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.

For pricing and how to order, see our Ordering page…

Hydronium Tabletop now comes in bright color choices, new for 2020!

Left: Phantom Black. Top row: Phantom Orange, Hydride Yellow, Di-Ethyl Orange. Middle row: Juice Purple, Hydride Black, Di-ethyl Purple. Bottom row: Hydride Blue, Juice Yellow

Hydronium Eurorack now features brushed aluminum control panels, in black anodized or silver finish.  The module width is 28 HP.

Hydronium Eurorack – 2020 edition featuring brushed aluminum control panel in black anodized or silver.


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.

Documents (PDF)

Video Demos

kit contents

Here’s a look at what you’ll find in the Hydronium kit.  The parts are organized in sealed sections of anti-static poly tubing.  The chips come packed with anti-static foam.


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. How hard is it to build?
A. This is not recommended as a beginner project.  You should have previous success building a synth DIY project before attempting to build Hydronium.  (Try our Tune in Tokyo or Lite2Sound PX!)  The parts kit is well organized, and the instructions are very clear.  A good quality fine-tipped soldering iron is necessary.  Don’t try it with an el Cheapo “woodburner” hobby store iron or some oversized 50-year old clunker from the vacuum tube era.

Q. How long does it take to build?
A. At a moderate pace, paying close attention to all the directions, probably somewhere around 4 hours… certainly not less than 2 hours !

Q. How many parts are in the kit?
A. There are about 225 parts in all (hardware included), including about 160 components on the main PCB.

Q. Are there any surface-mount parts I have to deal with?
A. No, any surface mount components will come pre-assembled.

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. Are some of the older Hydronium Eurorack modules too large to fit in Doepfer-style cases? 

A. Yes, Hydronium Eurorack with the original PCB (Main PCB revision C2) requires 113.7 mm (11.37 cm, 4.475 inch) between the rails. If your case has thicker rails, such as Doepfer, it can be a tight fit, and in some Doepfer style cases it may be too large to fit. If your Hydronium Eurorack does not fit your case, contact us and we will exchange it for a revised Hydronium that can fit.  This applies to units made in 2018-2019.

Q. Is this kit lead-free?
A. Yes, the PCBs and parts are all lead-free and RoHS-compliant.

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).

Q. Can I just have a bare PCB and the parts list?
A. No, it is only available as a full kit or pre-assembled.

Q. How can I use an Accent Gate (0 – 10V) with Hydronium?
A. Hydronium’s CV jacks are protected from damage by over voltage.  As is, you can connect an Accent Gate to Decay CV In, or Glide CV In, or Filter CV In, with useful results. But if you simply connect Accent Gate to Hydronium Velocity In, the output will be silent whenever the Accent Gate is off.

To patch an Accent Gate correctly to Velocity In for volume accents, insert a voltage processor module that lets you attenuate your Accent Gate and add a positive offset of about +1V.  The attenuation knob will serve as your Accent Depth.  The positive offset amount will vary the contrast between loudest and softest notes.

Q. I want to change the color of the LEDs. Which resistors control the LED brightness?
A. The blue LEDs included are rated 5000 mcd@20mA “super bright”, and the kit uses 20k ohm resistors to limit the LED current to about 0.1mA, resulting in a comfortable intensity of about 25 mcd.  If you substitute LEDs of lower brightness (less than 5000 mcd), you should adjust the LED resistors R16, R66, and R67.  Substitute lower resistance (4.7k, 2.2k, 1k … etc) if your replacement LEDs are less than “super bright”.