hexinverter.net ACXSynth MIDI2CV polyphonic MIDI-CV converter

MIDI2CV_ThumbCanadian company, hexinverter.net Electronics makes various modules for analog synthesizers. Almost every project is cross-platform and designed not only for Eurorack but also for 4U/5U formats. The model range includes noise-generating modules, sequencers, percussion, envelope generators and effects. This summer Hexinverter has released a new model: 4-voice polyphonic MIDI-CV converter.

MIDI-CV converter 5U module was designed by Alain Coupel of ACXSynth. In the year 2013 Stacy Gaudreau started to make his own version for Eurorack. Finally project became nice 8hp wide DIY module. Later I took a part in developing too by making new firmware.

Full kit (PCBs, components, front-panel and hardware) costs $120. (it’s not perfectly clear, USD or CAD, but rates are very close)

ACXSynth MIDI2CV has three modes, monophonic, multitimbral and polyphonic, which are selectable by switch. Analog CV/Gate outputs act differently according to selected mode. MIDI2CV_big
In Monophonic mode (last note priority), outputs act in the following way: Pitch CV is passed to CV1 and CV2 outputs. Range is 7 octaves with 12-bit resolution. The GATE output’s level going high on keypress and also the short pulse appears on the TRIG output. Each new note causes another Trigger pulses, but if you play legato, Gate is being held. Velocity goes to the VEL output and modulation goes to MOD output. Also MIDI sync function are available. High level on the RUN/STOP output corresponds to Play/Continue command of the sequencer and low level corresponds to Stop command. The short pulse goes to RESET output right after Play command. At the end, the pulses are goes to SYNC output at 24ppqn rate and the ACT LED blinks every beat.
In the Polyphonic mode (Poly4), eight outputs acts as four couples of the CV/Gate outputs for each of 4 available voices.

In the Multitimbral mode (Poly1), outputs also act as couples, but each part listening to its own MIDI-channel.

The very first note being sent to module after power-on defines working MIDI-channel and also first of the four channels in Multitimbral mode (others are respectively subsequent).

Logical events such as Gate, Trigger and Sync can be monitored by bright blue LEDs.


The kit is eye-candy! Chips and sockets are placed in separate tubes. Semiconductive components are separated from passive components and kept in anti-static pockets. Other hardware are also packed in several pockets. Front-panel is covered with special protective film.

Assembly and operational manuals are not included into package but you can download it at the manufacturer’s website. The assembly manual is simple to read and well-illustrated with colourful photos. Following this manual I have built my module in about 2 hours.

The module need to be calibrated before use. The calibration procedure is described in the operational manual. You need a good multimeter. I did the calibration in about 30 minutes using inexpensive Mastech MY68 multimeter, two Doepfer A-110 oscillators, A-115 audio divider and A-138 mixer.

Custom firmware

The way to assign notes to voices in polyphonic devices may be different on different devices. More traditional way can be denoted as serial or shifting. Notes are regularly and sequentially distributed to every voice on every keypress. It makes possible to let complete the release stage of the ADSRs when next note are taken right after previous note release if there is free voices. In the ACXSynth design, voices are assigned as stack: first taken note always goes to 1st voice and all subsequent voices are taking part only if previous ones are occupied (eg some notes are being held). Also if all 4 voices are used, any new notes are simply ignored.

I didn’t liked this behaviour and decided to make my own firmware using some old source published at ACXSynth website. you can read more about it at Project page.


The module is quality made, The components used are specially selected to provide best performance. it has unbeatable price/quality/functionality rate. The only weakness is buggy stock firmware, preventing me to rate this module as ideal, but it can be fixed.

Modularsynth.ru rating: 9/10

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