You’ve probably read our article about the free KERN CHECK spectral mono compatibility analyzer earlier this month.
We absolutely loved it. CHECK shows you where your stereo image collapses in mono across 40 frequency bands, and it’s a very useful little diagnostic tool.
The developer behind CHECK, Jonas R, also makes three paid mixing plugins that work as a chain alongside it.
KERN Audio Bundle includes SMOOTH, WARM, and WIDE plugins. Each solves one specific mixing problem, and together with CHECK they form a complete mixing toolkit from a single developer for under $60.
Jonas is a former touring musician from Denmark. An illness ended his performing career, and he spent the following three years reading psychoacoustic research papers, learning C++, and building the plugins he wanted to use but couldn’t afford.
Every KERN plugin starts from published academic research, has a maximum of five knobs, and runs under 3% CPU. There are no investors and no employees. Just one person in Copenhagen and a growing community of producers.
The bundle lets you get everything for $59 instead of $87 individually, which saves you $28 (32% off).
Let’s check out each one in more detail.
SMOOTH
SMOOTH is a dynamic resonance suppressor. It analyzes your signal across 40 psychoacoustic frequency bands (ERB scale), identifies which peaks are resonances versus musical character, and transparently reduces only the problems.
It works really well for cleaning up vocals, synths, drum buses, and full mixes.
I love the straightforward control setup here. Depth sets how much resonance is removed, Sensitivity determines how prominent a resonance must be before it’s caught, Attack controls reaction speed, and Mix handles wet/dry blending.
There’s also a Delta button that lets you hear exactly what’s being removed, M/S processing, and four draggable sensitivity nodes for frequency-specific control.
WARM
WARM adds harmonic saturation with three distinct characters.
Tape uses symmetric tanh clipping for odd-harmonic warmth. Tube uses bias-shifted asymmetric clipping with a strong 2nd harmonic for richer, more musical distortion. Transformer uses quadratic asymmetry for deep even-harmonic weight, ideal for bass and drums.
Each character uses Chebyshev polynomials to control harmonics independently rather than applying random distortion.
You also get an envelope follower that modulates drive with input level, like real analog circuits, and K-weighted auto-compensation keeps A/B comparisons honest.
WIDE
WIDE creates stereo width using a 40-stage allpass cascade with psychoacoustic frequency spacing. It works on mono sources too, creating real perceived width by generating subtle frequency-dependent timing differences between channels.
A per-band correlation constraint keeps every frequency mono-safe automatically. This is absolutely perfect when you’re working quickly and just want to add some stereo width quickly without destroying your audio.
Three modes are available: Stereo for allpass decorrelation, M/S for mid-side balance, and Haas for delay-based widening. Also, the Focus knob keeps bass centered while the rest of the spectrum gets the full width treatment.
Specs
All KERN plugins are available in VST3 and AU formats for macOS (Apple Silicon native) and Windows.
No iLok, no subscription, and no account required. Each plugin costs $29 individually, or you can get the bundle (SMOOTH + WARM + WIDE) for $59.
And, of course, CHECK remains free if you want to check it out.
More info: KERN Audio Bundle ($59)
Last Updated on March 30, 2026 by Tomislav Zlatic.
