Audiokit Synth One J6 is a new, high-quality, free Synthesizer app for iOS (iPad/iPhone) designed for mobile music producers and synthesis beginners.
When you start making music on iOS, you’ll probably first look for free apps on the App Store. Besides the pre-installed Apple GarageBand app, there is, for example, AudioKit Synth One, a community-favorite free, great-sounding synth app for iPhones and iPads.
The app debuted in 2018, and seven years later, it’s missing some key features as technology has advanced, especially the later-introduced AUv3 format. The app needs a successor—a fact that developer Matthew Fecher has also recognized. After a long period of development and fine-tuning, it’s finally here.
AudioKit has just dropped Synth One J6, the evolution of the “best-selling” free Synth One app. I was fortunate enough to participate in the beta testing of the Synth One J6 app, allowing me to share my impressions and thoughts with you.
Audiokit Synth One J6 Review
Like its predecessor, Synth One J6 is a free download from the Apple App Store. It runs on iPhones and iPads, standalone or as an AUv3 plugin. Unfortunately, it’s not available on macOS (either as a standalone or AUv3 plugin). That would be the cherry on an oversized chocolate-covered cream cake, but I hope for a future macOS release.
Audiokit Synth One J6 has been completely rebuilt, offering a new feature set and design. However, the dead-easy, inviting design, which even a schoolchild or a grandparent can understand, has remained. This is also one of the reasons why the first Synth One was so popular. The entry barrier was minimal, and you could start using it without having to take an introductory course in synthesis.
The Synth One J6 interface is divided into five tabs: DCO, ENV, VCF, FX, and LFO, allowing for navigation to different features of the engine. You can also easily swap elements by double-tapping on a tab.
Juno/Jupiter-Style Oscillators & Filter
The successor is once again based on a subtractive virtual analog core. This time, the legendary Roland Juno and Jupiter-8 analog synthesizers serve as the model. However, Synth One J6 is by no means an accurate top-notch emulation of these, nor does it aim to be.
But you can call it an analog-modeling synth because Matthew Fecher captured many aspects of these iconic retro analog synths.
At the core are two oscillators (DCO) based on the Juno-106 and Jupiter-8 synthesizers. The waveforms were obtained from AI algorithms that were fed with tons of recorded synth sounds, which analyzed every nuance.
Both oscillators feature classic waveforms, including sine, saw, square, and triangle waves, with Juno-style octave ranges, and independent level and tune controls. A square sub-oscillator adds extra bottom and richness to the sound. Plus, a noise generator brings extra spice to the sounds.
The sound generator structure is very classic and straightforward—no unnecessary menu diving or features you might not need anyway. The output sound pretty damn good for a free app. I get instant Juno/Jupiteristic vibes. As each section has its own volume control for mixing duties, the app doesn’t need a separate mixer.
Then, the oscillator flows into a Jupiter-8-inspired filter section with a resonant lowpass filter and a non-resonant high-pass filter. You can tweak it manually with the cutoff and resonance sliders or modulate the cutoff with both the envelope and LFO.
During testing, the filter revealed two distinct facets. At up to 50% resonance, it exhibits a beautiful, distinctive, and smooth creamy character. Above 50%, it unfolds its deep, growling character – delicious. You can’t hear any stepping or unwanted artifacts—a perfect filter, as it should be.
Flexible Modulation
We move on to modulations. Audiokit Synth One J6 hosts two classic linkable ADSR envelopes for modulating the filter and amplitude. The filter envelope also has adjustable keyboard tracking and velocity. Next to this, you can work with three LFOs: one super simple, and two more advanced.
For example, LFO 3 features a handy delay function that allows for modulation where the impact is felt in the sound, rather than at the beginning. Additionally, it features clever functions such as looping, restarting, and snapping back. These are advanced features typically found in larger synths. It’s nice to have them here.
Synth One J6 takes two approaches to mapping. The envelopes are fixed to the filter and amp. It’s a shame; more freedom would have been nice. The LFOs, on the other hand, offer complete freedom. In one of the few submenus, there is a mod matrix where you can route LFO2 and LFO3 to two different destinations.
What’s great and a big thumbs up is that these extend far beyond the classic cutoff filter parameters. Among other things, you can route it to the stages of the ADSR envelopes, the effects, the LFO itself (yeah, wild cross-mods), or even the individual steps of the sequencer. There are over 60+ destinations, says Audiokit co-founder Matthew Fecher.
A bit hidden in a submenu is the option to map the mod wheel to various parameters (LFO rate, cutoff…). For a free app, this is very impressive. Delve into these options, and you can get strange results like a sequencer with an organic touch.
Final Touches
To give the sounds the final shine, Synth One J6 also incorporates a multi-FX section with a prominent addition. The Supersaw, based on the Roland Jupiter 8000 supersaw, allows you to stack up to five voices with dedicated detune and stereo options. Use it to create thick and rich lead sounds, for example
Alongside the famous supersaw, there is a modeled Juno chorus with all three distinct modes (I, II, I+II). AudioKit has done a great job with this one, as it instantly evokes Juno vibes. You can also work with other creative effects, such as a widener, a simple bit crusher, and an auto panner.
The usual suspects: delay and reverb must not be missing. For the delay, Matthew opted for an analog-style delay that can be adjusted using the time, tap, and mix controls. It’s also easy to sync. It’s certainly not one of the most comprehensive delays you can imagine, but it complements the overall sound nicely.
A highlight is the built-in big reverb, which utilizes an algorithm borrowed from Sean Costello, also known as Valhalla DSP plugins. Technically, it’s a straightforward reverb with size, low-cut, and level controls.
There is also a second mode called Sparkle Hall. I have to admit that during the test, I had the reverb ON 98% of the time because it sounds lovely and dense. It can generate subtle halls in the first third, and if you max out the time, you open the door for ambient texture paths.
It’s positive that the jumps in the reverb sizes are very seamless and smooth. There are no artifacts, so you can also adjust the reverb sizes in a performance without creating unwanted noise. Lastly, there is an option to make your sound mode with a programmable glide and legato.
Not to forget, you can modulate the parameters using the LFOs through the mod matrix. For a free app, this multi-FX section sounds very professional and high-quality.
Playability
AudioKit has consistently prioritized playability in its apps. This is also the case with the new AudioKit Synth One J6.
You can play the app in the easiest and iPad-friendly way with your fingers using the built-in virtual keyboard. Of course, it also works with a MIDI keyboard or Bluetooth MIDI, which the app recognised instantly in testing.
For those who prefer automatic playing, there’s an analog-style arpeggiator/sequencer. It has up to 16 programmable steps, with rate and gate control. Individual steps can also be turned off, allowing for creative use to create varied patterns.
A highlight for me is the option to modulate individual patterns with the LFOs, which gives it a wonky, organic touch. Synth One J6 can also be easily used in a performance. The two built-in, freely assignable XY pads give you additional flexibility for interacting with parameters.
AudioKit Synth One J6 is just as easy to integrate into iPad music productions. Thanks to AUv3 support, you can load and play multiple instances of it in your iOS DAW or host. The app didn’t consume a significant amount of CPU on my iPad Pro M1 during testing, leaving me plenty of headroom for multiple instances.
Sound Browser
Synth One J6 is a free download. Everyone probably knows that by now, but what’s the catch? Do you need to purchase the sounds separately via an in-app purchase? Good idea, but wrong. These are also free of charge with every Synth One J6 download. By the way, if you would like to support the development, please consider using the in-app donation option.
The app ships with over 400+ newly crafted presets from the big Audiokit community, including Analog Matthew, Electronisounds, DMT Cymatics, Jakob haQ, Moby Pixel, and more. You will find an impressive, colorful palette of ready-to-use sounds that will inspire you instantly: arpeggiators, bass, pads, keys, leads, and more.
I joined the beta testing phase late, so many sounds were already finished. The ones I worked with were great fun and of high quality. Of course, there are the usual bread-and-butter sounds that everyone likes, but also more experimental and noisier ones. I’m sure the sounds will accompany you for many coming summer jam days or nights.
Lastly, the standalone app also has a handy built-in instant recorder with export function, and the engine supports full MIDI CC for mapping parameters to your MIDI controller.
Audiokit Synth One J6 Review Conclusion
All in all, Matthew Fecher has successfully brought his well-known free Synth One app into the 2025 iPad world. The Synth One J6 is an excellent mobile Synthesizer with character and countless inspiring sounds.
The simple operation invites people to make music with their iPads and iPhones. Whether you’re a child, a music producer, or a curious grandparent. Synth One J6 is understandable for everyone, even without knowledge of waveforms, filtering, shaping, etc. A Juno/Jupiter-style synth for everyone, and it’s just a free download away.
Audiokit has created an incredible deal for the mobile music-making community. You can criticise that it’s not the most comprehensive app (no FM, ring mod, etc.) Sure, but the app doesn’t need that, as it targets a different user group than flagship iPad synths from Waldorf and other companies. The simplicity and accessibility of the app are key factors in its success.
Audiokit Synth One J6 is available now as a free download from the Apple App Store. It runs standalone on iPhones and iPads, as well as an AUv3 plugin. If you enjoy the app, please consider using the in-app purchase feature to donate to the developer.
No PWM?
nope, strange indeed