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Spectral Audio Neptune

Analogue Synthesizer By Sam Inglis
Published August 1999

Spectral Audio Neptune

Swiss synth manufacturers Spectral Audio continue their journey into retro space with a new analogue monosynth, the Neptune. Sam Inglis finds out if it's out of this world...

It's now more than 10 years since the birth of house music kicked off the revival of analogue synthesizers. Despite continued demand for their old models on the second‑hand market, however, the big synth manufacturers like Roland have pooh‑poohed the idea of reviving the wholly analogue originals. This reluctance to retread old ground has left the field open for a number of enterprising smaller companies, such as Studio Electronics, Syntechno, MAM, Control Synthesis and Spectral Audio, to design brand‑new analogue synths which can do the same job as their vintage forebears without commanding their inflated prices. The benefits of hindsight and improved technology also mean that these new synths can offer features lacking on the originals, such as a decent MIDI spec.

Not all analogue synths are created equal, however. Early analogue synths, like the Minimoog, featured both a fully analogue signal path and a fully analogue control interface. This certainly gave them an uncompromisingly 'analogue' sound, and meant that every synthesis parameter had a dedicated knob or slider which could be controlled in real time. On the down side, however, it meant that they offered no means of storing patches, and at best only very basic external control from a sequencer.

Digitally‑controlled analogue synths like the SCI Prophet 5, on the other hand, still offered a fully analogue signal path, but recorded the values of the various synthesis parameters digitally. This eliminated both of the major disadvantages inherent in analogue‑controlled synths, allowing the incorporation of digital patch memories and fuller remote control via a digital interfacing system such as MIDI.

The march of progress soon meant not only that the values of synth parameters could be stored digitally, but that a number of elements in the actual signal path could be made digital. Envelope generators, for instance — which had been analogue voltage‑controlled devices — were replaced by digital chips that 'read out' levels in discrete steps. The unstable but rich‑sounding Voltage Controlled Oscillators used in early synths were replaced by more stable (but to some more sterile‑sounding) Digitally Controlled Oscillators, and eventually (in synths like the OSCar and Korg's DW8000) by digital 'oscillators' which also simply read out series of discrete values. Digital technology allowed the incorporation of an ever‑greater number of features, but at the increasing cost of unintuitive programming interfaces.

The dilemma facing modern manufacturers seeking to capture the 'magic of analogue' in a new design, then, is exactly how far along this timeline to go. Building an entirely analogue machine offers the best chance of nailing 'the' sound, but entails leaving out basic features, such as patch memories, which are taken for granted in other modern studio equipment. Designing a synth in which the control system and signal path are largely or wholly digital, on the other hand, allows the incorporation of a much more sophisticated user interface and feature set, but can lead to compromises — most often subjectively 'colder' sounds, lack of immediacy in programming, and audible 'stepping' between values through envelope or LFO cycles — which undermine the reasons why you might have wanted an 'analogue' synth in the first place.

The Full Spectrum

Spectral Audio Neptune

Swiss synth designers Spectral Audio have sought to please everyone, as much as is possible, by marketing products at both ends of the spectrum. Their Pro Tone (reviewed in SOS June 1996), was a fully analogue, two‑oscillator rackmount monosynth — with no patch memories and a fairly basic MIDI spec. Their Syntrack, on the other hand, (SOS review March 1998) is based around a digital oscillator offering 100 waveforms, though it retains an analogue filter stage, and provides comprehensive MIDI control and 100 memories.

The new Neptune, under review here, very definitely takes the former route, offering a traditional analogue VCO‑VCF‑VCA signal path with a few additional frills. Though the back‑panel socketry includes both MIDI In and Thru sockets, the former simply feeds a built‑in MIDI‑to‑CV converter which then distributes the analogue signals to the appropriate bits of the synth. Unsurprisingly, there are also direct CV and Gate inputs, though more control options are available over MIDI (see box on page 82). Also on the back panel is an input for processing external audio signals, along with a single mono output.

The continuing popularity of analogue synths is founded on two main features: their sounds, and the immediacy with which those sounds can be created and tweaked.

In terms of features, the Neptune addresses some of the shortcomings picked up by Paul Nagle in his review of the Pro Tone (more on these as we go on), but also sheds some of that instrument's features. Neptune is a two‑oscillator, fully analogue monosynth, offering an LFO that can double as a third oscillator, a switchable high‑/low‑pass filter, independent filter and VCA envelopes (a vast improvement on the Pro Tone's single envelope), a ring modulator and overdrive/distortion. The two VCOs each offer a choice of triangle and square waves; VCO1 adds the option of noise, while VCO2's signal can be substituted by an external audio signal. VCO2 can also be sync'ed to VCO1 for those classic punchy leads and tearing noises. Pulse‑width modulation (PWM), however, is not available — a definite step backwards compared to the Pro Tone.

The third oscillator can be used either as an audio‑frequency sub‑oscillator or as an LFO, offering saw, triangle, square, noise and random waveforms; alternatively, VCO2 can be used as the modulation source. MIDI sync is available, and the LFO can be used to modulate filter cutoff and the pitch of VCO1 and/or VCO2.

While the Neptune is by no means restricted to mimicking any one vintage instrument, some thought has clearly gone into providing accurate emulation of the distinctive features that characterise the sound of Roland's TB303 Bassline. The latter's Accent feature, for instance, is emulated (albeit in a fairly crude way!) by modifying the assignment of velocity to filter cutoff: in Neptune's 'TB303 Emulation Mode', MIDI notes with velocity values lower than 64 are treated as normal, while notes with higher velocities are treated as having velocity of 127, creating a distinction between normal and 'accented' notes. Other features, like the LFO doubling as a third oscillator, are more reminiscent of the mighty Minimoog.

Out Of Its Box

The Neptune I looked at (and listened to) was an early production model, and three things were immediately noticeable on removing it from its box. Firstly, it was extremely purple — but I'm told that as a result of market research in Germany, later models will be silver. Secondly, the external power supply (boo! hiss!) was one of those continental two‑pin affairs which only work in shaver sockets in the UK. Since not many recording studios are located in bathrooms, this could be a problem, but I'm told that later models will come with the right sort of adaptor (accept no less!). Thirdly, the review model's construction was less than rock‑solid: the top and bottom panels flexed even under gentle pressure, while the front‑panel knobs offered so little resistance when turned, it was initially hard to believe they were attached to anything at all, and one of the plastic push buttons appeared to be permanently jammed in.

Swiss Cheese?

But what matters most, of course, is whether the Neptune's sounds justify the decision to go with an uncompromising all‑analogue design. And I'm pleased to say that the answer is a broad 'yes'. It's child's play (well, childish play, perhaps...) to produce a good selection of typical analogue sounds, from rich leads to delicate plinks and plunks to apocalyptic bursts of tuneless noise. The two main VCOs are stable in pitch and pleasantly rich‑sounding, though the lack of fine‑tuning controls can make them hard to tune precisely. VCO1 offers a five‑octave range based on a fixed 16' footage, while VCO2 offers the same range but is switchable between 4', 8', 16' and 32' footages. I did, however, mourn the complete absence of PWM from both oscillators, which eliminates a whole swathe of sonic possibilities; given that the Pro Tone did offer PWM, it seems a little odd that it should be missing in action here.

On a brighter note, the Neptune's filter is powerful and reasonably flexible. Compared to that of the Pro Tone, the addition of a dedicated ADSR envelope more than compensates for the loss of switchable key follow and 24dB/12dB slopes. The influence of the envelope is continuously variable from full to fully inverted via a null point, and the filter whistles nicely when the resonance is cranked up. My only gripe is that the fixed degree of key follow (allegedly 100 percent) sometimes seems too extreme: filter settings that sound right on mid‑ and high‑pitched notes often leave bass notes almost inaudible, while settings that sound good in the bass registers become over‑bright as you move up the keyboard.

The switchable LFO/sub‑oscillator, which can be sync'ed over MIDI, is one of the Neptune's best features. Although there's no delay parameter for the introduction of modulation, there's a good variety of modulation possibilities, and the option of switching it to audio frequencies can yield rich, detuned three‑oscillator sounds. Happily, even when the third oscillator is being used as a sub‑oscillator, it's still available as a modulation source — a feature which opens up all sorts of possibilities for bizarre effects and nasty FM‑style clangs (audio‑frequency modulation of filter cutoff can be interesting, to say the least...). In fact, with the possibility of using noise and/or external audio as both a sound source and a modulation source, not to mention (yet) the portamento, ring modulator and distortion circuit, sound effects are perhaps the Neptune's strongest suit.

Portamento (or 'Slide' as Spectral describe it) is controlled by a single knob which, as you might expect, sets the portamento rate. The ring modulator also offers only a single knob, which seems to control the level sent to the mixer. The result is suprisingly subtle and musical when both VCOs are in action, while plumbing an external source in in place of VCO2 yields the familiar Dalek effect.

The Neptune responds to pitch‑bend and modulation over MIDI, though it's not possible to route the former to VCO2 alone (which would have been useful for those 'tearing' oscillator‑sync sounds), nor could I find any way of altering the pitch‑bend range: this seemed fixed at a frustrating not‑quite‑one‑octave.

At the end of the signal chain comes a circuit which is switchable between 'fuzz' and 'distortion'. Thformer, true to its name, adds a buzzing transistory distortion which seemed rather harsh to my ears. In 'distortion' mode, however, the circuit comes into its own. At low settings, it seems to fill out the sound without adding noticeable fuzz, while higher settings produce a great, warm‑sounding saturation which adds a lot of punch without ever disintegrating into noise. Be warned, though, that switching in the fuzz/distortion often causes the output level to jump alarmingly — my guess is that it simply overdrives the VCA...

In general, the Neptune strikes a reasonable compromise between simplicity and flexibility. The latter could be increased by incorporating the Pro Tone's dedicated external LFO input, an independent volume control for the sub‑oscillator (as it stands, the level sent to the mixer is controlled by the modulation depth) or some additional modulation destinations. Modulation of resonance would be nice, as would modulation of the VCA for tremelo and AM — and as they've very kindly included a ring modulator, why not make that a destination too? At £399, some might argue that the absence of PWM, a master volume control or even an on/off switch make the Neptune too simple a package for the price, but its powerful LFO/sub‑oscillator and aptitude for sound effects do much to make amends.


The continuing popularity of analogue synths is founded on two main features: their sounds, and the immediacy with which those sounds can be created and tweaked. Spectral Audio's Neptune achieves pretty well in both respects. Sound‑wise, it's a reasonably versatile monosynth with a few unusual features, which make it particularly suited to the production of weird noises. And although there are no patch memories, and the controls don't transmit MIDI data to be recorded in a sequencer, it's a breeze to use, what with every parameter having a dedicated (and clearly labelled!) knob, switch or button. Those seeking to add a bit of genuine analogue sparkle to their studios will not regret putting the Neptune on their auditioning list.

Neptune's CV

As the Neptune is a fully analogue synth, Spectral Audio have made it at home in the modern studio by incorporating a built‑in MIDI‑to‑CV converter. There are also CV and Gate sockets in the back which, if you're controlling the Neptune via MIDI, can be configured as outputs for driving a MIDI‑less vintage synth. This is handy, though less sophisticated than the converter built into Novation's BassStation Rack, which allows you to send separate monophonic MIDI sequences on two channels, with one controlling the BassStation and the other appearing at the CV and Gate outputs. The Neptune only accepts MIDI data on one channel, so the CV and Gate signals appearing at its outputs are the same as those going to Neptune itself. The BassStation Rack also supports Korg‑style Hz/Volt CVs as well as the more common octave/Volt standard, while Neptune implements only the latter.

In general, the Neptune's MIDI implementation is about as good as could be expected on a fully analogue synth. Though there are no memories to recall, Spectral have implemented Program Change messages to put the Neptune into its various different modes (switching on and off such features as 'Filter TB303 Emulation'). As well as Note On information, the Neptune recognises pitch‑bend data and allows you to assign MIDI controllers to filter cutoff and modulation; velocity data can also be sent to filter cutoff, but not to the VCA. The MIDI‑to‑CV converter can be set to last‑note or highest‑note priority.


  • Characteristic 'analogue sound'.
  • Very easy to use.
  • Some unusual features make it excellent for sound effects.
  • Built‑in MIDI‑to‑CV converter.


  • No PWM.
  • Slightly tacky construction.
  • External power supply.
  • No headphone socket.


The Neptune's features and sound (and, unfortunately, its price) are comparable to those of many of the vintage instruments it seeks to emulate. If you can live without PWM, however, definitely worth a listen.