Mostro FM, A Portable FM Synthesizer Without DX7 Backstory Is Available Now For Pre-Order

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It took a long time but the Outer Space Sounds Mostro FM, a portable FM Synthesizer is available now for pre-order

Already in July 2017, I reported about the company Outer Space Sounds and the new Mostro FM Synthesizer. It has now taken two years to complete this new portable synth. A little downer: you can only pre-order it now, shipping starts later. Outer Space Sounds Mostro FM is a 4-voice polyphonic Synthesizer that is based on a simplified form of FM synthesis, no classic DX7 operator algorithms, etc. You can operate it like a subtractive synth.

Each of the four voices features its own envelope, LFO, and sub-oscillator. Each voice is based on a sinusoidal carrier/modulator system that is accessible from the interface with carrier modulator ratio, modulation index, and feedback. Actually, the well-known parameters of FM synthesis.

Mostro FM

Modulations: Envelope & LFO

Each voice of Mostro FM features its own envelope that can be routed to the mod index, ratio, and feedback. The envelope generator routes can be set independently for each parameter or in all possible combinations.

Besides the envelopes, each voice also offers an LFO (freewheel or sync) that can be routed to the same three parameters (mod index, ratio, and feedback) but also to the carrier for a vibrato-like effect. LFO routes can be set independently for each parameter or in all possible combinations. So everything is very simple and easy to understand.

Colored Output

Mostro FM features an integrated delay effect processor at the end of the signal flow. It includes parameters for time, feedback and effect send.

There is also a glissando parameter that controls both the polyphony of Mostro and the time of the note slide in mono mode.


Mostro FM comes with a MIDI looper that allows you to record sequencer either from the internal keyboard or from external MIDI. Also, it offers an onboard keyboard made with thick black buttons that can be used for notes with different scales or chords. The keyboard can also be transposed by semitones.

A nice addition are the 5 CV inputs: gate, pitch, modulation index, radio & feedback with which you can control your synth also from other hardware devices like modular synths. For example, you can edit the engine further with other modulation sources such as LFO or envelope. Own sounds can be stored in up to 100 presets and copy them from/to a computer via USB. More space would have been better.

Further Specs:

  • dimensions: 25cm. x 13cm. x 7cm.
  • power consumption: DC 9v 1.5 amp.
  • USB-A: connect a USB MIDI device such as a controller to send and receive MIDI data
  • USB-B: connect to a computer to send and receive MIDI data, or to update the firmware and manage presets
  • MIDI in and MIDI out/through
  • Headphones output
  • TRS balanced audio output
  • Industrial grade knobs and buttons that allow for millions of interactions
  • Hardwood chasis
  • Stainless steel control panel coated with electrostatic paint, silkscreened and laser engraved

First Impression

An interesting portable FM Synthesizer that can score above all by its simple operation. For the demanded price, however, it will be a little harder to succeed, especially in times of Korg Volca FM, PreenFM2 and so on. Nonetheless, Mostro FM an interesting new synth that is different from a classic Yamaha style FM synth.

Outer Space Sounds Mostro FM is available now for pre-order for $429 USD that includes a $50 intro discount (regular $479 USD) + $30 shipping (EU + US). Shipping TBA.

More information here: Outer Space Sounds

Hardware Synthesizer News


    • Yeah. Saw that, but it seems that you can modulate preety much everything, even with CV. While you cant definitely do that on a Volca. I see its far more open and tweakable, which is what I always look for in my synths. Very good for an FM synth.

    • Yes for sure. And I think that it is not really expensive if you compare correctly.
      And so this is:
      Yamaha FM Chip we all know vs different architecture
      plastic vs hardwood chassis
      headphone out vs headphone and balanced out
      midi in vs midi in/out/thru und USB A/B
      no power supply included vs power supply included
      additional cv/gate inputs
      wobbly knobs vs high quality knobs

      I think you shouldn´t compare these two.

  1. PreenFM2 continue to improve or bugs fixe.
    Last firmware is 2.10 released this Jun 13, 2019.
    It also comes with a VST editor.
    And if you are very, very courageous, you can Do It Yourselself.

  2. The “DX7 back story” is actually fundamental FM theory as per Dr. Chowning, and the reason for multi-“algorithm” multi-operator FM is to be able to create the complex spectral contents (and content changes).
    So it is puzzling, that this synth is a mere 2-OSC synth that can have freq modulation between the 2 OSCs – the most absolute basic FM one can think of.
    To somehow see this as a differentiating feature compared to “DX7 back story” and “other FM synths” is, to put it very mildly, a stunning exercise in making a huge negative into a marketed positive.
    There is a very good reason for the “DX7 back story” and it is in the FUNDAMENTALS of FM synthesis for complex timbres… This gizmo at this price point with this type of hype marketing is ludicrous, at best.

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