Mutable Instruments Shruthi-1 Synthesizer

Image shown: Shruthi-1 with CEM3379 filter in a black case


The Shruthi-1 Synthesizer was originally manufactured by Mutable Instruments and is a hybrid digital/analogue monosynth. It was available from Mutable Instruments to customers in kit form for self assembly. This versatile synthesizer has the convenience of two digital oscillators and a sub oscillator, supplied via the 8-bit microcontroller. Classic analogue waveforms can be accessed as well as wavetable synthesis. Digital algorithms can be employed for bit crushing, phase distortion and XOR modulation. The mention of 8 bit may put some potential purchasers off, but many classic synthesizers have 8 bit digital oscillators.

The analogue part of the synthesizer is the filter. On launch, the Shruthi-1 featured an SMR-4 analogue filter. This 4 pole low pass filter sounds sounds very warm and not too dissimilar to filters used in some very sought-after classic Roland synths. The filter is not limited to only creating sound in the circuitry within the device, the filter can also be fed via the audio input allowing external sounds to be processed. The filter was later revised to an SMR-4 MkII version, retaining the sound, but reducing the part count.

A hybrid digital/analogue approach allows the synth to sound like a typical classic subtractive synthesizer but it can also be used to create digital sounds similar to those generated in 8 bit computers and consoles.


The firmware in the instrument is issued under the GNU General Public License. This allows the end user the legal freedom to modify the firmware. The C++ source code is available on github for download.


The Shruthi-1 only has five rotary encoders and four buttons on the front panel, yet programming the instrument is very intuitive due to the way the menu system is implemented. It is a very controllable device and includes a twelve slot modulation matrix. MIDI support is good and a large number of parameters are accessible for real time automation. An arpeggiator is built-in. Although it is effectively a monosynth, it can be chained together with other Shruthi's to add extra polyphony.


On launch, the Shruthi-1 featured an SMR4 filter. Mutable Instruments also released other filters for use with the main control board:

  • 4 Pole Mission - based on the SSM2164 chip
  • Dual SVF - combines two multimode filters
  • Polivoks - clone of the filter used in the Russian Polivoks synth
  • LP2 + Delay - filter inspired by the MS20 LPF with a digital delay
  • FX/Digital - lo-fi digital filter (not analogue)

A comparison of filters can be heard on Soundcloud.

Other filters have been added by third-parties, including:

  • To-LAD-4tr - transistor ladder design influenced by Moog products
  • Shruthacon - reinvention of the Steiner Parker filter
  • CEM3379 - using a Curtis CEM3379 chip
Image shown: Shruthi-1 Red Alert Edition


As the devices are sold in kit form, it is up to the assembler of the product as to which case they want to use. Many users are happy with the acrylic cases, available in many colours. Third-parties also construct metal cases. Some modders will modify existing synth cases to allow extra controllability and a dedicated keyboard. Mutable Instruments have in the past created limited edition runs of special edition kits and cases, which are:

  • Red Alert Edition - red Soviet style design with Polivoks filter
  • Midnight Edition - dark blue case with a ghost graphic using SVF filter
  • 4 Pole Mission - SSM2164 housed in a white casing with wolf graphic
  • Yellow Magic Edition- MS20 style filter with delay in a yellow casing


As circuit diagrams are available from Mutable Instruments and the firmware being open source, some users have modified their Shruthi's by adding extra rotary encoders and switches to allow more hands-on control. Recognising the wishes of customers to have more control, Mutable Instruments launched an XT version of the synth, which had a larger control surface with extra encoders.

Current Availability

The Shruthi-1 product line was discontinued by Mutable Instruments to allow the company to focus on modular synth hardware. However, the product is still available through Tubeohm, although is now rebranded as the Phoenix synth. At the time of writing, most filters are still available from Tubeohm, as are a selection of acrylic cases. An XT version is also available which allows improved hands-on control to more parameters, with a total of 29 pots.


Tubeohm have developed a new version of the Shruthi/Phoenix synth based on the EDP Wasp synth using the CD4069 filter. The new board also allows for CV control. Further details are available here.

Image shown: Shruthi-1 4 Pole Mission

Youtube Links

Shruthi-1 demonstration
by creator Oliver Gillet and Frank Hergott

Demo of Shruthi-1 Red Alert Edition
by Rodrigo Reyes

Shruthi-1 Custom keyboard build
by SynthProject

Shruthi-1 demonstration
by SyntheticMachines

Shruthi-1 Yellow Magic Edition
by Das Nut

Images by Danial Brien