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Life Of A Prophet

Prophet 600

While Sequential's Prophet 5 will always be regarded as a milestone in synthesiser technology, their Prophet 600 is almost forgotten already. Gordon Reid attempts to correct the history books.


THERE ARE MANY ways to classify keyboards - polyphonic, analogue, FM, LA... Try these: "classic", "a dog", "overlooked". Few keyboards fit the last category as, on the whole, musicians are able to ferret out new instruments of value. But occasionally something slips through the net - maybe because a competitor launched a superior machine at the same moment, possibly because the synth didn't subscribe to the current fashion, or perhaps it was simply too expensive. Instruments like the Roland JX8P, the Korg Trident II, the OSCar, and the subject of this article - Sequential Circuits' Prophet 600 - come readily to mind.


THE SEQUENTIAL STORY is one of the saddest in synthesis. You may not realise it, but if you own any MIDI device, multitimbral keyboard or expander, you owe a great deal to Sequential Circuits. We all know of the Prophet 5 - it's nearly impossible to read an issue of MT without somebody extolling its virtues - but do you know what the world's first MIDI keyboard was? What the first Mode 4 (multitimbral) instrument was? They were the Prophet 600, and the Sequential SixTrak. And a company with such foresight and technological expertise must become enormously successful, with a wide product range and penetration into all areas of music making, right? History tells us that the innovators spend all their money on development, take the risks, make the mistakes alongside the successes, and then go out of business. Consider the fates of Sequential, Moog, ARP, and Oberheim. (All of which were, incidentally, American.) None exist today.

So, what went so badly wrong that the most respected synthesiser company in the world in 1980 went out of business only seven years later? Just as Moog never equalled the Minimoog, or ARP the Odyssey, Sequential never managed to follow the Prophet 5. Flagship synths, the Prophets 10 and t8, though conceived in grand fashion, rapidly bit the commercial dust, and the Prophet 600 was probably the last chance that Sequential had to remain a mainstream synth manufacturer. The next series of machines they produced were well designed, innovative, contained hosts of useful features not found on competitors' keyboards, and were commercial disasters.

SCI eventually lost the imagination of the public during the period of the SixTrak, MultiTrak and Max. Most of Sequential's later dabblings in the market were disasters: low-end synths, rhythm units, even computer-based sequencers. The marvellous Prophet t8, and the less marvellous but more easily obtained VS synth and 2000/2002 samplers weren't enough to save the company. With no cash, no product, and no steady customer base to help it through the bad times, Sequential folded in Autumn 1987. And that's a shame, because we need the SCIs of this world to ensure that we don't all end up playing the presets on the latest Japanese megasynth.

The Prophet 600 had been discontinued for some time when Sequential folded. Never a commercial success, it caused a bit of a stir as a curiosity when it was launched in '82, but was discontinued amid total consumer apathy in '85. Two reasons account for this. Firstly, the original recommended price of the 600 was £1650, although the street price was usually £1395. In fact, in '82, Prophet 600s were more expensive than Prophet 5s - clearly ludicrous because, with the exception of MIDI, the 600 is a cut-down (and cut-cost) 5. Secondly, the years '83-'85 were almost the sole preserve of Yamaha's DX-series synths. At £1395 the 600 competed directly with the DX7, and that was a fight it couldn't hope to win.

Over-priced, designed around unfashionable technology, it started off life misunderstood and ended up an underpowered anachronism (lousy MIDI spec, unstable, no cartridge slot, no software). But the asking price of secondhand 600s is now creeping upwards as more players begin to realise its value.


THE 600 IS not a particularly large synth, the width being defined by its five-octave keyboard (which is neither velocity nor pressure sensitive and has no split facility), and a three-inch wide controller panel with Moog-style pitch and modulation control wheels on the left hand side of the keyboard. The steel case (finished with solid wood end-pieces) is quite shallow, and you're certainly not going to break your back carrying one around with you. All programming controls are on the top panel, and retain the general layout of the Prophet 5, although the so-called Bang and Olufsen quality switches had been replaced as part of the cost-saving exercise. The control layout reads as follows. The synth has 100 memories (00-99), selected by typing the number required on a nasty touch-membrane keypad. Although they've never given me any serious problems on stage, the touch-keys are liable to double-enter numbers. Above the pad is the screen - just two seven-segment displays I'm afraid, and par for the course in 1982. With 100 patches, no naming facility and no patch bank/number arrangement, how's your memory? Next along the panel is a second bank of touch-switches which control saving and restoring of voices from tape (totally reliable in my experience), tune request, a button to switch from presets to panel control, patch save to memory, and sequencer and arpeggiator controls. Don't get too excited - the sequencer is limited to real-time entry, has only two tracks, and no facility to sync to external devices. Mercifully, the speed of both the arpeggiator and the sequencers is controlled by a panel knob rather than by the LFO.

The remaining controls are definitely analogue, definitively Prophet, and sleek and laid out in exactly the way that digital synths aren't. There are six sound-building sections, each contained in its own "box' on the panel: Poly-Mod, LFO-Mod, Oscillators 1 & 2, Filter, and Amplifier. What could be simpler?

The 600 is six-note poly with two oscillators per voice. Both oscillators offer a choice of sawtooth, triangle and square waves, with independent pulse-width modulation. Unusually, the oscillators are not constrained to one waveform at a time, so all three waves may be selected on both oscillators simultaneously. Observing the oscillators and filters on an oscilloscope shows that the waveforms are remarkably faithful to the ideal. This wasn't a common quality in 1982 - you only found really square squares on synths like the Moog and the ARP 2600. But Sequential understood that the better defined the waveform, the truer the harmonic content, and the more flexible the synth - another reason for the power of the Prophets.

Osc B has both coarse (four octave) and fine tuning knobs, whereas Osc A has coarse tuning and a sync switch to lock the oscillators together. The philosophy of this is identical to that of the OBX and Jupiter 8 - no chorus, but controlled detuning of two fat analogue oscillators. The oscillators, which are mixed using a single knob (100% A through to 100% B) may be further fattened using the modulation sections. Poly-mod was a favourite feature on the Prophet 5, and had been retained in a more limited form on the 600. The filter envelope and oscillator B can be independently tapped (control knob for each) and sent to either or both of Osc A (two-operator analogue FM synthesis?) and the filter (modulating the cutoff).

Finally we come to the LFO, which offers triangle and square waves. Frequency is controlled by another knob and an Initial Amount control sets the minimum modulation even when the mod-wheel is set to zero. There are three possible destinations for the LFO - the frequencies of oscillators A and B, the pulse width of A and B, and the filter cutoff.

The 600 also offers polyphonic portamento (the speed of which is controlled by yet another knob, this time situated between the oscillators and the envelopes) and Unison. This has two modes: chord latching and 12-oscillator unison. In the former case, if you hold down a chord and then switch on Unison, the synth will latch onto the lowest note in the chord, and track the whole set of intervals around as you play a solo line. But for frustrated would-be Minimoog owners the real interest lies in Unison. Switch it on and mind the windows.

With so many sound creation options even before filtering and enveloping, the 600 shares an important quality with other classic analogue synths such as the Odyssey - it can sound fat just using the oscillators.

The filter section is par for the analogue course - low-pass cutoff frequency control, resonance, and envelope "amount'; keyboard tracking options (off, half, and on), and an ADSR envelope. The filters are 24dB/octave, will self-oscillate, and have the characteristic warmth associated with the Minimoog and Prophet 5. This isn't all that surprising because it was the idea of a polyphonic Minimoog that motivated Dave Smith to design the '5, and the 600 is son of '5.


The early 600s were primitive by today's MIDI standards - but what do you expect from the first MIDI device in the world? Initially blessed with only one mode and no way to distinguish between channels, MIDI could easily have become just a system to enable Prophets to communicate with each other. (Much like Oberheim's ill-fated OCI, or Roland's DCB.) However, by the time that the 600 was discontinued in 1985 it boasted Modes 1 and 3, 16 channels, program, dump, and mod wheel data. Updates are possible for early models (serial numbers pre-424 and possibly somewhat after) but I've never seen them advertised in the UK.


THE PROPHET 600 was one of the world's first hybrid digital and analogue synths. This means that, unlike in otherwise similar instruments such as the Prophet 5 and the OBX, the Z80 microprocessor is not only acting as a control device (storing patches, driving MIDI...) but it is also involved directly in the act of synthesis.

Most significantly, there are no separate analogue envelope generators in this Prophet. Instead, the envelopes are calculated by the computer and are then converted, via sample & hold devices, to dynamic voltages. There's no physical LFO either - the LFO is computed by the Z80, as are the outputs of the pitch and modulation wheels. The modulation that you hear is created by summing the outputs from the envelope generators, oscillators, and wheels.

It works something like this. Every 200th of a second the Z80 calculates the values of the envelope generators, calculates the LFO and the effect of Glide, refreshes the LEDs on the top panel, looks at either the pitch or the mod wheel, refreshes all the S&H control voltages, and finally checks one (and only one) top panel control knob. Once it's done all this it looks at the keyboard and, if you've played a note in that period, works out the voice assignments. You can detect the consequences of this digitisation by trying to adjust some of the control knobs by very small amounts. Listen carefully and the effect on the sound jumps from value to value.

The majority of the circuitry in the 600 is digital, but the sound created is unmistakeably analogue. This was yet another innovation which has helped the synthesisers of today be what they are - although, like their other developments, it didn't help SCI very much. The analogue sound is created in the output stages of the synth where all the digital calculations are returned to good old analogue voltages. The oscillators themselves (two per voice) are Curtis chips (as found on later Prophet 5s) and the filters, amplifiers, and mixers are also to be found on a single Curtis chip (per voice), yet another Sequential innovation. Finally, the output is analogue filtered to get rid of digital quantisation noise!


MUCH HAS BEEN written about the pros and cons of oscillator instability (not to be confused with tuning instability). On no synth is this debate more pertinent than the Prophet 600. Despite all the Sequential developments in digital control, the Curtis oscillators can be wild. Now, if you want each note to be an exact copy of the last one you're not going to like the 600. But this is an age where we are being bombarded with adverts for "humanisers", random pitch variations (see the Roland D70 launched later this month), and every digital effect under the sun to liven up the sound. If you play one note on a 600 using one oscillator only, and one waveform only, you get the result you would expect from an analogue synth - a steady tone. But the moment you use the second oscillator (even tuned to the same pitch) you get chorusing. Or rather, the movement within the sound that chorusing sets out to achieve. Unison mode also achieves this. If you're feeling curious, and have the facilities, set up a single note on a DX7 and a Prophet 600 - let's say, a two-oscillator sawtooth with no modulation or envelope, then listen...

Shortcomings? Well, the down-side to this is the extent of the uncertainty you have to endure just before you start playing. Oscillator movement is one thing, but the 600 takes the whole thing a stage further. Changes in temperature, humidity, or the rising price of beer are all likely to send the tuning wandering off into the stranger realms of Far Eastern atonal music. Worse than that, if it gets really excited, a Prophet 600 will jump out of its patch altogether and leave you in some decidedly weird sonic landscapes. Luckily, all is returned to normal by pressing the Preset button a couple of times - but if you don't have a hand free things can get mighty embarrassing up on stage. If things get really out of hand, oscillator re-scaling is possible, but only if you're confident about performing minor surgery on your synths. Otherwise, have the machine serviced professionally.


THE 600 IS an unusual synth, but few people are going to buy one for that reason alone. Some keyboards and expanders are purchased because they ooze facilities - effects units, splits, layers, multitimbrality, drum sections, sequencers - and I wouldn't be without these devices either. But, the bottom line with any keyboard has to be "how does it sound?". There's a good reason why synths such as the OBX and the Super Jupiter are so sought-after: their sound. Does the Prophet 600 live up to its Sequential pedigree, or was it "first of the dogs"?

The 600 is an enigma. Sometimes there's nothing to touch it, and you wouldn't separate me from mine with a crowbar. At other times I'm tempted to convert it into an ashtray. Firstly, its strengths. You want luscious, deep string-type sounds? Get a secondhand 600. This synth stands side-by-side with the Prophet 5 and OBX, and head and shoulders above most other analogue keyboards (which is strange considering its hybrid nature). You want lead and bass lines that send Minimoog owners diving for cover? This is your synthesiser. I've never come across a synth that sounded so much like a Moog, which is also surprising, since its architecture is far more reminiscent of an ARP Odyssey. Fast rise times, filter distortion, tortured resonance - it'll do most things a Moog will do, provided that you don't want three audio oscillators. The 600 Hammond impersonations can also be very interesting. The final strength of the 600 is sound effects. Although it's not the most configurable synth in the world, it will oblige you with a wide range of malicious graunches and thapwangs. You'll also find the 600 quite useful for brass sounds - from solo trumpet to polybrass stabs - although you'll need a reverb unit to make them listenable. Other groups of sounds which benefit from a generous helping of reverb are flutes and (synthetic) percussion.

The 600!s weaknesses ? Just about everything else; it doesn't take kindly to being asked to go (sonically) where no Prophet has successfully gone before.


SO, SHOULD YOU be interested in a 600, and how much should you pay for one if you are? Powerful as synths like the D50, M1 and VFX are, they produce "characteristic" sounds. A Prophet 600 could be just the synth to add that little bit of extra interest to your sound.

How much? There was a time when you could pick up a 600 for about £250, but the typical price today is closer to £400. That's got nothing to do with inflation - we, the keyboard playing public, have simply realised the value of the synth. And that's fair enough. If you're looking for an OBX, Prophet 5, or perhaps even a Memorymoog or Jupiter 8, check out the little Prophet 600. You never know, you might be pleasantly surprised and save yourself a bob or two in the process.

Thanks to Argents in Denmark Street, London for technical information.

Also featuring gear in this article

Previous Article in this issue

ART Multiverb III

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Cheetah SX16

Music Technology - Copyright: Music Maker Publications (UK), Future Publishing.


Music Technology - Aug 1990


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Retrospective (Gear) by Gordon Reid

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