Note: below the text and pictures you’ll find the original DIY article.
Powertran’s top of the range kit instrument offers a relatively inexpensive way for the home musician to make the step up to true polyphonic synthesis. This is an obvious move once the possibilities of string ensembles and portable polyphonic instruments have been exhausted, and there’s an added advantage to the Powertran approach in that the number of simultaneously available notes (and thus the cost of the machine) can largely be determined by the constructor. The instrument can be built with a single pair of oscillators (a single voice) and a second, then a third and a fourth voice, can be added as desired.
These additional voice boards, each containing two oscillators, a VCF and VCA, are easily installed inside the machine. The total capability is eight voices; the final four are available in an expander module, which matches the Polysynth in style and sits neatly by its side. Although the two units are controlled by the single four-octave keyboard, their audio outputs are not combined. This simply means that two channels of a mixer have to be used, and gives the possibility of some fascinating stereo placement effects due to the unusual voice assignment system described later.
The general styling of the Polysynth resembles that of the monophonic Transcendent 2000, which has sold in large quantities and has proved a versatile starting point for many a home musician. The synth is not unattractive, with wooden end cheeks and white legending on a black aluminium front panel, but it is on the bulky side. Controls are a combination of rotary pots, rotary switches and two-position slider switches, with silver caps on the rotaries which also have deeply knurled edges and a white line mark to indicate the setting.
The controls are laid out in a fairly conventional left to right pattern to indicate signal flow, with the exception of the oscillator tuning controls which are on the right hand side on the section marked Voices.
In the four voice version which we examined, the tuning controls are arranged in four pairs, each with an on/off switch and LED status indicator, with the top banks of controls representing VCO 1 in each voice and the bottom bank representing VCO 2 in each voice. In contrast to the Korg Polysix or Juno 6, which have a single tuning control, all eight controls have to be adjusted independently.
Tuning can therefore become a little laborious; the oscillators need to be locked together and the VCA and VCF held open before this can be done, and although the advantages of being able to obtain unusual interval tunings are considerable these can’t be obtained quickly. This brings up the whole question of making instant sound changes on stage, which will be considered shortly.
On the left hand side of the control panel are the portamento, wave shape selection and synchronisation controls. Each of the two banks of oscillators can be switched to sine, sawtooth or square. On square wave the mark/space can be adjusted manually, or automatically by a slow oscillator operating at a fixed speed of about 5Hz with a choice of sine or square modulation. Use of mark/space modulation thickens up the sound considerably, as of course does a slight detuning of each pair of oscillators. Volume of the two oscillator banks is independently adjustable and Synchronisation locks the tuning of each pair of oscillators together.
It’s not possible to produce the kind of harmonic distortion found on the Moog Prodigy by de-tuning when in sync, and in fact a single oscillator alone sounds fairly weak — certainly no better than that on the Transcendent 2000. It’s when they’re played together that a quantum jump in sound quality takes place, and the sound when all eight are running together with a little detuning, pulse width modulation, vibrato and resonance is a revelation.
The vibrato section has its own speed, depth and sine/square controls, and there is a plus and minus one octave rotary switch and a Transpose pot. This functions over the rather unusual range of an octave and six semitones and would be useful for deep bending effects were it not for the fact that it doesn’t have a dead band and so can only be returned to any given tuning with some care. In the centre of the control panel are a green power LED and the white noise level pot.
Filter and Envelopes
The filter and envelope controls are fairly conventional, but have a couple of interesting extras. The filter is a standard 12 dB/octave low pass type which just falls short of resonating at the highest Q setting, and has a sine wave sweep oscillator with its own speed and depth controls. In addition there’s a VCF track switch which opens up the filter as higher keys are played, simulating the response of acoustic instruments.
It’s a pity that more treatments aren’t available for the filter; the square wave modulation on the pulse width would be better placed here, and since a white noise source is present it seems odd not to include a sample and hold for random effects. In addition a fully resonant filter could provide some interesting sounds.
The VCF can be swept by an ADSR envelope which has positive and negative options and a Track switch on the Release mode. This also exists on the VCA envelope and works in inverse proportion to the keyboard voltage, so that release is longer at the lower end of the keyboard. This is useful in simulating acoustic piano sounds, which can be done extremely accurately on the Polysynth, because lower and heavier piano strings tend to sustain for longer than higher and lighter ones.
The VCA also has ADSR controls and a Continuous switch which holds it at full volume. The state of the gate to the two envelopes is indicated by an LED under each voice, and so a good visual indication of the method of note assignment can be gained during normal playing.
Note assignment is cyclical, with a single note played continually on the keyboard using each oscillator in turn, two notes or three notes stepping around in order and all four notes using a continuous cycle of assignments. There are two performance advantages to this system; the first is in the stereo placement of the voices if the expander is being used, in which case the stereo picture is always changing. The second is when using the polyphonic portamento; even if the same chord is played continually, the oscillators glide in intricate patterns up and down the keyboard to provide a fascinating overall texture.
The controls, then, are fairly basic, but with a couple of unexpected extras, so that with everything flat out the sound can be quite breathtaking. The Polysynth’s problems lie, if anywhere, in ease of use and expressiveness. The pitch bend lever mounted to the left of the keyboard and sprung vertically is pleasant to use, but doesn’t cover a great enough range. There is no facility for modulation by foot pedals and although there are two modes of note assignment neither of these offers the New Pitch Detection of the Transcendent 2000, and so new triggers cannot be obtained if four notes are already held down.
Because of the tuning method it’s not possible to obtain interval tunings quickly, and many of the controls are very ‘toppy’, acting only in the highest one-third or so of their travel. A band pass mode on the filter, which again exists on the Transcendent 2000, would have been a useful and inexpensive addition, and the range of speeds available on the filter sweep oscillator, for instance, would benefit from being extended.
Happily, a lot of these problems are the sort of things which can be adjusted to personal taste during construction. The sensitivity of the pitch bender could be increased, and as the panel is generously laid out it should be possible to add controls for modifications to the modulation circuits, such as a simple sample and hold.
As a studio instrument, then, the Polysynth can be extremely powerful and impressive with a little thought applied to getting the best out of it. The keyboard is pleasant to use and the limitations of four notes are far from apparent, while the construction is sturdy enough to ensure long service.
It would be a brave man who would attempt to tackle the Polysynth on a darkened stage under concert conditions. Memory presets may be an expensive facility, but to obtain a given sound on a totally variable polysynth like this requires a good degree of familiarity. On the other hand, there’s no denying that some startling sounds can be obtained, including Hammond organ, strings, acoustic piano, clarinet, reversed piano, eight-note monophonic lead and a host of others. Regard it as a challenge — build it, modify it, learn to get the best out of it.