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Spectral Audio ProTone

It may be Swiss, but there's nothing cheesy about the idea of a synth that lets you rekindle your rusty analogue programming skills. Paul Nagle dons his boots and goes hiking through the peaks and troughs of the ProTone's analogue waveforms...

Continuing the retro trend of 'new analogue monosynths', Swiss company Spectral Audio enter the fray with their ProTone, a 2U module generously laden with controls and ready to do your bidding. The first thing you'll notice is its colour. The review model was a cherry red, and because Spectral Audio are industry striplings, they're still able to offer purchasers the personal touch, and supply the ProTone in the colour of your choice.

The unit is compact, a mere six inches deep, and a look at the back panel reveals MIDI In and Thru, but no Out. This is because there is no processing between the synth's controls and the sound generator, and so MIDI codes can be neither generated nor received by the controls. Stereo outputs, CV and Gate sockets, and external VCO and LFO inputs complete the picture. Happily, the power supply is internal.

Programming & Internal Architecture

Programming involves getting to grips with a combination of stylish knobs, slightly flimsy switches and small black 'liquorice imp' pushbuttons. With the latter, only a few millimetres separate the on and off positions, which sometimes makes it hard to tell the status without an experimental prod. And perhaps it was a lingering effect of the colour, but I felt there was something of a 'kit' feel to the controls.

These niggles aside, you've got instant access to everything, and a view of the synth's setup which beats any LCD hands down. This is fortunate, as Spectral Audio have totally omitted any means of storing patches. The positive side to this is that the position of the controls always reflects the sound that the synth will make; the negative side is that many of the advantages of MIDI are lost, especially in live situations where you might need a particular sound instantly.

The ProTone uses highest‑note priority at all times. This is a shame, as it prevents certain solo techniques being developed with lowest‑ or last‑note priority. If, however, you use a synthesizer more for basslines and sequencing than for expressive solos, it won't worry you too much. I have to say I wasn't immediately blown away with the sounds I was able to coax forth. The filter doesn't seem to have the range for those classic sweeps, and the resonance (or 'resonanz' as it is labelled) comes tantalizingly close to self‑oscillation without quite getting there. With no means of saving patches internally or via MIDI, you need to take a note of all settings on the thoughtfully‑provided patch charts. Through rose‑tinted spectacles, I reminisced about the joys of reprogramming sounds one parameter at a time from precious, if slightly grubby, pieces of paper.

Fortunately, the ProTone is very simple and direct to learn, and if you spend a little time with it, you'll soon be rattling through its repertoire without a safety net. Taking the front panel from left to right, there are two Voltage Controlled Oscillators (VCOs), a small mixer section, the filter, Low Frequency Oscillator (LFO), ADSR envelope generator and an overall level and pan control. The two VCOs each have a tuning range of approximately plus or minus a sixth. This really isn't enough, and I'd suggest at least an octave would have been better, especially for bass and sequence lines. It also lacks an overall tuning control and the option to fine‑tune between the oscillators.

With no superfluous facilities, this is subtractive synthesis stripped to its barest essentials.

The VCOs both have sawtooth or square waves in their raw sound palette, with VCO 1's square wave being continuously variable from a sort of hollow clarinet through to a narrower, oboe‑like pulse wave. VCO 1 also has a switchable noise source suitable for percussive or sound effects, whilst VCO 2 has oscillator Sync and an external input. This allows other synthesizers, drum machines or suitably amplified instruments to be processed through the synth's filter, drastically altering their tone: a useful facility for you bold explorers out there. By enabling Sync, VCO 2 locks to the pitch of VCO 1, and by then applying modulation or manually altering the pitch, you can create some rich timbral changes. Sadly, there is no way to route the envelope shape to oscillator pitch — it's the LFO or nothing. Slide (portamento) is provided for those gradual pitch changes, letting you glide smoothly between notes, as with fretless bass or violin.

A simple mixer balances the levels of each oscillator and the ring modulator. Ring modulation produces the sum and difference of two incoming pitches, resulting in metallic, complex tones. You can't tune the oscillators very far apart, so this is slightly limited, but it does add to the richness of the sound. I found myself constantly mixing it in a little to thicken things up.

Filter & Envelopes

The filter features both high‑ and low‑pass modes, and is switchable between 12 and 24dB, giving a wide range of tones. I think it's fair to say that analogue synths stand or fall by the quality of their filter. In the past, Japanese companies such as Yamaha and Korg produced monosynths with 2‑pole (12dB) filters, whilst Americans Moog and Sequential used the much steeper 4‑pole (24dB) roll‑off. Thus we tend to think of the Japanese sound being alternatively 'thin' or 'delicate', whilst the Americans are 'fat'. Admittedly, I'm generalising quite a bit here, and ignoring several important exceptions, but the point is that even when switched into 24dB mode, the ProTone doesn't match the fatness of a Pro One or Moog Prodigy. I'd say its sound was closer to that of the Roland SH‑series. This isn't necessarily a bad thing — in fact it is easier to fit the ProTone into a mix — but there are going to be times when you want that gutsy sound. Experimentation with filter, accent and envelope settings provides plenty of usable analogue material, from thunky bass noises to strange space zaps and warbles. Switching in the high‑pass filter aggressively removes any lower‑frequency components, and by cranking up the resonance, the synth achieves quite a sweet, distinctive character.

Having but one envelope is a little restrictive, but being of the ADSR variety, it's a cinch to program. An envelope amount control decides how much of the envelope is routed to the filter. Modulation is derived from the single LFO featuring a three‑way range switch, and a rotary knob to fine‑tune the rate. The available waveforms are: square, pulse, two different sawtooths, triangle, sine, random, noise and the external option. Unfortunately, there is only one overall knob for controlling modulation amount, which is then routed according to on/off push‑buttons. If you wanted a little gentle vibrato but a deep pulse width modulation, well, you're sunk. Routings are to VCO1 pitch, VCO2 pitch, VCO1 Pulse Width, Cutoff and pan position.

The external LFO input is rather an unusual feature, allowing an outside source to be patched in and used as a modulator. I used the output from another synthesizer, patching in both low‑ and high‑frequency sounds with interesting results in the weird noise department. Switching to the fastest LFO speed available and applying it to pitch or cutoff generated some screaming alien effects, and by using noise as a modulator, I managed to come up with a remarkably good aural impression of frying bacon.

With no superfluous facilities, this is subtractive synthesis stripped to its barest essentials, and I mean it as no insult when I say this would be an ideal tutor for someone who has only ever seen more complex and daunting 'modern' instruments. It's also great for us oldies, who can instantly feel at home with its layout and functionality. Forget any worries you may have about using 'other people's' sounds, since everything you program will contain all those tiny variations that real knobs bring. Somehow, seeing a filter's range represented as numbers from 0 to 127 never quite feels the same as turning a knob freely and using your ears!


At the time of this review, there is no English version of the manual, so I was thankful that the front panel was mostly self‑explanatory — and setting up the MIDI reception didn't take too long to suss out either. Nevertheless, I hope by the time you read this, an English manual will be available.

I'm forever seeking synths with improved accessibility, and with more than 30 real‑time controls, the ProTone certainly delivers in this respect. It strikes me that the lack of in‑depth MIDI specification might not be such a drawback in these enlightened times. Armed with one of the affordable hard disk recorders now on the scene, all experimental tweakings can be captured. It's then a simple matter to cut and paste all the good bits, saying goodbye forever to worries about generating too much MIDI performance data.

The ProTone's no‑nonsense, informative layout will appeal to synth newcomers who want to get to grips with programming right away. The raw sounds aren't muscular enough for my taste, however, and there aren't really enough options to make them unique. Having said that, I know the high esteem in which some people hold the (much simpler) Roland TB303 Bassline, and how much they're prepared to pay for one. The ProTone can easily pass for a TB303...but then so can my ageing grandmother after eating sprouts!

Synthesizer sound is very subjective. I frequently felt I was on the verge of programming something classic, but despite my best efforts, was unable to get there. A second ADSR or wider oscillator pitch intervals would have helped significantly. Ultimately, I think the ProTone is a little expensive for what it delivers, but perhaps the blend of MIDI, knobs and rackmountability will be enough to sway potential buyers who might otherwise have chosen something second‑hand.

MIDI & CV/Gate

Setting up the ProTone for MIDI operation is simplicity itself. Push the Learn button and the synth automatically sets its MIDI channel to that of the next incoming message it receives. Key ranges are also set by incoming notes, and the intriguing‑sounding controller x, which governs filter cutoff, is similarly assigned to whichever controller number the ProTone first receives when the Learn button has been depressed.

Although not usually an advocate of vivisection, the first ProTone I received was dead on arrival, so I opened it up. Whilst nosing about inside, noticed two jumpers labelled in and out. I discovered that these switch the function of the CV/Gate jacks to either input or output mode, so provided you have an old synthesizer which uses the 1 Volt‑per‑Octave convention, and a triggering voltage of 0‑4.5V, you can use it to drive the ProTone.

Once I received a working unit, I tried running the ProTone from CV/Gate using an old analogue sequencer, and this rattled along so well that I found myself wishing for a voltage‑controlled filter input, so I could really make the most of the ProTone. I also tried operation the other way around, plugging the ProTone's CV/Gate output into my faithful Korg MS50 module and running everything from MIDI. After boosting the trigger signal slightly, the results were pretty accurate. I'd say as an add‑on, the MIDI/CV converter is useful, but it is not a replacement for a dedicated unit, since there is little available other than notes and trigger information.

MIDI Implementation

The ProTone has a somewhat spartan MIDI implementation, responding to note on/off (with or without velocity), pitch bend, controller 7 (volume), controller 64 (sustain), controller 65 (sets accent on or off) and controller x (see the 'MIDI & CV/Gate' box for more on this). This is then used to control the filter cutoff, usefully enough.

Since the ProTone has stereo ouputs, I was surprised that CC10 (pan) wasn't implemented, and that modulation depth could not be controlled remotely at all. Due to the way that the accent feature of the filter works, setting MIDI volume to 0 won't necessarily mute the instrument. You'll need to ensure that accent is off too, or you may still get output directly from the filter. This is confusing at first, though you might find a creative use for it.

Six MIDI program changes are recognised, which act as switches for the few options not available from the front panel. This is a rather messy process, since you may have to send up to three program numbers in succession, depending on your needs. The MIDI program changes received are as follows:

  • Program 1: The ProTone does not recognise velocity, legato playing is recognised (default at power up).
  • Program 2: Recognises velocity, but not legato play.
  • Program 3: Velocity does not affect the filter (default at power up).
  • Program 4: Velocity affects the filter.
  • Program 5: Envelope retrigger on — ie. the envelope retriggers even during legato playback (default at power up).
  • Program 6: Envelope retrigger off.


  • What you see is what you get, as there are no patch memories.
  • A dedicated knob for every function.
  • Built‑in CV‑MIDI converter.
  • Looks good.


  • No patch memories — what you see is what you get! [I've got this funny sense of déjà vu, Paul — Ed]
  • Simple architecture.
  • Limited MIDI control.
  • Relatively expensive.


It's surprising to see the resurgence of the monosynth, presumably in response to the need for blippy filter sounds in dance music. The ProTone delivers an adequate range of analogue noises, with the added advantage that you can instantly 'see what it will sound like'. It's a brave move to bring out a MIDI synth with no patch memories and practically no MIDI control over parameters, and although this might restrict its usefulness live, it encourages familiarity and mastery in a way that is too often ignored.