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ASM Hydrasynth

Digital Synthesizer By Paul Nagle
Published May 2020

ASM Hydrasynth

Seldom has a new synth from a new company excited so much anticipation as the Hydrasynth. Should you believe the hype?

One of the highlights of SynthFest 2019 was the opportunity to get hands-on with the ASM Hydrasynth — and I wasn't the only old-timer gushing about the return of ribbons and polyphonic aftertouch. Afterwards, the long winter months dragged on endlessly, but finally one arrived at my studio. Two, in fact, because the Hydrasynth Desktop was released at the same time.

Pressure Points

To understand some of the excitement, it's worth stepping back and pondering what makes polyphonic aftertouch so enduringly fascinating. Whenever the topic arises, there's almost always mention of a certain classic synthesizer. In the 1970s, many of us lusted over that instrument despite the fact that it was impractical for any bedroom studio, even if a rich aunt died and bequeathed the necessary dosh.

I'm talking about the Yamaha CS‑80, of course. This mighty instrument, in the hands of virtuosos such as Vangelis, proved that synthesizers really could be responsive, expressive musical instruments after all.

Polyphonic aftertouch, or poly pressure, allows the player to modulate notes individually from within a range already held down. This requires sensors for every key and is therefore more complex to produce than channel or mono aftertouch. Typically based on a strip beneath the keyboard delivering a single value to the whole synth, channel aftertouch is ideal for monophonic instruments but less effective when applied to a whole chord. This is where the CS‑80 — and just a handful of other synths — score so highly.

Ashun Sound Machines are a new Chinese company determined to tackle the expression deficit. Their polyphonic pressure implementation is pretty thorough, and includes user tailoring of parameters such as delay, onset time, fade and release, to reduce bounce, jitter and so on. The Hydrasynth response should therefore be adaptable for most players.

The four-octave keyboard feels great. I particularly like the squared-off black notes, which lend it a more piano-like feel and assurance. You can choose from a selection of velocity and aftertouch curves, but there's no denying it lacks the fifth octave regarded as essential by most two-handed players.

Length issues aside, you'll be pleased to hear that the Hydrasynth is not just an expressive controller with any old synth engine tacked on. It is a digital eight-note polyphonic instrument with three analogue modelling oscillators, two of which can perform 'waveform scanning'. These are ably assisted by two filters, five LFOs, five envelopes, an arpeggiator, multiple effects and a comprehensive 32-slot modulation matrix! It's a monotimbral experience, though, with no layering or split capabilities.

These are substantial instruments, weighing in at 10kg and 3.6kg respectively, with no skimping on I/O. MIDI is available over USB and a full complement of 5-pin ports. The keyboard model has a ribbon controller running almost its full length, and on both versions, every knob, button and encoder feels solid and trustworthy.

The Colour Of Magic

A chunky encoder is the gateway to selection from the five banks of 128 patches available. Each patch has its own colour, which is reflected in the encoder and left-hand controls. On shipping, only banks A-C are populated, but ASM offer a further bank on their web site, plus manager software to move data around, make backups and so on. This might be worth exploring sooner rather than later, because the synth always powers up at Bank A's first patch, while your own will probably begin in the empty Banks D and E.

Getting around isn't as easy as it might be because there's no method of going directly to a particular location. Initially, you change patches by spinning the encoder or pressing the L/R arrows, holding Shift to jump in 10s. To speed things along you can define 32 'Favorite' patches which serve as more efficient entry points. Additionally, you can browse by name and category, but there are no User categories.

It's hardly surprising that the Hydrasynth's factory patches set the scene with some impressive Vangelis tones, paying particular homage to the Blade Runner score. Scrolling on, you'll encounter usable brass, strings, bells, basses and arpeggios — sounds that wouldn't seem out of place on a synth from the 1990s. I was most taken by the wavetable sweeps, glassy digital textures, pads and electric pianos, many demonstrating the excellent reverb. While there are a host of worthy bread-and-butter patches, very few give a sense of how strange and 'out there' the Hydrasynth can be.

Hydra Eclectic Power

Let's face it, if the Hydrasynth had a control for every parameter, few of us could afford one. Instead, the main interface is built around encoders, buttons and a crispy clear OLED display. A second display provides patch names, envelope and waveform graphics and an oscilloscope that reflects the real-time output. The architecture is represented as an interactive panel of 26 Module buttons, each an entry point for further pages. Repeatedly pressing a button progresses through the pages, and there are dedicated page buttons too. Further visual assistance is provided by the dynamic lighting of the LFO and envelope buttons.

It's fast and logical: you select a module to work on and tweak the parameters with LED-ringed encoders. Indeed, it's possible to get so fixated on the encoders you entirely forget the dedicated controls. Incidentally, all controls send and respond to NRPNs, rather than the lower-resolution CCs. The resulting parameter changes are smooth and not constrained to 128 possible values. However, NRPNs aren't a perfect fit for step sequencing or generic controller duties, for example, while some expected CCs are ignored, such as CC7 (volume).

Fortunately, other than a few minor inconsistencies (there are, for instance, three different methods of copying objects), the Hydrasynth is a dream to navigate, whether you're making minor tweaks or dishing up entirely new patches.

The Hydrasynth is a seriously impressive opening salvo from a fledgling company; hopefully it heralds a more expressive future for synthesis.

The Oscillators

All three oscillators can access any of the 219 available waveforms; these include the expected analogue modelled waves, plus families of more exotic fare, with names such as Cluster, Flux, Vokz, Klangor and Scorpio. The tonal qualities of each are rich, varied and best experienced by trying them out while watching the oscilloscope. I did initially wonder whether a couple of hundred waves would be enough, but since the Hydrasynth offers myriad ways to twist, squash, mash and trash them, it should be ages before you try every combination.

As well as delivering single waveforms, the first two oscillators can switch to WaveScan mode, a quick and easy variant of wavetable synthesis. If you recall from earlier wavetable synths, you're usually presented with a list of fixed wavetables, each containing chosen waves. You progress through each table using controllers or modulators to produce the distinctive timbral movement.

Rather than fixed tables, the Hydrasynth permits the user to create lists of up to eight waveforms, with gaps and silences available for punctuation and fades. The Wavelist Edit page simplifies the selection process in various ways, and if you have no particular plan to start with, the Random button will choose waveforms for you.

You might expect that just eight waves couldn't compare with the longer tables of older synths, but in practice I didn't find this to be the case. One of the common misconceptions concerning wavetable synthesis is that it's all about endless transitions, but in my experience, the coolest results are achieved by morphing between one or two markedly different waveforms. In such cases, the in-betweens are often as interesting as deliberately prepared waves.

Having created a table, you probably want to hear some automated scanning. This is as easy as pressing an LFO button, then an oscillator button, to set up a basic connection. You then pick a specific parameter to modulate: in this case, Wavescan. It's so fast I wished the selection process could be extended to include other sources, such as the mod wheel, aftertouch and so on. As of firmware version 1.3.1, you select these with an encoder.

Having animated one oscillator, you can build an entirely different wavelist for the second, and maybe modulate that with an envelope for a bit of variety. And while the third oscillator lacks the scanning functionality, it can still select any waveform and you can modulate the selection too. I found it great for thickening duties, sub-bass and miscellaneous tinkly splashes.


The first two oscillators are treated to a pair of 'Mutators' in series. Their names are shortened to Mutant on the panel (presumably so the text fits the buttons) but whatever you call them, Mutants are the Hydrasynth's ace in the hole. Each offers linear FM, wave stacking, oscillator synchronisation, three varieties of pulse-width modulation, or a rather nifty harmonic focus. A Dry/Wet parameter is ever-present, ensuring you can always fade between the unprocessed sound and mutation effect.

Linear FM is, on the surface, basic two-operator FM with feedback, but you aren't restricted to the built-in sine or triangle waves — you can choose any oscillator, the ring mod or noise as source. It doesn't end there, either: the source could be taken from another Mutant or either of the Mod inputs. As you can imagine, this can generate way more than DX-style bells and log drums!

Wavstack layers five copies of the waveform, with variable detuning. Applicable to any waveform, scanned or otherwise, it can serve up Supersaw-type effects and so much more. If you position Wavstack after an FM Mutant, those icy FM tones become something altogether more lush.

The keyboard version of the Hydrasynth measures 80 x 35 x 10.3 cm and weighs in at 10kg.The keyboard version of the Hydrasynth measures 80 x 35 x 10.3 cm and weighs in at 10kg.

Oscillator Sync offers synchronisation to any oscillator and it's fitted with a couple of novel extras: Window and Ratio. The former is used to shave off the clicky, glitchy artifacts of synchronisation, while the latter specifies the number of times resynchronisation occurs within a cycle. This wasn't a feature I'd met before but it's a cracker, and at its extremes, is capable of further variations on the theme of 'nasty and noisy'. This is a common trait of most Mutants when pushed, and ASM are to be applauded for not limiting their baby to tame or 'normal' results.

Three varieties of PWM are present. In ASM Warp, you pick eight sections of the source wave and modulate them individually. As with oscillator sync, Ratio sets how many times the process occurs within a single cycle, leading to mad wave-folding flavours if you depart from the standard ratio of 1.00. Standard PWM is on hand too, as well as 'Sqeez', a weird 'squash the waveform sideways' alternative.

Finally, the Harmonic Mutator picks out harmonics, highlighting some over others. Sweeping its depth parameter adds shimmer and sparkle to any patch — a kind of inverse filtering.

Before moving on, it's worth mentioning that the ring modulator can make use of any oscillator, Mutator, Mod input or noise (white, pink or brown). It therefore opens up either Mod input as a potential source of external audio, as we'll see later. For now, it's worth knowing that all audio sources — internal and external — may be panned and processed by a pair of filters, a balance control setting the amount for each.


The filters offer serial or parallel operation, with filter one the most comprehensive. It includes 11 types, ranging from Moog ladder homages to impressions of the MS20 LPF and HPF, plus a Doepfer Low-pass Gate and a trio based on a boutique modular from Threeler. Last in the list is a vowel filter complete with animated mouth. It shapes the vowels AEIOU in various orders and, when fed with suitably rich waveforms, delivers spooky vocalisations, throat-singing robots and comparable delights.

The 12dB and 24dB ladder filters are smooth and effective; they're accompanied by bass-compensated versions (Fat12 and Fat24) that don't sacrifice their bottom end to high resonance. And while you're unlikely to mistake the MS20 filters for the real thing, they offer worthy alternate responses. I don't know how accurate the Threeler filters are, but I liked the band-pass mode in particular; some of its cutoff and resonance combinations give strange, buzzy but interesting results.

The second filter is a 12dB state-variable design with smooth morphing between low-pass, band-pass and high-pass modes. Potential uses include shaving the excesses from the first filter or, when connected in parallel, applying completely different filtering to selected sources.

For the first filter, the drive control is on hand to boost and fatten. Positioned either pre- or post-filter, drive can significantly change filter character, although with a corresponding hike in volume. Both filters have pre-configured LFO and envelope modulation connections, plus keyboard tracking and velocity routings, thoughtfully saving time and mod matrix slots.

Modulation Matrix

With 32 slots to fill, the Hydrasynth's matrix is the tool for programming the interaction with those expressive controllers. It can reach parts of the synth not always open to modulation, such as the effects, arpeggiator parameters, envelope stages and even the levels of other slots, with only a few omissions as of the current firmware (such as Glide and envelope shapes).

As well as performance controls, we saw earlier how straightforward it is to rope in conventional modulators. Of those, the five LFOs are identical, having all the expected waveforms, noise, plus a user waveform with up to eight steps. Each step of this 'almost sequencer' is programmed by those ever-present encoders, with a smoothing option if necessary.

LFOs can be retriggered by every note played, run freely or be switched to one-shot mode. Activate tempo sync and the LFO locks to a wide range of intervals, from 16 measures to a lively 64th triplets. This is fab for wavesequencing effects or programming the sync'ed automation of Mutants, filters and so on. In non-sync'ed mode the LFOs stretch from 0.02Hz right up to 150Hz — and yes, you deal with them in real-world values, not meaningless numbers.

The real-world principle applies to the envelopes too, in what must be one of the best implementations I've seen in years. Hydrasynth envelopes have continuously variable shapes for every stage, so if you need a shallow logarithmic attack, a steep exponential decay and a long linear release, this synth can deliver them. The times for each stage are generous too — up to 60 seconds for decay and release — and a couple of extra stages have been added for further versatility: delay and hold have maximum times of 32 and 36 seconds respectively.

If you are pondering applications for such glacial shifts, another parameter might offer clues: Freerun. Touch any key briefly, and a free-running envelope will progress through all its timed stages without further intervention. In a live situation, you can initiate complete ambient soundscapes while you nip off for a glass of sweet sherry.

The envelopes can loop, infinitely or a specific number of times. They can be tempo-synchronised too, with each stage having a musically quantised length, adding up to a whole new flavour of groovy. I reckon timed envelopes are almost justification for owning a Hydrasynth by themselves! The only trick ASM have missed is the ability to trigger envelopes independently of the keyboard, say from a Mod input, LFO or Macro button.