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Rare Waves Hydronium is an excellent analog synth for electronic music producers. This is a great-sounding music instrument with MIDI and CV-Gate control. Its available as a DIY kit, or pre-assembled — in tabletop format and Eurorack — and your choice of colors! Its brand new for 2019, created and supported by Eric Archer behind all of our well-known Grendel gear.


The Rare Waves Hydronium is a semi-modular synthesizer with unmistakable true analog tone. Its rich array of CV patch points can interconnect with other analog synthesizers to inspire your music deeper and further in the creative zone.

While it is designed to fulfill the role of a TB-303 inspired acid synth, Hydronium is not a clone circuit. Instead it is an all-new design with its functions re-imagined and extended a little beyond what you might expect from the clone genre.

Its tone is based on a single VCO, a resonant low-pass filter, and a simple decay envelope. And as you’d anticipate of any acid synth, it has programmable glide (pitch slide) and dynamic accents for individual notes.

Hydronium’s classic tone is a natural choice for sequenced basslines. Switch the envelope to gate mode, and it also serves as a lead synthesizer when you need a bright, clean sawtooth or pulse waveform in the mix.

It can be played either by MIDI control or CV-Gate. In addition, its built-in MIDI-to-CV converter with post-glide v/oct output lets you use it as a bridge between your MIDI controller and other analog synthesizers.

Hydronium features an audio effects loop that is positioned between its filter and VCA. This is a good place for distortion effects, phaser, and certain other effects that tend to increase the noise floor when patched to a synth’s line out. The pre-VCA effect return silences the self-noise from the effect, preserving full dynamic range even when you bend its tone with a high-gain fuzz pedal, for example.


 Monophonic (1 voice)Oscillator Waveforms

 Sawtooth, Square, Pulse, PWM


 Voltage-controlled 12dB/oct state-variable low pass filter with variable Q and self- oscillation capability

Envelope Generator

 Velocity-sensitive decay envelope. Triggered (decay) or gated (hold-release).Dynamic Accent

 Bipolar accent control (+6 dB boost, -infinity dB cut)Glide (Pitch Slide)

 Legato glide, full glide.MIDI

 MIDI Input x 1 (DIN-5 jack)CV-Gate Inputs (1 volt per octave)

 V/oct In x 1 (100k impedance), Gate In (20k impedance)MIDI-CV Outputs

 V/oct Out (post-glide), Trigger Out (5ms @ +5V), Velocity Out (0..+5V)Other CV Inputs

 Filter V/oct In, Velocity In, PWM In, Decay CV In, Glide CV InAudio Inputs

 Audio In (VCO cancel), Audio Mix In. Both are pre - filter.

Audio Line Output

  •   1/4” mono unbalanced (Hydronium Tabletop)

  •   3.5 mm mono unbalanced (Hydronium Eurorack)


     Eurorack: Width = 28 HP (5.6”, 142 mm), Depth behind rails = 1.0” (26 mm)

 Tabletop: W x D x H = 4.8 x 6.0 x 2.2” (122 x 152 x 56 mm)


Q. So it’s a TB-303 clone, just like a Roland in every way?
A. No, its not a circuit clone, and the controls aren’t meant to exactly copy the 303 controls either.

Q. What sort of filter does the Hydronium have?
A. Hydronium has an OTA-tuned 12dB/oct state-variable filter. The filter is derived from a reference design published by National Semiconductor in the LM13700 OTA datasheet. It also contains improvements compiled from various articles published in the Electronotes journal several decades ago.

Q. I’m located outside N.America. Can I use the included AC adapter?
A. Yes. The AC adapter is rated for 100-220VAC input, 50-60Hz. It has N.American style pins. To connect it to a European or UK outlet, you just need a simple slip-on travel adapter. This adapter is not included, but they are commonly available on eBay for next to nothing. See the reference photo:

Or if you already have a 12VDC adapter, with 2.1mm center pin positive, then that is OK to use with Hydronium.  It only needs ~50 mA of current.

Q. What is controllable by MIDI?
A. Note On/Off, Velocity, Accent (accented note = MIDI velocity > 100), Glide On/Off (legato phrase), Decay time (mod wheel)

Q. What is controllable by CV?
A. Pitch V/oct, Filter V/oct, Gate/Trigger In, Velocity (0..+5V) *no accent gate in*, Decay CV In, Glide Time CV In, PWM In.

Q. What CV outputs does it have?
A. V/oct out (post-glide), MIDI Velocity Out (0..+5V), Trigger out (5 ms pulse @ +5V)

Q. What are the audio patch options?
A. Pre-filter audio in (mix), pre-filter audio in (VCO mute), Effect send/return (between filter and VCA), Main out (1/4” unbalanced).

Q. Why is there a MIDI- CV converter in Hydronium Eurorack? I already have a MIDI-CV converter module, so isn’t it a waste of space?
A. Hydronium’s MIDI-CV converter is tightly integrated with the synthesizer. It provides velocity control of the envelope generator, and also lets you control decay time with the MIDI mod wheel. Also, it is designed to play the correct pitch of each MIDI key without requiring you to painstakingly adjust a VCO’s Coarse and Fine Tune controls every time you set up a patch. Once Hydronium’s coarse tune trimmer is set up, you’ll always get the correct MIDI pitch – just use the Tune knob for fine tuning. Additionally, Hydronium’s V/OCT output has the legato pitch glide effect applied to it. (Most MIDI-CV converters don’t give you an easy way to put glide on the V/OCT CV output.)

Q. Will the Hydronium run on batteries?
A. No.

Q. Does it take USB MIDI?
A. No. Hydronium uses standard MIDI cable only (DIN-5).

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