Review: Sequential Prophet-6 Analog Synth

October 12, 2015

It’s hard to believe it was about 20 years ago when some of us first started missing the great sound and instant-results interfaces of analog synthesizers. “Why doesn’t someone make a fully-analog mono synth?”. . . “Wouldn’t it be great if modular synthesis came back?” . . . “Companies should recreate the great analog synths of the past, but with modern stability.” We asked for all these things and, amazingly, we got them. But one plaintive cry, especially present in online forums has gone unanswered: “Will someone make a classic polyphonic slab synth based on actual voltage-controlled oscillators?” Until now.

Concept

Who would do it? Of course, we all looked to Dave Smith, but he seemed ambivalent about VCOs, and with the success of his Prophet 12 and Pro 2, both of which employ digital oscillators feeding analog filters, he had every reason to be.

That is, until the Prophet-6 was announced at NAMM this past January. Right in the midst of the success of his other new synths, he granted our wish for a VCO poly, in the form of a successor to the iconic Sequential Prophet-5. That even extends to its “Sequential” branding, as Yamaha, who had owned the name, returned the rights to Smith at the beginning of this year. But this is no mere reissue. The Prophet-6 draws on the foundation of the Prophet-5 (let’s call those the P6 and P5), but is greatly expanded in functionality and expression.

The Prophet aesthetic is delightfully preserved with similar graphics, similar LEDs, and similar buttons. It’s definitely a “vintage” interface and yet gives all the control necessary to effectively access all of the new features.

Oscillators

Historically, Dave Smith Instruments has used digitally-controlled and outright digital oscillators in their synthesizers. But for the Prophet-6, they designed a brand-new VCO that boasts a substantially-expanded feature set compared to the P5. Plus, it’s stable enough for you not to need to worry about tuning, but analog enough to create slop between the two oscillators if desired.

The tuning knobs for each oscillator are quantized to pitches. As such, the synth is largely in tune at any given moment. Dialing in octave and interval changes is very easy, even if some might prefer a continuously free range of frequency to choose from.

The P5 had buttons to select the somewhat limited choices of a sawtooth or a square waveform for either oscillator. The Shape knobs in the oscillators of the Prophet-6 define the waveform, and they show a square wave, a sawtooth, and the new inclusion of a triangle. But unlike the P5, these waveforms are continuously variable. As you turn the knob, one analog waveform morphs into the next with an effectively infinite amount of intermediate shapes in between. This may be one of the few examples of continuously-variable waveforms on a VCO-based synthesizer.

Plus, the waveform of Oscillator 1 can be modulated by Oscillator 2 in the Poly-Mod section (more on this later), putting this continuous variation on musically useful auto-pilot. Throw pulse width modulation into this mix, and you can modulate the waveform with a waveform that has itself been modulated. I don’t need to belabor how “meta” that is.

The second oscillator can still be set a lowfrequency mode, not to mention used as a modulation source in the Poly-Mod section. Using the second oscillator as a modulation source in Poly- Mod, and then modulating various aspects of either oscillator using the LFO, can result in some astounding and unpredictable sounds, especially if Oscillator 2 is in the audio range.

These days we have very high expectations about tuning stability, and the Prophet-6’s VCOs certainly stay in line. However, some of us crave a more vintage sound that was the result of wellmeaning- but-unruly oscillators drifting. The Slop knob on the P6 allows oscillators leeway in regard to pitch—anywhere from subtle phasing to fullon cacophony. Artful usage of this feature allows the vintage synth lover in all of us to have our cake and eat it, too. Yes, hard sync is still here, so you can play Cars covers for days.

Filters and Mixer

The Prophet-5, like most other vintage analog synthesizers, featured a lowpass filter. But as the owner of a Yamaha CS-50, I’ve always loved the versatility that comes with also having a highpass filter. So I was quite excited when I saw a resonant highpass filter on the Prophet-6.

I was a fan of the lowpass filter on the DSI Pro 2, so I was happy to find out it’s that very filter (times six) that’s in the P6. It’s warm, rich, and has a pleasing character. The highpass filter is a new design, and works very well alongside the lowpass to create bandpass effects, nasal sounds, serious low-end emphasis, formant simulation, and more.

The filter envelope is shared by both filters, but can be modified by the Amount knob (which is bipolar) and Velocity buttons, which determine the intensity of the envelope from the velocity of your playing. Features like this make the P6 capable of expressive feats the P5 could only dream of.

Keyboard-based intensity includes “half” and “full” options for each filter. “Half” is great for producing traditional dark basses when you play softly and bright funky synth stabs when you play hard. “Full” does this even more, but can get extreme enough to turn the self-oscillation of the lowpass filter into a further sine wave generator.

In addition to the filter envelope, each filter can be independently modulated by aftertouch, the Poly-Mod section, or the LFO, allowing a tremendous diversity of simultaneous modulation possibilities.

The mixer in the P6 is similar to the P5 with one addition: a sub-octave tone, which is a triangle wave derived from Oscillator 1. Suboscillators are popular in monophonic synths for bringing in low end, but this works well in a polyphonic context, too.

LFO

On the Prophet-6, the LFO (low frequency oscillator) and “Wheel-Mod” of the Prophet-5 have been combined. The waveforms “Reverse Saw” and “Random” (equivalent to sample-and-hold) have been added as modulation sources, and the new highpass filter and amp have been added as destinations.

On the P5, you could direct LFO modulation to each oscillator’s pulse width independently. On the P6, the two have been combined via a single button. However, the option to control the amp with LFO modulation is new, and greatly increases the synthesis possibilities. The LFO can affect the lowpass and highpass filters either simultaneously or independently via the multiple-select button; LEDs help you keep tabs on which filters you’re modulating. An LFO Sync button syncs the frequency of the LFO to the Arpeggiator/ Sequencer clock.

Most commonly, you’d bring in the LFO using the modulation wheel, but if you’d like LFO-based modulation to occur at a fixed amount without using the wheel, you can use the Initial Amount knob that Sequential has added to the LFO. This can be used in tandem with the mod wheel to change the degree to which either of them modulates.

Poly-Mod Section

One of the huge claims to fame of the original Prophet-5 (alongside polyphony and patch memory) was the Poly-Mod section, and it has returned in the Prophet-6 (see Figure 1). The term is short for Polyphonic Modulation, which is the ability for multiple sources to modulate multiple destinations at once, in varying amounts.

Fig. 1. In the Poly-Mod section, you can dial in positive or negative amounts of the filter envelope or oscillator 2 as modulation sources, and hit the buttons to route them to destinations. It looks simple, but can sound complex.

On the P5, via two amount knobs, you could use oscillator 2 (which could operate either in its normal audio range or as a secondary LFO) and the filter envelope to modulate any combination of the pitch of oscillator 1, the pulse width of oscillator 1, and the (lowpass) filter cutoff.

The P6 adds more to this section, namely bipolar positive/negative knobs for the sources (the P5’s amount knobs were positive only), and more destinations. These include the continuously-variable waveform of oscillator 1 and, since there are two filters, either the lowpass or highpass filter or both.

The possibilities for modulation are very deep here. Beyond the ability to route multiple sources to multiple destinations, you can circle back and modulate some of the sources themselves, which again has the “meta” result of modulated modulation. Poly-Mod may not be as multifarious as the mod matrix on a soft synth such as Omnisphere, but because of how cleverly this seeming handful of sources and destinations is implemented, it’ll be a long time before you exhaust its sonic diversity. After playing a prototype at DSI headquarters, editor Stephen Fortner remarked, “Between the sequencer and the Poly-Mod, this thing can practically do generative music like a large modular synth if you want it to. You can just let it rip and not hear a timbre repeat for over an hour.”

Aftertouch

The keyboard of the Prophet-5 (and those on most competing slab synths of the era) sensed neither velocity nor aftertouch. In keeping with modern expectations, the Prophet-6 senses both. The Aftertouch section lets you apply positive or negative pressure-based modulation to a number of destinations. It’s “channel” aftertouch, so the effect is uniform on all depressed keys.

Want upward or downward pitch-bend on one or more oscillators? The aftertouch can do that. I found that setting one of the oscillators to bend up a full octave (the rough limit on the positive side of the Amount knob) led to some beautiful CS-80-like glides against the static pitch of the other oscillator.

A common application of aftertouch is to add vibrato even when both hands are engaged in playing. The LFO Amount button works in conjunction with the Amount knob to let you bring in just the right depth without excess pressure overshooting the mark.

The Amp button lets you control volume with pressure; with a negative amount setting, more equals less. Plus, similarly to other sections, a dual button routes aftertouch to either or both of the filters.

Choosing multiple destinations, e.g., amp and filter in order to get louder and brighter (or quieter and darker given negative amount settings) can add subtle but powerful expressiveness. Finally, you can fine-tune the aftertouch sensitivity in the Global section. Players who find themselves going to fullblast pressure too quickly will find this very useful.

Unison

In general, unison functions stack all the oscillators of a polyphonic synth into one huge monophonic voice. While there have been many terrible implementations of this concept in the past, it sounds wide and warm on the Prophet-6— mainly because it’s not the perfectly-in-tune proliferation of PCM-based samples you’d get on a modern workstation.

Furthermore, the P6 lets you define how many oscillators will speak in unison mode: anywhere from a single one to two layers of six. Best of all, even if you pummel your listeners with 12 stacked-up VCOs, you have the Slop knob to dial in the warm tuning and phase variations of those VCOs, as opposed to the more robotic sound that can occur when this stacking is imitated digitally. Add some portamento, and presto—you’ve got yourself a monster of a mono synth.

Effects

In general, I’m the kind of analog purist that doesn’t use many effects, but I have to say that the ones in the Prophet-6 really complete the process of creating iconic sounds. While they are digital effects, they are implemented in such a way as not to detract from the synth’s overall character. They’re immediately accessible, controllable via a knob-per-function setup, feature true analog bypass, and sound simply great.

Two effects channels are available simultaneously. Effect A includes bucket-brigade-style and digital delays, chorus, and two phasers. Effect B has all of the above plus a reverb with hall, room, spring, and plate simulations.

There’s actually one analog “effect” present, namely stereo analog distortion, which has its own knob on the far left of the panel. While it’s fun to create guitar-like distortion sounds (and make guitarists’ eyes widen as you play “Eruption”) for me the real joy of the distortion knob is using subtly, which can add even more to the richness and warmth that’s already on hand.

More Features

The Prophet-6 includes an arpeggiator with all of the standard note values and note-playing orders you’d expect from a mid-’80s poly synth. Alternately, this section can act as a 64-step polyphonic sequencer, where each step can play up to all six voices if desired. The sequencer is dead simple in that you simply hit Record and play the keyboard, and it’s pretty straightforward to add rests and ties via a secondary mode of the patch storage buttons. The tempo of the arpeggiator and sequencer is controlled by the Clock section, which includes a Tap Tempo button and can sync to any external MIDI clock source.

The P6 can output monaural or stereo audio. If it’s set to stereo, then you can use the Pan Spread knob in the Misc. Parameters section to disperse note events across the stereo field. This adds interest as well as perceived warmth. Also in this section are pitch-bend range (up to an octave with the same range in both directions), note priority for unison mode, and a per-program entry volume that’s independent of the master volume. This last one eliminates unpleasant surprises when switching patches during live performance, as the nature of analog synthesis can make for variations in volume from one patch to another.

Last but not least, there’s a chord hold mode and several musically useful alternate tunings are supported.

Conclusions

We’ve had to wait a long time for a great contemporary iteration of a truly voltage-controlled polyphonic analog synth. It’s hugely challenging to combine completely analog tone generation; a thoughtful, attractive, and immediate interface; the warmth that people who never gigged with vintage synths reminisce about; and the reliability and stability that people who did gig with them appreciate. The Prophet-6 has most definitely been worth that wait.

Of course, it may or may not be for you depending on your needs. Synths with digital hearts such as the John Bowen Solaris, Roland JD-XA, or Dave Smith’s own Prophet 12 can create many more different kinds of sounds and offer multitimbral capability—for starters. And if you’re looking to interface to your modular gear via CV/ gate, you’ll find plenty of I/O on the Pro 2 but none here. But for the hardcore poly-analog enthusiast, I struggle to imagine a more ideal realization of a traditional yet modern synth than the Prophet-6. Even if you’re not that hardcore, how quick and fun it is to dive into the control panel and whip up sounds makes the P6 ideal if you’re just getting into synthesis. Dave Smith Instruments— um, Sequential—has another winner on their hands. A Key Buy winner, in fact. Check out our hands-on video demo here!

Snap Judgment

PROS Your days of wishing for an all-analog polyphonic synth from an historic designer are over. Powerful and expressive functionality. Interface is immediate and fun. True VCOs make for unparalleled analog warmth.

CONS Some may wish the keyboard had five octaves. Not multi-timbral, so no splits or layers. Lack of CV/gate connections makes it a non-starter for integrating with modular rigs.

Bottom Line

Combines vintage and modern virtues so well that we’re wondering what will happen to demand for the Prophet-5 on the used market.

$2,999 list | $2,799 street
sequential.com

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