This new addition to the Moog semi‑modular 60HP range is a little hard to pin down, so what exactly is the Spectravox?
Along with the Labyrinth (reviewed in our August 2024 issue), the Spectravox is Moog’s newest semi‑modular standalone and Eurorack‑compatible device, joining the well‑known Mother‑32, DFAM and Subharmonicon, with similar compact form factor, styling, and high‑quality construction.
It would be easy to assume the Spectravox is just an analogue vocoder — not that there are very many of those out there — but in fact it’s a multi‑function unit that should have much wider appeal. Under the hood it has a true analogue vocoder’s requisite pair of filter banks and accompanying facilities, and I note with some kind of miserly delight that it undercuts the currently available, hefty Moog 16‑channel vocoder reissue by about £4500.
However, it’s also possible to use the Spectravox only as a filter bank, routing into it mono audio from elsewhere and enjoying tone‑shaping possibilities that lie somewhere between those of synth filters and EQ. In this role there’s a direct lineage to the Moog 907 Fixed Filter Bank, used in various full‑size modulars. In fact, a Spectravox can do things a 907 can’t. The first is to introduce resonance in each filter band, and the second to move or ‘shift’ the filter frequencies, jointly and relatively, making the Spectravox more of an ‘unfixed’ bank, if you like. Spectral Shift (as it’s called) can be done manually, or an onboard, hardwired LFO can do it for you, with scope for liquid, phaser‑like sweeps or resonant sirens.
Finally, for distinctly good measure, the Spectravox is a simple synth voice as well, which can act as the harmonically rich carrier component for the vocoder, or can be utilised independently. It has a single oscillator with sawtooth and variable‑width pulse waves alongside a white‑noise source. The provided envelope generator is a basic thing, with an instant attack, variable decay, and no sustain phase, but it can help to create percussive sounds. Pulse‑width modulation, vibrato and more can also be achieved using the shift LFO, via the patchbay.
Say ‘Uhh’?
The way these elements interact when you approach a Spectravox for the first time may not be glaringly obvious. The only non‑modular connections (other than for the 12V 2.0A mains adaptor) are a quarter‑inch rear‑panel mono line‑level output that also doubles as a headphone out, and a front‑panel XLR/jack combi ‘Program Input’. In contrast with many digital vocoder alternatives on the market there’s no MIDI, so the onboard synth cannot be played that way. The included quick start guide isn’t a great help either, being extremely light on practical, useful information. The downloadable full PDF manual is much more what we’ve come to expect from Moog, includes some fascinating historical context, and has a vital ‘Exploring Spectravox’ section with some setup walkthroughs. A clutch of patch overlay cards are useful too, but you’ll still get the most out of those after you’ve read the PDF. In the end, and after some experimentation, it all makes sense, and there’s a lot to enjoy.
Like the other models in the range, the Spectravox’s front panel is 60HP wide and can be removed from the case and fitted into a Eurorack system.
Filter Fabric
Much as I’d love to (yet again) pull on my full flower‑power get‑up and dive into the Spectravox in its role as a vocoder, it makes more sense to consider it as a filter bank first. So the lab coat it is...
Getting into this mode, and others, is a matter of appropriately juggling the three switches at the lower right. Then, with no external patching, you’ll hear the internal oscillator routed through the filters. External signals go into the 3.5mm Carrier patch point on the front panel and break the internal synth connection. Actually, you can also use the XLR/quarter‑inch combi socket: on its own it won’t get routed to the right place, but patching Program output to Carrier input solves that. The mic input does not supply phantom power, incidentally, but its gain knob was able to tease out healthy signals from various dynamic mics I tested it with. An accompanying LED indicates signal level with varying brightness, but is a blunt tool as meters go.
In no time it emerges that the Spectravox’s filter bank — the one used to sculpt and shape external signals — is flexible, can pull off some interesting and remarkable tone transformations, and has plenty of character. It’s distinctly unlike the vast majority of single‑band synth filters we’re all used to and is full of practical and creative potential.
At the same time, it is not a Moog 907 emulation. Where the rare handmade module has filters tuned to two harmonically‑related stacks (250, 500, 1000, 2000 Hz, alongside 350, 700, 1400 and 2800 Hz, plus low‑ and high‑pass filters), the Spectravox offers the funkily inharmonic line‑up of 230, 320, 560, 830, 1200, 1700, 2300, 3200, 4500 and 5400 Hz when the Shift knob is at its default central position. You can then swing frequency centres sufficiently that the lowest band will go almost subsonic at one extreme, and the highest band ultrasonic at the other. A 907 can only dream of that.
Where they are similar though is in the way that the pots provided for each band are all attenuators. You can only cut the level for a band, never boost it, but attenuation can be complete in that if you turn down all bands at once you’ll get silence. On the Spectravox attenuator pots are of the skinny love‑’em‑or‑hate‑’em kind, but I understand an official knurled pot kit is available to purchase to help those of the latter persuasion.
Because of the inherent coloration associated with the Carrier filter bank, a huge timbral range can be teased out of a simple single oscillator. To illustrate the point, the three waveforms here are all Spectravox pulse waves. The first is sourced direct from the VCO output, and is the raw, ‘accurate’ pulse straight from the oscillator. The second is from the main output with Shift in its central position. The third — basically a triangle wave — has Shift turned 90 degrees left. The crucial thing here is that all filter bands were wide open at all times: a further range of wave shapes emerge when you start to use them.
Filters 2‑9 are band‑pass types, with overlapping 6dB/octave slopes (as far as I can make out, squinting at my spectrum analyser). The slope flattens and broadens a touch in the upper 30 degrees or so of pot travel. Moog call filter 1 a low‑pass, and 10 a high‑pass, and on the control panel that’s reinforced by little response curve graphics. In practice they behave much like additional band‑passes in their relative positions, albeit with filter 1 exhibiting more of a shelf‑type response over low‑end frequency content. You could even argue that they’re named the wrong way round, according to the way they often behave. Certainly filter 10 removes high frequencies, pure and simple, which is not most people’s definition of a high‑pass filter... It’s a matter of semantics more than anything, so work with the band controls intuitively and all will be well.
Turning up the Resonance knob (yep, eat your heart out again, 907!) steepens the filter slopes and tends to make the frequency centres ring. Not to the extent they’ll self‑oscillate, but enough that they can and will contribute harmonic content, almost like a resonator.
How you employ band attenuation, in conjunction with the Shift knob to ‘tune’ the bands, is up to you. Remarkable vocal tract‑like and nasal sounds are available when you turn down all bands except perhaps say 2, 5 and 10, increase resonance and set Shift quite high. Try it with your Moog Subharmonicon for a Trautonium‑inspired nerd‑out. Or you could sculpt a ramp going from a fully open band 1 to a closed 10, to create something more like a conventional low‑pass response, with Shift opening and closing the array like it was a cutoff control. Raise band 10 and high frequencies can start to bleed through, somewhat reminiscent of the smooth sparkle of an Oberheim SEM or other continuously variable filter between its LPF and notch states. Yet another approach could be to turn down all odd‑numbered bands (for example), raise resonance, and start sweeping your Shift. The resulting moving peaks and phase interactions are reminiscent of some phaser or flanger sounds.
Which brings us nicely to the onboard Shift LFO. Hardwired to the Shift function, it has a fixed triangle wave and rates from approx 0.05Hz (one cycle every 20 seconds) to 500Hz. Even with the Amt (Amount) knob at maximum the LFO can’t quite visit the extremes of Shift that can be explored manually, but the range is still more than sufficient for classic LFO‑driven phaser effects amongst others. It’s also great for adding subtle movement and minimising any sense of sterility associated with fixed filtering.
Rounding out this first section, I’ll take this early opportunity to point out that the flip side to Spectravox’s uncommon filtering chops is a certain inherent, strong, baked‑in colour that’s impossible to escape. The carrier filter bank skews every signal that goes through it, no matter what. Routing in a test tone or a mix, for example, and then twisting the Shift knob — even when all bands are fully open and resonance is down — will still result in obvious, radical filtration zooming across the audible spectrum. This is rather different to most low‑pass filters of the modular synth world, which when they’re fully open (and assuming they’re not distorting) should spit out pretty much what you put in. The Spectravox’s ever‑present harmonic zing is certainly not a failing, it’s definitely a feature, and is very much part of its character. Often it imbues everything with a scooped quality, warm and buzzy.
Voco Loco
Turning to Vocoder mode now (please feel free to turn on your lava lamp...), this way of working with the Spectravox brings the additional Program (or analysis) filter bank into play. It receives signal from the front‑panel combi jack or patchbay Program input, and in keeping with its analysis role its filters are fixed in frequency and unaffected by the Shift knob.
Used conventionally, with vocal program and synth carrier signals, things can certainly sound rich and smooth, recalling those earliest Wendy Carlos vocoder tracks. However, it has to be said, intelligibility of speech or singing is very poor indeed. Now, this arguably makes for an accurate emulation of that first, iconic Moog‑Carlos 10‑band vocoder, which is reputed to have required an almost absurdly over‑enunciated vocal style. It’s not that there’s no clarity at all, but there’s frequently nowhere near enough for words to be made out, regardless of how extreme your delivery. Consequently it may be safer to think of this vocoder as more of a ‘singing synth’ than something that can get near an ‘Autobahn’, ‘Mr Blue Sky’ or ‘O Superman’.
Still, in an effort to improve speech intelligibility, Moog have employed the historically‑rooted scheme whereby white noise replaces the Carrier signal fed to filter bands 9 and 10 when they’re triggered by fricatives, sibilance and other ‘unvoiced’ elements in the Program signal. The Hiss position of the Hiss/Buzz switch enables this, and it works well in principle, albeit with a rather hard‑edged on/off quality. However, the use of band attenuators can tame these noises and integrate them much better into the overall effect.
Of course, the Carrier filter attenuators and Shift knob are in play here for vocoding just as they are for filtering, and that allows for a kind of easy, in‑the‑box tone shaping and articulation of the vocoded signal that I’ve certainly never seen before in hardware, and is almost as rare in software.
Additional niceties include a Hold switch (with an accompanying patchbay trigger) that will freeze control voltages arriving at the Carrier filters: it’s a sort of spectral ‘shutter’. Engage it while singing a vowel sound, say, and there’s no requirement to keep on singing. It can also make for some fascinating effects when applied rhythmically to an incoming signal. Then there’s an option to have the onboard decay envelope trigger from a certain threshold of Program signal level. Finally, huge flexibility is offered via the patch points on every band for both the envelope follower outputs and the VCA inputs. Mind‑boggling possibilities open up here, not always easy to predict. One is simply to cross‑patch bands to scramble the normal spectral response: bass sounds could open the highest filters. But you can also attempt tricks like patching band 1 and 10 envelope followers to external Eurorack envelope or sample triggers, letting you beatbox into the Spectravox and ‘play’ kick and snare elsewhere. Or with a suitable sequencer, with multiple CV gate outputs, you could rhythmically open and close individual frequency bands. This aspect of the CV connectivity is lavish, and in fact was completely missing from the prototype, engineer‑built Spectravoxes that emerged from the 2019 Moogfest. So thank you, Moog, for pushing that out to us general punters.
Synth Forever
The inclusion of a simple oscillator and noise source on the Spectravox is a nice bonus, and adds surprising flexibility. In its most basic role it’ll support static‑pitch robot‑voice vocoding, acting as an internal carrier. But it’s also a genuinely playable, useful modular sound source in its own right, which in my tests had accurate 1V/oct pitch tracking over a wide range and decent tuning stability too. The VCO Frequency knob sets static tuning from a clicky 10Hz to dead‑on 4kHz, and with such a span it makes accurate tuning for musical use with external CV controllers quite tricky to dial in, though it’s possible with care.
The Spectravox’s back panel, as with all the 60HP range, is a sparse affair, with just a quarter‑inch audio output and a 12V external power input.The switchable sawtooth and variable pulse oscillator sound is classic, lively Moog, and supported by both a dedicated pulse‑width knob and patchbay PWM input can offer movement and girth. Just as fruitful though is the continuously variable VCO/Noise mix knob, rather like the one offered on the Mother‑32.
Now, for use in a modular rig you could source the oscillator and noise signals via their dedicated patchbay outputs, where they emerge pristine and energetic. But part of the charm of the Spectravox is that baked‑in filter bank coloration I mentioned earlier, and using the post‑filter VCA output (or indeed the main rear‑panel output) makes for a very different sound. It’s the same sawtooth, pulse and noise, but even with all filter bands open they’ll now be far from pristine. The scooped quality is evident once more, wave shapes are nowhere near their starting points, and the noise profile becomes unusual but attractive, with somehow quite hollow, throaty‑sounding low‑mid frequencies and smoothed‑out highs. With all those band attenuators in play I can’t think of another modular‑world oscillator/filter combo that is quite like this one. It’ll produce uncannily human vocal timbres on the one hand, and beautiful synthetic snares and hi‑hats on the other, for which the little decay envelope and front‑panel (or patchbay voltage) trigger come into their own. Alternatively, max out noise level and resonance, enable one or two filter bands only, and patch a 1V/octave musical controller to the Spectravox’s Shift input, and you can tease out all sorts of pitched noise timbres that track perfectly and can be played ‘in tune’, in their way.
Vox Aeterna
As with some other Moog semi‑modulars, what you might first assume the Spectravox is all about is not quite how things work out in reality. I’ve described its application as a filter, vocoder and synth voice, and there is quirkiness associated with all three. As a filter it excels at tonal sculpting bordering on resonator territory, but isn’t perhaps one most people will choose for bread and butter synth patches. As vocoders go it has a strong flavour and certain quite magical qualities, but vocal intelligibility is not where it excels. As a synth it’s fundamentally basic but can sound distinctive and unique.
It’s when you look beyond obvious mainstream uses and get creative that the Spectravox seems to bloom into something really special.
What emerged for me is that the whole of this unusual box of tricks is greater — and weirder — than the sum of these parts. It’s when you look beyond obvious mainstream uses and get creative that the Spectravox seems to bloom into something really special. An example is the potential as a noise source: it’s a wonderful generator of all kinds of interesting beds, sweeps, special effects and indeed synthetic drums. Then there are the sounds that emerge through feedback, such as when you patch the VCA out to the Carrier in: unpredictable outcomes emerge at the turn of every band pot. Not to mention the modulation possibilities offered by frequency‑constrained envelope follower outputs, or the broadband Program level follower, which exists only in the patchbay. To think out of the box with the Spectravox is to discover its real essence I think. The more experimental I got with it the more it gave back.
Could it be better? Yes. It’s unfortunate the vocoder doesn’t have the scope to be more intelligible than it is, because for mainstream use (and who isn’t up for that once in a while?) you’ll need something else besides. We could wish for more CV ins too: LFO Amount is the one that is conspicuously missing. There are also no general attenuators or mults, which will have you looking elsewhere for some jobs.
However, the Spectravox is both unique (as far as I can see) and entertainingly versatile. It carves niches that could be irresistible to devotees of synth history, filter fetishists, effect aficionados and exploratory synthesists alike. It’ll make for an iconic, absorbing and great‑sounding enhancement in many a semi‑ or fully‑modular setup.
Levelling Up
Audio inputs and outputs are vital for an external filter bank and vocoder, and the Spectravox has everything it needs on the face of it. But I was surprised to find that Moog have designed Program and Carrier patchbay inputs for line level (specifically 1.7V) and not Eurorack (typically 5V) signals. Of course, you can attenuate your favourite Eurorack oscillator’s output before it hits them, but that’ll require an external attenuator or two (there are none built in here) and it means gain‑staging can become a bit hit and miss. Certainly with complex vocoding using external carrier and program signals I frequently ran into distortion, especially with high levels of resonance. But since backing off levels sometimes made no difference I suspect it’s just part and parcel of the Spectravox’s internal summing, and I’m minded of the various Moog synth mixer stages that are revered because they are not clean or linear.
The other gotcha is that sounds using heavy attenuation of many program filter bands simply end up very quiet. This is easily dealt with if you’re able to raise gain to compensate on a mixer or audio interface, and you might even choose to make the 12 o’clock position of the Spectravox’s volume control represent a normal working level, allowing for compensatory boosts above it when required. It’s again potentially more of a bother in Eurorack, where you’ll need to commit something with a gain or boost stage to compensate. Signal‑to‑noise ratio is eroded in these circumstances, inevitably, though not unworkably so. Oscillator and noise bleeds can be audible sometimes, though on the whole signal‑to‑noise performance is respectable, and plenty good enough for real‑world use.
Vocoders Revisited
Vocoders have always piqued interest in the general listening public, but their roots are in the purely functional, as devices that made the best of severely limited bandwidth in early long‑distance voice communications, as far back as the 1920s. One of the very first musical uses came through a collaboration between Wendy Carlos and Bob Moog in 1968: two modified 907 Fixed Filter Bank modules were used alongside a clutch of envelope followers and VCAs. The rest, by way of Switched‑on Bach and the soundtrack for A Clockwork Orange, kicks off a particularly colourful part of synth history that saw many competing vocoder products released through the 1970s.
Vocoders work by pairing two complementary banks of (mostly) band‑pass filters. In the first, the program or analysis bank, there’s an envelope follower on each band, which together form an approximation of the harmonic make‑up of an input signal. The voltages generated from the followers open VCAs for a set of corresponding filters in the second bank, called the carrier or synthesis bank. A harmonically rich signal like a pulse‑wave tone is usually used as the audio input for that, and for musical use it can be melodic or chordal. Voila! The energy signature of the first signal is imposed on the second, and if you’re using your voice as a program input it won’t really matter at what pitch you sing or speak: the pitch that’s heard is that of the carrier.
Crudely, the more bands a vocoder has the more intelligibly speech and other articulate signals will be rendered when used as a program input: some digital vocoders offers hundreds of bands. But it’s interesting to note that another vocoder classic, the Roland VP‑330, also had only 10 bands and manages to be highly intelligible. Other factors such as band tuning, filter slopes and envelope follower response characteristics come into play too. Of course, there’s a question as to whether speech intelligibility should always be the end goal, or if other characteristics and strengths could or should be prioritised: the Spectravox is one answer amongst many, and perhaps there are no wrong answers...
Alternatives
Genuine analogue vocoders are rare, and apart from the expensive, 16‑channel Moog reissue the currently available alternatives are Behringer’s VC‑340 (a largely faithful copy of the classic Roland VP‑330) and in Eurorack format AnalogFX’s VXC‑2220. Both offer vastly better vocal intelligibility than the Spectravox, but no equivalent of its Shift control or band attenuation (other than in the module, if you were to equip it with banks of external attenuators). In the digital realm the Electro‑Harmonix V256 pedal is a dependable choice, adjustable between eight and 256 filter bands, and sporting a MIDI‑driven polyphonic synth. The Boss VO‑1 is good too, though you’ll need both external program and carrier signals for that. Beyond that there are vocoders built into synths (Arturia Microfreak, Korg MicroKORG) and also plug‑ins, often fabulously flexible. Arturia’s Moog‑inspired Vocoder‑V is a particularly potent one.
As for filter banks, in the modular world Doepfer’s A‑128 is a 15‑band fixed bank, pure and simple. Somewhat more complex is Tiptop Audio’s Eurorack recreation of the Buchla 296 Spectral Processor. Also check out the ADDAC 601, with eight bands and lots of voltage control, and of course Behringer’s ultra‑affordable copy of the 14‑band Moog 914. In software, a standout option is the 914 MkII plug‑in by Audio Damage: it’s a 14 band Moog 914 emulation and has some equivalents of the Spectravox’s desirable resonance and Shift features.
Pros
- Unusual tone‑shaping potential for external signals and internal tones.
- Patching extends creative potential exponentially, blurring traditional oscillator/filter boundaries.
- The simple synth oscillator and white noise source punch above their weight.
Cons
- The vocoder offers poor vocal intelligibility, despite undoubted character.
- External gear required for some basic CV tasks, such as level matching.
Summary
A historically resonant analogue filter bank, vocoder and synth rolled into one. Satisfyingly adaptable, it can also be delightfully weird.
Information
£599 including VAT.
$599
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