It took me a while to get hip to the benefits of dynamics processing in Eurorack and was of the mindset that it was something best left out of my case and taken care of [if at all] later on in my music-making process. I’ve had great results and a lot of fun with other dynamics focused modules [DPW’s Shape comes to mind] but never included a full-fledged compressor in my case. After living with Messor, a compact and versatile stereo compressor from Cosmotronic, I’ve been kicking myself that I waited so long.
The Messor packs a ton of utility into its small footprint and is remarkably versatile. Messor has manual control over the compressor’s attack [time ranges from 1 to 500ms], release [from 5ms up to 3.5 seconds], and ratio, as well as threshold and make-up gain. There is a sidechain filter [switchable between low-pass/high-pass] with frequency control, a dedicated bypass switch, and a toggle to switch between “pure” and “warm” modes, with the latter introducing a great sounding tube-like soft clipping to the signal.
In addition to its stereo inputs, Messor has CV ins for attack and release, gain, filter cutoff, and an input for an external sidechain signal. There is an output for the gain reduction envelope, which can be utilized in other parts of your patch, as well as the stereo outputs. Both the attack and release times can be modulated with a positive signal and their respective knobs will act as offsets when CV is present.
The threshold knob sets the point at which the compression is triggered and signals above the threshold generate a gain reduction envelope which is shaped by the attack and release settings and there is a handy [and nice looking] LED ring around the threshold knob that provides some visual feedback of the generated envelope.
Messor’s compression ratio can be set from completely off, with the knob fully counter-clockwise, and increased smoothly through various signal-to-reduction ratios into hard limiting and even further into negative ratios, at which point signals above the threshold are reduced to levels below the threshold—making loud sounds quiet. This over-limiting is perfect for transient shaping and messing about with the dynamic spread of your input signal.
Getting a classic sidechain pump going is simple with the external sidechain signal input, but the interesting part of Messor’s approach begins when utilizing the internal envelope follower and sidechain filter. Being able to precisely dial in which frequencies will trigger gain reduction allows for quite precise compression effects, especially within a patch that has a wide frequency range. You can avoid having certain overpowering frequencies [such as the low-end of a kick or a bass line] trigger compression.
Messor’s “warm” mode’s tube saturation emulation sounds great and having its progressive soft clipping goes a long way in adding a bit of analog warmth to the end of chain output. I basically keep it on all the time, but it is handy to be able to bypass if needed.
The Gain knob can go from silence to an additional 17db and is helpful not only for make-up gain needed due to compression, but also pushing signals into the Messor’s warming circuit or even into harsher clipping in “pure” mode. With the Gain knob fully counterclockwise the output signal is muted. Patching an envelope into the Gain CV input allows you to use Messor as a stereo VCA and you can almost get VCA like behavior out of the side chain filter as well, and throwing envelopes into the external sidechain input, the gain, and the cutoff inputs is an interesting space to explore.
Messor demonstrates Cosmotronic’s clean and attractively sleek aesthetic, and the build quality is top notch with great feeling knobs and solid, smooth pots. The LED rings around both the Gain and Threshold knobs not only provide welcome visual feedback but also give the module a great visual aesthetic, and despite its compact width, Messor maintains great ergonomics and excellent panel legibility. Having a Eurorack compressor with CV control over many of its parameters opens up a lot of potential for creative sound design, timbral exploration [and devastation] and I love the ability to filter out frequencies for precise sidechain control. If you’re looking for versatile dynamics control in your case, then Messor is a module definitely worth checking out.

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