Kjaerhus MPL-1 Pro SE

Do you remember that really cool Golden Compressor GCO-1 plug-in that Craig Anderton reviewed in the February 2005 issue of EQ? Well, the company that created that sweet little product has put something new on the market—the MPL-1 Pro SE. In short, it’s a cross-platform plug-in that plays nice with both Windows and Mac operating systems, is available in VST, RTAS, and AU formats, and is targeted for mixing and mastering applications. Interested? Sure you are. And we got the intimate details for you, too.


There are a few features that make this limiter unique, with the most significant being Kjaerhus’ over-sampled peak detecting algorithm (Figure 1). This new approach ensures that high frequency peaks are more accurately detected, resulting in both better high-frequency performance and elimination of output clipping. Another highlight is the stereo linking function, which is a kind of stereo/not stereo/stereo again deal. When enabled, the linking code allows peaks to be limited individually on each channel, while longer-lasting gain reductions remain linked. The benefit in the real world is that peaks in one channel do not produce artifacts in the other channel (“not stereo”) while the average compression still remains the same in each channel to avoid disturbances in the stereo balance (“stereo again”). Other nice-to-have features with the MPL-1 Pro SE include silent knobs, A/B comparisons, and parameter control via a “MIDI learn” function.

The GUI for the MPL-1 Pro SE has a clean, streamlined feel. Controls are neatly spaced, while values and meters are easy to read. There are four stereo meters (with peak-hold and AES 17 compliant RMS output), and the entire window takes up just enough space to get the job done. This is worth mentioning as lately I’ve encountered what seems a disproportionate amount of plug-ins that seem to take up too much or too little screen real estate. I guess we must be in a state of flux when it comes to common display resolutions, but it can make day-to-day work a pain.


I tried the VST version of this plug-in in mixing and mastering settings with Magix Sequoia. In general, the MPL-1 Pro SE exemplifies how much better limiter plug-ins are getting. Translation: The plug-in imparts little, if any, coloration to the sound.

I was completely sold on the MPL-1 Pro SE as a mixing limiter, as I found it to be unobtrusive on almost any instrument I threw at it. On electric bass guitar, the plug-in was able to tighten up an inconsistent performance on a rock track with ease. On piano it stayed out of the way until the occasional peak tried to sneak through. On drum overheads, the MPL-1 Pro SE retained much of the space and room sound while refusing to be pushed around when limiting time came. I suspect this is a direct result of Kjaerhus’ research and implementation of the MPL-1 Pro SE’s peak detection algorithm. To test my hunch, I tried similar settings using another popular plug-in limiter. While the results were similar (the benchmark plug-in utilizes a well-conceived auto release control), the MPL-1 Pro SE was just a touch more open and less “in the signal chain.” And even with this strong performance, the plug-in was more efficient in terms of CPU usage than most of the limiter plug-ins I use. It would not be out of the question to use a dozen or more mono or stereo instances and put little strain on today’s computer systems.

For mastering, the plug-in worked best when each setting was evaluated for each track (meaning preset settings should not be trusted blindly). At light settings (threshold of –1 and ratio of 1.5 to 1), the MPL-1 Pro SE is clear, and, at times, difficult to hear working—and this is good! At these values, the compressor should just smooth out the sound. In terms of the stereo linking function, I found it best to leave the feature off, or to fine-tune the stereo link milliseconds value to the given music. To my ears, using too short a release seemed to pinch the natural width of most tracks, resulting in a tighter, but more mono feel. As any mixing engineer probably spent a lot of time setting up the left to right landscape (at least I would hope so), I don’t think it would be a good idea to destroy that work without good reason. However, finding the correct link time avoided this problem.

Another parameter to keep an ear on is the program dependent release (PDR) feature. I’ve always been critical of any “automatic” compressor setting in a mastering situation, and the same holds true with the MPL-1 Pro SE. In short, this PDR varies the release time depending on the dynamics of the piece. When applied to a single instrument (say in mixing), this can be great. For example, the PDR function used on drums could decrease the release time after a dynamic snare hit, but increase the release during a busier segment. To my senses, this adjustable release can increase apparent loudness while avoiding pumping. [Note: When dealing with a full mix, I caution users to choose an appropriate release value or do several tests with the PDR before trusting it.]


The only limitation of the MPL-1 Pro SE, as far as I can tell, is that it isn’t as invisible or transparent as flagship digital limiters from TC Electronic’s System 6000 or Weiss—but one could hardly expect it to be. The over-sampled peak detection still places this at the top of the pile of currently available plug-in limiters. And I would definitely make this a first-call device for tracking or mixing situations. Given its efficient CPU requirements, I don’t think this should be a problem for most users.

When all is said and done, this plug-in is the crown jewel from a company that already has a slew of impressive titles in their catalog. And at this price, you can probably afford to take a chance and give it a spin. I recommend it.

PRODUCT TYPE: VST/AU/RTAS limiter plug-in.

TARGET MARKET: Recording musicians lacking a high-quality, yet affordable, plug-in limiter for mixing and mastering applications.

STRENGTHS: Clean sound. Revolutionary peak detection functions. Not a CPU hog. Cost-effective.

LIMITATIONS: Not as transparent as flagship limiters costing much more.

LIST PRICE: VST (Windows) or VST/AU (OSX) $118; VST/AU/RTAS (Windows and OSX) $148.