ESI Xsynth review: super-slim all-in-one MIDI keyboard with polysynth - February 1, 2026

ESI Xsynth review: this super-slim keyboard packs everything you actually need—a MIDI controller, polyAT keys, an audio interface, and a polysynth.

There was a time when companies offered MIDI keyboards with built-in synthesizers and audio interfaces at no extra cost. All-in-one devices like the Novation XioSynth or X-Station come to my mind.

Such clever devices had been unavailable for a long time, but last year they made a comeback: the MWave SMK-37 Pro (now also sold as the Donner Starrykey-37) and the Xsynth from the German company ESI Audio.


ESI Xsynth review
I’ve had the ESI Xsynth in the studio for some time, and now it’s time for the full review. This will show if it lives up to its promises.

Disclosure: ESI sent over an Xsynth for review. They sent it to me for free, but have no say over the review and don’t get to see it before I publish it.


Unboxing
Unboxing
The Xsynth arrived in a slim package. Inside is the instrument, a USB-C cable (not in the picture), and documentation, including licences for DAWs like Bitwig Studio 8-Track, Cubasis LE, and more.

A carrying case to protect the Xsynth when you’re on the go is not included. More on why this is important later.

ESI Xsynth review
The Xsynth is an all-rounder: a MIDI controller keyboard, an audio interface, and a polyphonic digital Synthesizer. A complete package like this is rare in 2026.

The housing is made of lightweight aluminum and weighs just 700g, making it perfect for taking with you. It has 25 keys, which have been slightly shortened (in comparison with the Xkey) to accommodate the other control elements.


That’s the Xsynth
In the upper area, there is a bright OLED display, five endless rotary controls (4 left, 1 right), and a small, switchable knob that controls audio volume and serves as a soft power ON/OFF switch.

I would prefer both functions separately, but one can understand the decision here, as the keyboard has a very compact design. Additionally, you have 20 soft buttons for navigating through the menu system.

Connectivity
Let’s take a look at the back, where the ports are located. They are very comprehensive and offer everything you need, all in a small and compact form factor.

From left to right, you have a stereo input (AUX in) and a stereo output (line out) on TRS 3.5mm sockets, a headphones input, full MIDI in/out on TRS minijacks, and a USB-C port for MIDI and audio.



I don’t usually like TRS MIDI, but the design here doesn’t allow for anything else, so it’s good and positive that it has both MIDI IN and OUT. The sockets are solid and high-quality and caused no problems during testing.

I’ve structured this review into three parts, covering the key features of the ESI Xsynth: MIDI controller, audio interface, and, to keep things interesting, the synth engine.

MIDI Controller: The Keyboard
If you looked at the Xsynth and didn’t know what it was capable of, you would say it’s a classic MIDI controller keyboard. That would be partly true.

The Xsynth is a classic USB-C bus-powered MIDI keyboard. It was recognized immediately upon connection on macOS and iOS and was ready to use. No drivers or extra software.

It has 25 full-size keys with velocity sensitivity and, as a highlight, polyphonic aftertouch. The key design is super slim, yes (case dimensions: 387mm x 148mm x 27mm), and you can’t expect the same keyboard feel as a regular one.

If you’ve never used an Xkey before, you’ll need to get used to the feel of the keys. The key travel is very short. You can barely feel the difference between the beginning and the end of the keystroke.



I had the chance to test the keybed on the Xkey 25 many years ago (around 2015) and wasn’t a big fan of the playing feel at the time. That hasn’t really changed, to be honest. I prefer mini keys such as the Arturia Keystep or the Novation Launchkey 25.

However, I had the same experience as any beginner. You have to get used to the feel. At first, it was a question of whether to trigger the sound, but once I got into the groove, it kept getting better.

Polyphonic aftertouch worked well with the integrated synth engine and with external sources. The keybed is not my favorite, but when it comes to compactness and portability, it’s unbeatable.



There’s no built-in chord or scale function, though. Unusual, since we live in a time when every Synthesizer and MIDI controller has that kind of feature. Maybe it’ll be added later.

Important: Get yourself a carrying case. Many years ago, a key broke on my CME Xkey 25 because I transported it without protection.

ESI doesn’t yet offer a case (but hopefully soon), but according to them, the CME Supernova at 19,99€ should fit, which is still available but often sold out.

MIDI Controller: MIDI CC Controls
Besides the keyboard, there are also pitch and modulation controls. Yes, on soft buttons, and they worked perfectly. Plus, you have buttons for octave up/down, keyboard hold, glide, and a built-in arpeggiator. More on this later.

The Xsynth has four parameter encoders. They are well-built and have a good grip. These are primarily for sound settings. When used as a MIDI controller, these four encoders correspond to standard MIDI CC parameters.


The display works fine, but the camera has the wrong frame rate
The menu provides access to 4 banks of four encoders, giving you 16 MIDI CC assignments.

Clever: the developers have arranged the MIDI CCs so that the freely assignable ones don’t conflict with the synth engine’s MIDI CCs. Otherwise, there would be a huge mess. Don’t worry, you can freely adjust these settings in the hardware at any time.

Thanks to the MIDI input, you can also play the internal engine with another keyboard or sequencer.

Audio Interface
MIDI and Synthesizer keyboards with built-in audio interfaces are rare in 2026. Here is a list of keyboards from August 2024 that offer both. I know I must update it, and I will do it asap.

The ESI Xsynth has one, too. A look at the back confirms it: it has a stereo input and a stereo output with 3.5mm jacks. Additionally, you have a headphone output.

Although I was initially a little worried about the closely spaced jacks, all my 3.5mm stereo cables fit into all of them simultaneously. So you don’t need any special cables like with the Teenage Engineering TX-6.



On macOS and iOS, the 24-bit/96kHz audio interface was automatically recognized and installed. I can’t say how it works on Windows, as I haven’t used Windows in 20 years.

You can record an external stereo signal to your computer via the AUX input. This works without any problems. However, the signal bypasses the entire synth engine, meaning it doesn’t go through the filter or effects.

I asked Lukas Schuette, the developer, about this on the SB 25, and he said it might change in the future. Additionally, the synth engine can be conveniently digitally recorded to a computer or tablet via USB-C. In my setup, however, this led to random jitter. I haven’t found a solution yet.

If you connect a drum machine, for example, you can record both signals simultaneously, but only as a stereo mix, not separately. Tapping the mixer button twice opens the audio interface settings, where you can adjust everything.

A Digital Routing Issue
The audio interface sounds great. However, there is one problem. If you try to add effects to the digital synth engine signal coming in via USB-C, you’ll experience jitter/distortion at the line/headphone output. It’s not a bug, but a routing issue.

This is because the ESI Xsynth routes the signal twice: once directly from the engine and once via the audio interface to the output. Adding effects here causes these signals to collide, resulting in glitchy clippings.

When I brought up this issue, the ESI team said it could be fixed, but it has not yet made a decision. I understand it’s a very minor problem affecting only a few people.



Both the hardware settings and the editor offer additional audio interface and audio routing configuration options. For example, you can change the routing of the AUX in or out of the USB signals.

I would be happy if there were an option to mute the engine’s direct audio when the Xsynth is functioning as an audio interface on a computer or tablet, such as an iPad.

The audio interface function performs well in testing. I didn’t notice any problems that would disrupt recording or daily workflow. Audio in a MIDI keyboard is a special feature. It’s become rare, and it’s great that someone has implemented it.

That’s a bonus point. If this minor routing issue were fixed, it would score the full points.

Synthesizer With VA Rompler Vibes
A MIDI keyboard with a built-in audio interface is already very convenient. But now, the whole package is rounded out by a fully-fledged Synthesizer. Not just a preset player, but a complete synth engine.

The Xsynth engine features a subtractive, virtual-analog Synthesizer supporting up to 10 voices. Depending on the programmed patch, 8 voices are guaranteed, 10 in the best-case scenario.

The engine has a classic structure, perfect for beginners: three oscillators, multimode filter, three LFOs, three AHDSR envelopes, a modulation matrix with 16 slots, three effects, and an EQ.



Sounds can be controlled using the four display parameters and the top-level buttons. Four parameters don’t sound like much, suggesting extensive menu diving. That’s true, but the developers have made it incredibly easy.

I’m not a fan of menu diving synths, but with the buttons that serve as shortcuts to features (OSC, filter, etc.), navigation is super easy. For example, pressing the OSC button lets you easily cycle through the three oscillators without using any key combinations.

The flat menu structure makes programming very fluid and easy. This is reinforced by the limitation to four simultaneous parameters. Each feature UI displays the maximum available hardware parameters. Yes, just four, not more.

If a section needs more parameters, you step to the second page and then tweak it using the same hardware encoders. This is a clever design decision. This makes the Xsynth very hands-on, even with a minimum of knobs.

I just wish the display time could be set to a longer duration than 4 seconds. The UI often switches between elements too quickly.

Synth Details
Let’s get to the nerdy stuff in my ESI Xsynth review. It uses three oscillators as sound generators, with samples/single-cycle waves rather than real-time-generated waveforms. Not certain, but it’s possible they tried to squeeze as much as possible out of the available CPU.

This option probably consumes less power than real-time generation. A wide variety of waveforms are available. The built-in environmental field recordings and acoustic sounds, such as a piano, are unusual and add a unique touch to the synth’s character. Custom file import is not supported, but hopefully will be in the future.

Each oscillator has only a limited set of controls, including waveform selection, coarse and fine tuning, and keytrack. Additionally, you have FM between oscillator 3 and oscillator 1, PWM, sync, unison, drift, and more. Ring mod is available in the oscillator mixer.



For such a small Synthesizer, the oscillator range is pretty impressive. They have a certain VA and rompler retro charm, reminiscent of another era. Features like FM, PWM, and sync don’t blow me away in terms of quality, but they are solidly implemented.

Everything exists in a better and higher-quality form. That’s clear, but not in this particular form factor. Then, you have a simple oscillator mixer with an adjustable ring mod.

From here, it goes into a multimode filter with 12dB and 24dB slopes on the lowpass, bandpass, and highpass. It is adjustable with four controls: cutoff, resonance, keytrack, and drive. On the second page, you also have a shortcut to the envelope.

The filter sounds great. At medium resonance, it gains character, and at maximum resonance, it enters self-oscillation. However, I find the “drive” label somewhat confusing, because when you set the parameter to 0, it becomes quieter.



The other way around, it doesn’t get richer or crunchier. To me, it’s not an overdrive in the classic sense, but rather the gain of the signal fed into the filter. It should be called “gain”.

More Details You Should Know
Modulation isn’t bad either and should be highlighted in the ESI Xsynth review. It houses three LFOs and three AHDSR envelopes, plus a mod matrix with additional modulators.

The LFOs offer features we can only dream of in other, more expensive synths: In addition to a multitude of waveforms, there’s sync, rate control, as well as LFO delay, attack, and start phase.

A big plus is that the LFOs operate at audio rate, allowing for very lovely FM sounds. This is often not the case in modern synths. Equally useful and welcome is the hold function in the ADSR envelopes, making them AHDSR envelopes.

Programming the modulation works like a breeze. Twice-pressing the “MOD” button brings up the 16-slot modulation matrix, with classic source, destination, and amount parameters.

Here you will also find other modulators such as unipolar and fine versions of the LFOs, velocity, aftertouch, polyphonic aftertouch, and more.



Everything at the synthesis level is modulatable, including oscillators, filters, and even LFOs and envelopes, enabling cross-modulation. Especially lovely with the audio-rate LFOs. It’s a shame that the effects can’t be modulated.

Three FX slots are available to polish your sounds. Good-sounding basic delay, reverb, distortion, and more FX algorithms are all on board. Nothing extraordinary, but good sounding, and they bring every sound forward with extra character, width, and depth.

Plus an EQ in the fourth position. I find the arrangement of the effects a bit odd. Not every effect slot offers the same algorithm list, which forces you to use a specific slot because it’s only available in this one. Plus, you also limited how the effects are routed.



Handy Tools
For those who dislike menus, ESI delivers the Xsynth with a fully featured editor and librarian. Currently, it only works in standalone mode, but hopefully it will become a plugin in the future.

Once launched, you can use it conveniently to program and manage your sounds. Alongside this, it manages your firmware updates and deeper features, such as audio interfaces. Cool, it also has a real-time MIDI monitor built in.

Xsynth doesn’t feature a sequencer but a sophisticated arpeggiator that also works with other MIDI gear. It offers plenty of modes to explore, tempo, and octave control, and MIDI synchronization. One page further, you can go deeper with patterns.

Two banks of 128 patterns give you instant new arpeggiator shapes and make you almost forget the absence of a sequencer.

Sound Design
A synth engine invites you to design your own sounds. For this, it has 512 slots, where you can save your creation, although ESI ships the synth with 256 ready-to-use patches.

These can easily be overwritten in the hardware or in the editor if you need more space than the remaining 256 slots. I like that there are also classic sound filters, making it easier scroll through the patches.

Lots of features, but how does the Xsynth engine sound? The Xsynth doesn’t sound bad, but it’s not the best digital Synthesizer out there either. It’s a lovely-sounding digital engine that performed well in the test. The sound spectrum is wide and colorful.

I was very surprised by how beautiful and characterful the pads and sound effects can sound. Much of that is also due to the built-in field recordings that give the sounds an organic touch. Also, the audio-rate modulation makes some wild stuff.

I like to describe the Xsynth as the child of a 90s ROMpler and an early-2000s virtual-analog synth. It has the charm of both worlds. This is also confirmed by the included presets, which really highlight this.

ESI Xsynth Review Conclusion
With the Xsynth, ESI offers an impressive all-rounder keyboard synth designed for mobile setups. Super thin, light at only 700g, space-saving, and fits in any bag, even the smallest one.

And it’s everything in one device: a MIDI keyboard, a MIDI controller, an audio interface, and a polysynth. It’s perfect for setups where mobility is preferred, but without sacrificing the essentials of mobile music production.

Xsynth pairs perfectly with a laptop, especially with an iPad, as they are just as slim and make a very elegant, powerful pair. And with far fewer cables to travel with. A single USB-C is all it takes to record the engine in your mobile DAW.


Xsynth vs setup with individual elements: MIDI keyboard…
At the same time, one must also accept that individual components, larger ones, often offer more and may even be better. There are certainly better audio interfaces or digital polysynths than the Xsynth that sound richer and offer even more features.

The Xsynth should be viewed as a complete music production solution, not just as a Synthesizer. If you consider only the synth alone, there are better options within its price range.

However, as part of a bundle with all other goodies, it’s a charming polysynth offering extensive tweakability and a distinct retro-flavored digital sound.

This is the end of the ESI Xsynth review, but I hope this is the beginning of a new wave of all-in-one MIDI keyboards. With the technology we have today, it only makes sense to put everything in one package.

Pro

• excellent all-in-one package (full synth, MIDI controller, MIDI keyboard, audio interface)
• mobile solution
• polyphonic aftertouch keys
• not the best, but a charming, full-featured synth engine with retro vibes
• menu system
• patch editor

Neutral

• keyboard feel (personal)
• no official ESI carrying soft case

Contra

• audio over USB-C issues (dual audio stream + USB-C jitter (macOS)) (probably fixable)
• fragile keyboard (CME Supernova transport bag recommended)

ESI Xsynth is available now for $429/359€.

More information here: ESI Audio

Source: https://synthanatomy.com/2026/02/es...-all-in-one-midi-keyboard-with-polysynth.html

 

Jun 2, 2025:
https://audiosex.pro/threads/nice-portable-synthesizer.80294/