be consistent from note to note.

The third modulation panel is called Aftertouch, which is self-explanatory. The response is bi-polar and can be applied to the pitches of VCO1 and VCO2, the loudness, the filter cutoff frequency and mode and, most important of all, the LFO amount, which allows you to control the amount of modulation applied to all of the destinations specified in the LFO section.

Next in the signal path lies an analogue distortion effect, which can add anything ranging from a mild overdrive to massive signal distortion. Many users might be tempted to crank this up, but you should be aware that, when used heavily, it makes sounds significantly louder, requiring a decrease in the patch volume to keep driven sounds at equivalent levels.

Finally, we come to the dual digital effects units. Both host the same selection of nine effects, although unit B adds a choice of four reverbs to the standard set. Some of the effects impressed me, and the ring modulator — which can track the keyboard, and locks to the first note played and held — stands out. Some of the others I might use more sparingly. Editing is straightforward since, in addition to their individual on/off statuses and wet/dry mix, each effect offers just two parameters. Nonetheless, you can synchronise the delay effects to the arpeggiator, the sequencer or MIDI Clock for the usual range of rhythmic effects. Alternatively, if you bypass the effects section, the audio signal passes to the outside world without ever leaving the analogue domain, which explains DSI’s claim of an all-analogue signal path.

The Rear Panel

Dave Smith Instruments OB6

It should come as no surprise that the rear panels of the OB6 and Prophet 6 are the same. This means that, in addition to the quarter-inch left/right stereo outputs and the quarter-inch TRS headphone output alongside them, you’ll find two quarter-inch TRS expression-pedal inputs for loudness and the filter cutoff frequency, and two quarter-inch TS inputs for sustain and sequencer/arpeggiator on/off.

Next to these, you’ll find the standard MIDI In/Out/Thru on five-pin DIN sockets, and MIDI In/Out via USB. Finally, there’s an IEC power input for the internal 100-240 V AC 50/60Hz power supply.

The Arpeggiator & Sequencer

It’s not surprising to find that the OB6 shares the Prophet’s arpeggiator, offering rates from 30 to 250 bpm or synchronisation to MIDI Clock, with timing divisions ranging from half to eight times the bpm. Arpeggios can cover up to three octaves using any of five modes: up, down, up/down, random and assign, the last of which is the order in which you pressed the keys to create the pattern. If you press Hold, the arpeggio will continue to play until you release it, and a rather nice touch is ‘Relatching’, which means that, as long as you keep one note held from the initial pattern, you can add additional notes as the arpeggio plays, creating more complex patterns than would otherwise be possible. However, while the manual mentions that you can set ‘repeats’, which I take to mean ratcheting, that’s not available.

Likewise, the sequencer is common with the Prophet 6. This offers 64 steps, each capable of holding up to six notes so that you can sequence chords as well as melodies. You can also program rests and ties, start and stop the sequence using a footswitch, and transpose sequences while they’re playing. You can also play over the top of sequences using any unallocated voices. If that sounds a bit basic, you’ll be surprised at how much you can achieve with it because you can clock the sequencer using a gate signal or percussive audio generated in the outside world. So, while the sequencer has no real-time recording or playback capabilities, this raises the possibility of much more interesting timings than is possible using a clock. To test this, I programmed a chord progression into the OB6 and then directed the Gate from my SH101 into its Sequencer jack. When I played a solo on the SH101, the OB6 accompanied it and, if I wanted to hang back a little, or push forward against the beat, I could do so. Alternatively, I could record a sequence into the SH101 and, on playback, this would drive the OB6. The results were pure techno. Since every patch can be saved together with its dedicated sequence, there are no fewer than 500 sequence memories, which should be enough to keep you engaged for a while!

Moving on, unison mode allows you to turn the OB6 into a monosynth with up to six voices under each key, and you can determine how many notes are in the stack, the amount of detune and the spread of voices. This is where the synth’s Key Assign modes become important. There are six of these — lowest, highest and latest priority, with and without multi-triggering — so you can tailor the OB6 to respond in the same way as your favourite monosynth. Unison also offers chord memory, and the key assign mode then affects how the chord is played; if low-note priority is chosen, the pressed key is the lowest in the defined chord, whereas if high-note priority is chosen, the pressed key is the highest in the chord. That’s a nice touch. Also, the chord is saved with the patch, which is useful.

Finally, the OB6 offers polyphonic portamento, and you can choose from four modes that add another set of performance options. Happily, and unlike vintage synths based on SEMs, the slew rates are the same for all voices, so you can play in unison with portamento without creating weird (although sometimes wonderful) detuning effects. However, the slew is applied only to the CVs controlling the oscillators so, if you play a key at the bottom of the keyboard followed by one at the top, or vice-versa, the filter frequency jumps instantly if keyboard tracking is on. This means that upward and downward glides of the same patch can sound very different.

Dave Smith Instruments OB6

In Use

Despite being so diminutive, the OB6 looks great, and feels both solid and robust. But since it’s based upon the Prophet 6’s chassis, it’s restricted to a four-octave semi-weighted keyboard, which is pretty much guaranteed to divide the synth community. I would never choose to use a four-octave synth if it could be avoided but, if it’s sufficient for you, or you’re wiling to play the OB6 over MIDI from a larger keyboard when necessary, I’ll say no more about it.

It offers 500 factory patches in ROM, which are duplicated in the 500 RAM locations ready for editing and overwriting. Stepping through the first few of these, it soon became apparent that the OB6 excels at, well, almost everything. From aggressive lead sounds to monstrous basses, from polyphonic patches that would cut though any mix to the gentlest of aetherial pads, from vintage sounds to the most modern freaked-out EDM, it was hard to find an obvious weakness. But what of the underlying sound? Does it really have the character of the early, SEM-based Oberheims?

Before answering that, I’d like to dispel two myths; the OB6 doesn’t sound like a Prophet 6 with blue stripes, and neither does it sound like an OB8. To be fair, I could program some patches on all three that were almost indistinguishable, but those were relatively rare. So let’s now return to the main question. To test this, I compared the OB6 with my 4-Voice. The results were as I had expected. The OB6 occasionally lacks the depth that can occur when each of the SEMs in the 4-Voice do something slightly different from one another, but its underlying tone is undoubtedly SEM-ish, albeit with a modern sheen that is probably nothing more than all the voices doing what they’re supposed to, rather than doing something approximating what they’re supposed to. So, while the two synths’ characters are not identical, they’re nowhere near as different from one another as they are from my OB8, let alone something like a Prophet 5, or a Jupiter 8 or a Memorymoog, or any other analogue polysynth that’s not based on SEMs. Furthermore, the OB6 is far more flexible and more playable than its venerable forebear and, the more I compared them, the more I realised which I would choose today if someone asked me to add some ‘Oberheim sounds’ in a session. (Hint: it’s not a vintage synth!)

I proceeded to create a library of new patches, and it proved simple to obtain the sounds I wanted. While it’s not strictly a ‘one knob equals one function’ interface, there are no voicing menus or additional pages of parameters, so everything fell to hand and there were no nasty surprises lying in wait. Some people have already criticised the OB6 for lacking the programming depth offered by (for example) the OB8’s page 2 functions, but to do so loses sight of the bigger picture. DSI could have poured all manner of additional functions into the OB6, but that would have destroyed its immediacy and turned it into a different instrument. While creating my new sounds, performance was catered for by the combination of velocity and aftertouch sensitivity, together with the pitch-bend and modulation wheels, plus the four pedal inputs. However, I didn’t get the opportunity to try something that I think would be a standout capability on this synth; although it’s not mentioned in the descriptive part of the manual, one of the MIDI message tables in the manual implies that the OB6 responds to Polyphonic Key Pressure. If true, this opens up yet another layer of possibilities.

But despite its good points, there are several things that could have improved the OB6 still further without altering its underlying character. The first of these is the addition of a small screen to enable patch naming. I know that vintage synths didn’t have these, but neither did they contain 1000 patches. Secondly, I would have added a dedicated vibrato LFO (as on the Oberheim TVS-Pro) to free up the global LFO. Next, I would have tweaked the CVs generated by the LFO section; currently, a modulation amount that’s suitable for vibrato is too delicate for tremolo. I might even have been tempted to add a limited amount of analogue connectivity because there’s currently no way to integrate the OB6 into analogue setups except through its pedal inputs. I would also have tried to make it possible to run the sequencer and arpeggiator simultaneously, and for them to transmit what they’re playing to the outside world as MIDI Notes. Being bolder, and if I were designing the OB6 from scratch, I would have made it (at least) an eight-voice synth and incorporated a five-octave keyboard. But I accept that, had this been the specification, it’s unlikely that the OB6 would ever have been built, so I’d better shut up now!

As for faults (rather than omissions) I discovered just one; I was unable to transpose the sequencer when driving it from an outside source. It’s a minor problem, but one that needs addressing.

Finally, I just have a word of caution regarding the audio outputs. If you increase the pan setting, the synth’s voices are spread left and right in increasing amounts unless mono mode is selected in the global parameters. Don’t fall into the trap of connecting it with just one cable while in stereo mode. The OB6 doesn’t use the mono/left output convention and I know of one chap who was convinced that his OB6 was broken because half of the voices were so quiet.

Conclusions

Acquiring the ‘SEM sound’ has become very expensive in recent years and there’s no guarantee that, having bought a vintage synth, your pride and joy will always work properly, or sometimes work at all. So the introduction of the OB6 is significant, and we should applaud Dave Smith and Tom Oberheim for working together to produce a polysynth that simultaneously feels familiar but produces a sound that is quite different from anything previously to emerge from the DSI stable.

I hate to eulogise but, the more I used the OB6, the more I discovered how much I loved the sounds it was producing. You may claim that it’s not the same as a bunch of vintage SEMs in a box, and I agree — for me, it’s better than that. And, while it’s not a low-cost synth, neither is it expensive, and it costs far less than you’ll spend on a fully functional 4-Voice. So put aside any prejudices that tell you that modern synths aren’t as good as vintage ones; if you were to ask me which to buy as a musical instrument rather than as a collectors’ item, my brain would outvote my heart, and the OB6 would win every time.

Alternatives

The obvious competition for the OB6 is the Prophet 6, and if you prefer something that sounds like a Prophet, this may be the synth for you. But if you want the SEM sound and require more than two voices, your only alternative in the analogue domain is currently a vintage Oberbeim, with all of the size, weight and risks that that entails. Alternatively, if you’re prepared to consider a digital synth, there are numerous VAs that can approximate the SEM sound and, in the software realm, there are packages such as Arturia’s SEM V that may surprise you by how well they emulate the sounds of the synths that inspired them.

The Soundtower Editor

Dave Smith Instruments OB6

While the OB6’s sound may be vintage, its MIDI specification is not, with many parameters sent and received as MIDI CCs and the full complement available as NRPNs. The forthcoming Soundtower editor takes advantage of this, allowing you to view and edit patches, view sound banks and perform all the usual librarian functions. In addition, it provides powerful morphing and ‘program genetics’ operations to create new sounds using selective mixing, morphing, mutating and randomising. You can also edit the sequencer with a range of drawing and text tools.

It’s an extensive package although, at the time of writing, it was unfinished, with a number of omissions and bugs to be ironed out. Mark Lanoszka at Soundtower confirmed that a production version that addresses all of these points should be available long before you read this review.

Calibrating The Synth

When the review OB6 arrived, its sound was inconsistent from one voice to the next, so I ran the calibration routine to bring the oscillators and filters into line with one another. The manual recommends that you repeat this procedure multiple times over the first few days of use so that it can “learn the range of temperatures at your location and keep itself in tune over this range”. I’m not sure what this means for lobbing the synth off the back of a truck on a snowy January afternoon, but the routine only takes a few seconds and, once the OB6 has warmed up, a couple of calibrations tend to bring everything into line without ever making it sound dry or sterile.

Global Parameters

Although most of the voicing is laid out in front of you, there are 19 global parameters relegated to a parameter access system. These include transposition, master tuning, MIDI Channel and Clock, the input used for MIDI, MIDI Clock and SysEx, whether front panel changes are transmitted and received as MIDI CCs or NRPNs, and whether the synth responds to MIDI controllers such as pitch bend and modulation. A second row includes Local on/off, how the synth responds to signals received at the sequencer input, the knob mode, the sustain pedal polarity, alternative tunings (a choice of 16, including microtonal scales), the choice of velocity and aftertouch response curves, and whether the output is mono or stereo.

Published August 2016