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This quality analogue processor offers precise and intuitive control over your compression.

Founded in 2004, Oliver Gregor’s Dusseldorf‑based Rockruepel Pro Audio now offer three products: the Comp.two, which is a high‑end, dual‑mono/stereo vari‑mu compressor; the, a stereo/dual‑mono analogue mastering limiter; and, launched fairly recently, the The last, which is slightly more wallet‑friendly than the others, is a mono, analogue VCA compressor, and is named for its interesting side‑chain filter, of which more below. In developing the, Gregor worked closely with two Grammy‑nominated mastering engineers, Germany’s Stefan Heger and Las Vegas‑based Luca Pretolesi, whose personal contribution to the project is acknowledged by the presence of his StudioDMI logo on the’s front panel.

Freq Out

As is the case for all Rockruepel products, the is designed and manufactured in Germany. Its high quality is evident throughout — in the mechanical design, hardware, and electronic components, of course, but also in the fit and finish of its physical construction, both internal and external. It all inspires precisely the sort of confidence one should expect of a device in this price bracket.

The most obvious clues to the’s headline functionality are two horizontally‑mounted faders on the left‑hand side of its front panel. The upper fader sets the corner frequency of the side‑chain high‑cut (low‑pass) filter, and the lower one sets that of its low‑cut (high‑pass) filter. Each is a 12dB/octave filter, and its frequency range can be switched to any one of three bands, labelled High (1.5 to 20 kHz), Mid (150Hz to 2.3kHz), and Low (20 to 200 Hz). A pair of tri‑colour LEDs indicates which band is currently selected for each filter. This dual‑filter arrangement obviously gives you the option of applying variable high‑pass, low‑pass or (when both faders are in play) band‑pass filters, but also means the faders can be set very precisely around the desired frequency.

The two filters, which can either be used on their own, or to shape the compressor’s control signal, each have a fader and a switch to focus the fader’s range in a specific part of the frequency spectrum.The two filters, which can either be used on their own, or to shape the compressor’s control signal, each have a fader and a switch to focus the fader’s range in a specific part of the frequency spectrum.

The remaining four switches, each with ‘illuminate when active’ LEDs, form a line at the bottom of the front panel. The first takes the in and out of hard bypass, and the second activates the Listen function. This routes the side‑chain signal directly to the outputs, which obviously allows you to monitor the filtered side‑chain signal being sent to the compressor, but also means the can be used as a high‑quality high‑pass/ low‑pass/ band‑pass filter. If side‑chain filtering is not being applied, it also offers a means of comparing the compressed and uncompressed signals.

Next comes the Link switch, which connects two compressors for stereo operation. Once linked, whenever the threshold of either unit is exceeded, both channels will be compressed simultaneously. The compressor and side‑chain filter controls are not linked and therefore should normally be set to identical values to avoid anomalies in the stereo soundstage. Completing the row of four is the three‑position toggle switch that marks the start of the compressor control section, and selects between three compression ratios (2:1, 4:1 and 10:1). Its orange LED illuminates only when compression is actually taking place.

A trio of chunky, ribbed, circular knobs set the threshold (+15 to ‑44dB), attack (0.1 to 120 ms) and release (0.1 to 1.2 seconds). A fourth such knob sets the output gain (0 to 15 dB), allowing you to compensate for any drop in level due to the action of the compressor.

The sparsely populated rear panel carries the balanced XLR audio in/out connectors, the Link function’s quarter‑inch TS jack socket and the four‑pin locking XLR socket that connects to the unit’s outboard switch‑mode power supply. Presumably due to the generous side‑chain filtering on the front, there’s no provision here for an external side‑chain input or side‑chain insert point.

Two units can be linked for stereo operation, using a TS jack socket on the rear.Two units can be linked for stereo operation, using a TS jack socket on the rear.

More Flow, Less Work

Sonically, the delivers a clean, neutral and uncoloured musical performance that doesn’t impinge on or change the character of the source audio. The characteristics of its compression and the level of control available over the threshold and attack and release speeds are as you’d expect to find in a high‑end, modern, feed‑forward VCA compressor.

In a situation in which a low frequency is triggering the compressor and causing unwanted overall compression, a high‑pass side‑chain filter that offers fixed frequencies in that area (60, 125 and 250 Hz are pretty common values) can help prevent the bass and low midrange component of kick and snare drums, basses and large‑bodied acoustic guitars from dominating the compression. However, filtering out those frequencies in the side‑chain can do little to prevent, for example, the mid to upper midrange components of a snare drum, or the higher frequencies of cymbal splashes, sibilants and acoustic guitarists’ finger squeaks from causing problems.

The’s extremely intuitive, continuously variable, yet precise approach to side‑chain filtering enables you (to continue the kick and snare example) to high‑pass the kick and snare in the low band to prevent unwanted compression, while also assigning the low‑pass filter to the mid band to create a band‑pass filter, which could for example be used to put the emphasis on the snare’s mid to upper midrange frequencies. While the ability to focus compression on a specific frequency bandwidth allows you to control wayward frequencies, it also affords you an opportunity to explore the more creative aspects of midrange compression. For instance, you can use compression and make‑up gain to bring the midrange content of a signal more to the front, or push it backwards, whilst leaving its treble and bass relatively uncompressed and able to breathe.

When I first began working with the I didn’t quite ‘get’ it, and that ‘aha moment’ arrived only once I’d studied some of Luca Pretolesi’s many YouTube videos.

Good as it obviously is, when I first began working with the I didn’t quite ‘get’ it, and that ‘aha moment’ arrived only once I’d studied some of Luca Pretolesi’s many YouTube videos, which gave me a much better understanding of his approach, the equipment he uses and how he uses it. Pretolesi is known, amongst other things, for his use of midrange and Mid‑Sides compression, and a video of his mastering setup (as at June 2022: demonstrates the extent to which the has been influenced by his “hybrid digital, analogue, in‑the‑box and summing” mixing and mastering workflow. Check out the video, and you’ll notice that a pair of Sidechain.ones is connected, via a Dangerous Music Liaison switching system, to an SPL Hermes mastering router, which can route a dual‑channel/stereo audio signal through a chain of up to eight hardware processors. The Hermes can also assign single processors through either of its two parallel mix stages to allow comparison of, for example, the effects of two different compressors. Also present is an analogue SPL Gemini M‑S encoder/decoder that can be inserted by the Hermes directly into a position in the processing chain. In essence, this configuration allows very flexible push‑button assignment of Pretolesi’s pair as an insert or parallel compressor on the master stereo bus for either stereo or M‑S compression.

Having understood Pretolesi’s signal path and workflow, the’s role in his very personal mastering process really began to make sense. His use of midrange compression to move the centre‑panned elements of a track forward or back, and his utilisation of M‑S based processing to widen or narrow a stereo sound stage by altering the relative Mid‑Sides levels, and/or adjust the dynamics of the Mid signal without affecting the dynamics of the Sides (and vice versa) is creative, highly‑skilled, and often impressively subtle.


The is a lovely device. To me, first and foremost, it is a mastering‑grade compressor, whose intuitive side‑chain filter implementation allows quick but surgically precise control over which frequencies do or don’t trigger compression. A dual‑mono pair, linked or unlinked, could be used to great advantage on the stereo output bus of, for example, an analogue summing mixer or, for more detailed enhancement and adjustment, on a M‑S encoded version of that bus. To make the most effective use of it, though, you’d need a patchbay or routing matrix to combine it efficiently in series or parallel with other hardware.

Personally speaking, I’m drawn to outboard equipment that leads me to explore new workflows, and the Rockruepel does that. If I earned my keep mastering tracks in the analogue domain, as Pretolesi does, a pair of these would be high up on my list of potential purchases, and there’s a very high probability that the review pair would already be sitting permanently in my rack!  


  • Mastering‑grade audio performance.
  • Equally high‑quality construction.
  • Precise, versatile and intuitive side‑chain filtering.
  • In Listen mode it doubles up as a high‑quality ‘bracket’ filter.


  • Side‑chain insert and wet/dry control would extend its versatility as a standalone unit.


This single‑channel mastering‑grade compressor has an intuitive high‑pass/ band‑pass/ low‑pass side‑chain filter, which that allows the precise tailoring of the frequencies which trigger compression. Best used in pairs!


£1649 each or £3199 per pair. Prices include VAT.

MasteringWorks GmbH +49 2236 393731.


MasteringWorks GmbH +49 2236 393731.