PreenFM2 Digital FM Synthesizer

  • 6 Oscillators
  • 2 Envelopes
  • 3 LFO
  • Polyphony & Tuning
  • up to 14 voice Polyphony
  • 4 channel Timbrality
  • Patches RAM: 128
  • Multipatches RAM: 256
  • Storage: USB
  • Editing: USB
  • Arpeggiator +Arpeggiator syncable to MIDI Clock
  • Effects +1 Slot with LP, HP, Bass boost, Band Pass, crusher
  • Sequencer +2 Step Sequencers
  • DX7 preset compatible through sysex.
  • Open Hardware and Open Source design by Xavier Hosxe.


FM synthesizers were the all the rage back in the 1980s. The most famous, and prevalent was the Yamaha DX7 series. The DX7 could deliver incredible bass and bell tones from its FM engine. The downside was that 90% of the users, never ventured into the configuration menus to make their own patches. They stayed with the presets, because programming the DX7 was just too difficult. Jump ahead to 2016, and Xavier Hosxe has designed an awesome beast of an FM synth. Playing this side by side against my own DX7II, it is impossible for to tell the difference with some of the presets. Even more astonishing is that the PreenFM3 has 14 voices of polyphony and timbrality while the DX7II had 16 voices of polyphony, and 2 channel timbrality.

Confused, no worries. If you had read the previous pages about some of the analog synths built on this side, you might recall they are almost always monophonic. That means, they have one voice (or sound) that can be played at anyone time. Press one key, get one sound, press three keys forming a chord, you get one, and only one sound or note. That means they are also mono-timbral, they can make one sound at a time. In a 4 voice polyphonic system, you press one key, you get one sound. Press two keys, you get 2 sounds and 4 or more keys, you get 4 sounds simultaneous. But, they are also mostly mono-timbral. That sound is the same patch or has the same shape, tone, harmonic for each key pressed. It is just that each key pressed will have a different musical key, which means you can play basic chords. So, 14 voice polyphony means I can play 14 keys simultaneously. However, because it is 14 channel multitimbrel, each one of those keys can be a completely different sound. I can even setup the voices so that each one answers to a different MIDI channel.

Here is how is sounds….. The ones I built sound identical, here they are with an Ambika hybrid synth. Demo of 2xPreenFM and an Ambika

Ok, the coolest thing is that this is a DIY project. Oh sure, you can order a built one, or even order a complete kit from someone. This site however is about building 100% DIY and we can use the designs of others, but we have to put the whole thing together ourselves. In fact you can download:

  • The PCB files
  • Firmware and source code files
  • Files to make the laser cut case

You can even modify any of them provided that you do it under the open source rules, and make any derivative open source to anyone else who wants it, free of charge. You can certainly build these and sell them if you want but first, we need the circuit boards and a schematic which we can get from the GitHub repository:

Then, send the board files out to your favourite online PCB manufacturer and ‘presto’.. you have them delivered in a couple of weeks:

The PCBs purchased online.

You may have a struggle with the Preen405 MCU board as it is 100% SMT components and the STM32F405 processor is really small. I destroyed my first attempt even after practicing on several inexpensive SMT practice kits. I also used a USB microscope cam to assist. Remember though, you can always buy the Preen405 already built from Xavier’s store and then build the rest yourself you are not comfortable. But if you shop around, and source all the components and PCBs, you can build this synth for just under $135. Compare that to the Korg Volca FM at $200. But the PreenFM blows the doors off the Volca. Hands down. The PreenFM is comparable to many higher end FM polyphonic synths in the $750 up range.

Yes… it is really small to hand solder. Those are 1/2 millimetre pitch.

There is tons of information on Xavier’s site, including sound examples. Here is the rest of the images of this build: