“The story began 15 years ago when I was repairing Soviet synthesizers”: Eternal Engine’s tube-powered MARS is the wildest synth we’ve seen in ages – here’s how it came to beEternal Engine has unveiled a new polyphonic synth based on vacuum tubes, and it claims it is the first of its kind.
The synth, called MARS, is a hybrid instrument offering the power of oscillators based on the famous R-2R DAC TDA1387 digital-to-analogue converter, combined with the original fully tube audio path based on the APPARATUS synthesiser circuit.

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Though vacuum tubes were first integrated into synths all the way back in 1939 with the Hammond Novachord, few modern synths have integrated them into their designs, and where they have, these have been monophonic – like the Korg Volca Nubass, for example.
MARS’ sounds can range from punchy aggressive FM leads and acid basslines, to soft classic pads and strings. It also supports full software control and easily integrates into any DAW via plugin. Each one is assembled by hand, and they are produced in limited quantities. Hear it in-play below:

It has a compact rack format, which is five units high, and four oscillators operating in modes: Basic (Sine, Triangle, Sawtooth, Fixed Pulse Width 1/2, 1/4, 1/8); Pulse; Ensemble and Noise with Hi-Pass and Low-Pass filters.
Further features include:

Voltage-controlled tube filter of the second order with self-oscillation with overload
Voltage-controlled amplifier, following the traditional architecture of tube opto compressors
Triode asymmetric overdrive
Indicator based on Nixie lamps
Four channels of 4xCV and Gate for integration with Eurorack format synths
External Input for processing the audio signal of external audio devices
Two HADSR envelope generators with pre-delay and multiple retriggers
Sub-audio frequency generator LFO with restart and synchronisation Clock, smooth waveform transition sawtooth-triangle-sawtooth, pulse-square-pulse, as well as Sample & Hold and Noise with reconfigurable Hi-Pass and Low-Pass filters
Auto and continuous modes of portamento / legato
Mono, duo, and four-voice polyphony modes
Flexible four-channel modulation matrix
PC connection with galvanic isolation

A polyphonic tube-powered synth is a wild thought, so MusicTech reached out to Alexander at Eternal Engine to learn more.
“The story of MARS began 15 years ago when I was repairing and modifying both Soviet and imported synthesizers,” he recalls. “I worked on iconic models like the Polivoks, Aelita, Alisa 1377, Minimoog, Jupiter-8, Prophet-5, Yamaha CS-80, and many others.
“By studying their circuits and designs, I learned what made them great, as well as their limitations. This experience helped me understand how different engineers approached sound design and problem-solving.
“Besides repairs, I also worked on synth modifications – fixing design flaws, adding new features like MIDI, and expanding their capabilities. Over time, I started to imagine what my ideal synthesizer would be and how I could build it.”
Alexander says he began developing his first tube filter in 2017, a process which took him over a year, and one which led to his first tube synth, Apparatus, which arrived in 2019. But while MARS only arrives now in 2025, he had been working on it for some time before the launch of Apparatus.
Credit: Eternal Engine
The first version, MARS-1, was a hybrid synthesizer with a modified Polivoks filter and an analog three-phase chorus. It had a lot of potential, but the project was too complex for one person, so I focused on developing modules for the tube-based Apparatus.
“In 2020, during the pandemic, I finally had time to return to the MARS-1 project. With all the experience I had gained from Apparatus, I was confident that I could combine powerful digital oscillators with a rich tube sound. The result exceeded all my expectations.”
Alexander says designing MARS’s interface was one of the biggest challenges of the process, as it “had to be compact enough to fit in a standard rack while staying intuitive and easy to use – without making the musician feel overloaded with too many controls”.
“I always liked the idea of having one or two knobs with parameter indication for each section,” he says. “This way, you can always see the stored value, have instant access to any parameter, and avoid overcrowding the front panel with unnecessary controls.
“It took several years to refine the design, and now MARS is finally ready for production. I believe it offers a unique mix of modern digital flexibility and rich analog depth. This synthesizer can create everything from deep, evolving textures to aggressive, expressive tones while keeping a dynamic and organic sound.”
MARS is priced at $2199. To find out more, head over to Eternal Engine.
The post “The story began 15 years ago when I was repairing Soviet synthesizers”: Eternal Engine’s tube-powered MARS is the wildest synth we’ve seen in ages – here’s how it came to be appeared first on MusicTech.