This month we conclude our series of articles on polyphonic synthesisers with a review of the Sequential Circuits Prophet. Sequential Circuits is a small and relatively new company based in Sunnyvale, California which owes its existence to electronic designer Dave Smith. He began manufacturing electronic musical instruments on a part-time basis in 1974, producing firstly the Model 600 sequencer, and later in 1976 an updated Model 800. Then, in 1977, the highly successful Synthesiser Programmer was introduced, and it was with the profits, realised mostly from this programmer, that Dave Smith and Sequential Circuits were able to finance the design and production of the Prophet. If you read my review of the Oberheim 4-voice (SI May 1978), you may notice some striking similarities in the way in which both companies got off the ground.

The review instrument is the Prophet 5, and at the time of writing is the only one in the UK. However, by the time this magazine reaches you, several top musicians will be using them and if you keep on reading you will soon see why.

The easiest way to describe this instrument is to say that it is equivalent to a five note polyphonic Minimoog, and that it will remember 40 different preset sounds, each of which can be easily recalled and also modified. Impressive, eh? So now let's look at the Prophet a little more closely.

The first thing that strikes you when confronted with this instrument is how small and neat it is. To be exact it measures 37in x 16in x 4½in (a little smaller than a D6 Clavinet), and incorporates a five octave C-C keyboard. The case is mostly real wood, beautifully finished. In fact almost too well finished; if I owned one I would worry about every little scratch.

The Prophet 5 is a 5-voice instrument, ie only up to five notes at a time may be played. At this point it might be as well to introduce the term 'homogeneous'. With reference to the Prophet, this means that all the oscillators, filters and envelopes on the separate voices are linked to a single set of controls on the front panel, and therefore, unlike the Oberheim 4-voice, each of the voices will have the same settings. The only parameters exclusive to individual voices are a keyboard control voltage, internal oscillator bias voltage (for tuning) and a gate. The front panel is therefore basically similar to a standard monophonic (ie 1-voice) synthesiser, but in fact each control affects all five voices simultaneously.

Each voice consists of two voltage controlled oscillators; VCO 1 has sawtooth and pulse (variable width) waveshapes, VCO 2 has sawtooth, triangle and pulse (variable width) waveshapes. Other features include noise source, a mixer, a 24dB/octave voltage-controlled low-pass filter with ADSR (attack, decay, sustain, release) envelope, and a voltage controlled amplifier with ADSR envelope.

There are basically two types of modulation. One is common to all voices and provides even, synchronised modulation which is routed through a modulation wheel controller situated to the left of the keyboard. The other is called Poly-mod. This is separate for each voice, and provides a more natural sound, since each modulation source is not synchronised voice to voice. Situated next to the modulation wheel is a pitch bend wheel with centre stop, which will bend all voices simultaneously, and associated with this is a master tune control. The individual tuning of each voice is done automatically so that all the voices are constantly in tune with each other. The instrument can be played in unison mode, whence the keyboard becomes monophonic, each voice running off the same gate and pitch control voltage. In this mode all voices are producing the same sound, but are not synchronised, so the sound produced is very rich. In the unison mode the glide facility can be used.

The only remaining section is the programmer. Using this all the settings of the voice controls and switches can be remembered and up to 40 different programs can be stored. This is backed up by a battery with a ten year life so that your programs can be stored even with the instrument turned off. The programmer, as with the rest of the signal switching and routing, uses momentary pushbutton switches with a light emitting diode (LED) incorporated to indicate the state of the switches. There is also a LED numeric display to indicate at a glance which program is being used. To record your own program, you first set up the required sound, then the memory location in which the data is to be stored, then press the record button. It's as simple as that! A record disable switch on the back of the instrument prevents you from accidentally erasing any programs. Also any program recalled can be modified without losing the original preset sound, so that if necessary the instrument can revert to the original preset.

These, then, are the basic features of the Prophet 5. So what can you do with these ingredients? Answer: One hell of a lot! Imitations of instruments can be produced easily. The factory programmed presets included electric piano, clavinet, electric organ (very rich indeed), strings, bass and flutes; all excellent. The programmer can also be used to store sound effects: for example a thunder clap can be set up, stored, recalled and activated at a specific moment. This instrument is extremely versatile. As the promotional blurb goes... 'the industry's first completely programmable polyphonic synthesiser provides unparalleled ease of use while retaining the sophistication required by the most demanding synthesist.' This seems to me to be perfectly true.

There are, however, limitations to the instrument, the major one being the ability to play only five notes at once. This is quite a disadvantage. Originally there were two models, the Prophet 5 and the Prophet 10, both identical except that the latter could play ten notes simultaneously. I had the pleasure of trying a Prophet 10 a few weeks ago and was very impressed, but unfortunately it seems that the ten voices cause the power supply to run at too high a temperature, leading to tuning problems. For that reason the Prophet 10 was discontinued a few weeks ago, and I must admire Sequential Circuits for doing this as they will undoubtedly lose some potential customers by only providing a five note capability. It nevertheless demonstrates that Dave Smith and his team are perfectionists and not prepared to produce equipment even slightly below spec. (Incidentally the tuning on the Prophet 10 I tried seemed only fractionally out on one voice.)

The only other facility I find to be absent is the triangle waveform on VCO 1. I consider this to be important in order to get that really mellow sound when beated with the VCO 2 triangle wave. It would also have been nice to incorporate the glide in the polyphonic as well as unison mode. One last problem is that the instrument came without any legs. I can understand this as it is an ideal shape for standing on top of another instrument, but which one? It seems to make other keyboards obsolete. Anyone got a grand piano?

Over the next few months I shall be occasionally including in this column details and hints about modifying various keyboards. Most keyboard instruments can be customised to suit individual requirements, so if you have any specific queries drop me a line c/o the SI editorial address, I will see what I can do. I'm afraid I won't be able to reply to every question personally, but I shall hope to cover as many items as possible. I also welcome details of any modifications that you have done to your own instruments, and if suitable will pass information on. I look forward to hearing from you.

rrp £3000/$3995

Dave Crombie is resident electronic design engineer at Rod Argent's professional keyboard store in central London.