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Stereo EQ & VCA Compressor By Rory Dow
Published February 2023


Billed as an instant mastering device for on‑stage use, the DOCtron IMC (Instant Mastering Chain) could well be an entirely new class of audio processor.

The DOCtron IMC is a collaboration between German manufacturers DOCtron and electronic musician and YouTuber Martin Stimming. Essentially, it’s a high‑quality stereo mastering chain in a portable package — it measures 15 x 15 x 6cm. The reason it’s so compact is that it’s designed primarily for live musicians (although it will work just as well in the studio). Imagine you are performing live with electronic equipment; maybe some modular, a drum machine and a synth or two. Your set follows directly after a DJ who has been playing finished, mastered material. Your setup is unlikely to have the loudness to compete. Adding that ‘mastering sheen’ to a live performance can be difficult. Do you choose a laptop running plug‑ins, which adds latency and unnecessary complexity? Or do you go for rack equipment — compressors, EQ, limiters, and so on — which are bulky, heavy and require multiple power sources? The IMC (Instant Mastering Chain) aims to solve this problem.

You’ll probably already have noticed that the diminutive size of the box doesn’t translate to the price. That’s because this box is full of no‑compromise electronics, including Lundahl transformers, DOA op‑amps and THAT VCAs. All of which help to make it sound fantastic: the ‘mastering’ part of its name is not a lazy marketing description.

Watch our SOS video chatting to Martin Stimming about the DOCtron IMC at Superbooth 2022.


The IMC weighs less than a kilogram and is supplied in a tough ‘road‑proof’ carbon case. The power supply is separate, which I know some dislike, but it’s perfectly understandable for a small box like this, and it comes with a soft carry case that should prevent it from getting scratched in transit.

Audio comes in and out via pairs of balanced quarter‑inch input and output jacks (apparently, some early units were unbalanced). The EQ comes first in the signal chain and offers low and high shelves. The circuitry is based on a Trident EQ design and does an exceptional job of teasing out the sub and adding air. Each of the shelves can add or remove up to 14dB of gain, which is far more than you might expect from a mastering chain but can be great when using the IMC in the studio. The EQ can be bypassed using one of the three illuminated buttons on the front panel, which is handy for quick comparisons.

On the rear of the unit, you’ll find an on/off switch, 24V DC power supply input, quarter‑inch balanced inputs and outputs, quarter‑inch side‑chain send and return jacks, and a mini‑jack headphone socket.On the rear of the unit, you’ll find an on/off switch, 24V DC power supply input, quarter‑inch balanced inputs and outputs, quarter‑inch side‑chain send and return jacks, and a mini‑jack headphone socket.

The EQ’ed signal then moves through a discrete op‑amp for the purpose of lowering the gain before the solid‑state VCA compressor. The gain stage allows you to put the signal in the compressor’s sweet spot, and compensate for any gain that might have been applied when EQ’ing. For the compressor, you can adjust the threshold, ratio, and attack and release times. If you’ve ever used the classic SSL mix bus compressor or one of its many clones, you’ll be right at home with the ratio, attack and release options on offer here.

For further flavour, the compressor can be switched between feedback and feed‑forward modes. Feedback is the ‘normal’ option for an SSL mix bus compressor, but both have a distinct sound and offering both styles is a nice touch. Side‑chain send/return jacks on the rear allow you to process the compressor’s control signal, and the return jack acts as a key input if not using the send.

After the compressor, the signal hits the output transformers, which is where a lot of the ‘magic’ happens. You can adjust the gain of the signal going into the transformers, and higher levels cause the signal to be overdriven in a very pleasing manner. The front‑panel control goes from ‘off’ to ‘mad’ to ‘nuts?’. I’m not quite sure why the last is a question... Essentially, anything in the mad‑to‑nuts range causes noticeable distortion, which isn’t what you’re looking for when mastering but could well be useful in the studio. Setting the transformer gain control more conservatively, though, results in added harmonics, saturation and analogue limiting. A final output attenuator allows you to compensate for any level added when driving the transformer, and make the level suitable for whatever mixer you’re plugging the IMC into.

In Use

Before we move on, I want to leave you in no doubt about the most important thing: the DOCtron IMC sounds fantastic. The combination of high‑end circuitry, EQ, compression and those lovely Lundahl transformers means that any unmastered audio you put through it will instantly sound warmer and more coherent. It’s great at adding sub, glue, and air, as well as giving you a louder signal without increasing peak levels.


Martin Stimming’s original concept of a portable mastering device is an interesting one. Electronic musicians often tour with a minimal amount of gear and, crucially, without a laptop (at least on stage). A single musician can do an entire set with equipment that fits in a backpack, and the IMC is designed with that person in mind. This level of portability is a relatively new thing. Samplers, synths and drum machines have got progressively smaller over the last couple of decades, but while high‑quality mastering‑grade processors have become available in slightly smaller packages (eg. the 500 series) they haven’t really followed the same trend.

In a box that weighs no more than a kilogram, you get just enough circuitry to turn a completely unmastered signal into something that sounds ‘like a record’.

Overall, the IMC nails its goal. In a box that weighs no more than a kilogram, you get just enough circuitry to turn a completely unmastered signal into something that sounds ‘like a record’. Most importantly for the gigging musician, it will mean your live set can follow a DJ and compete in volume. And one design feature I particularly like, given the intended use, is the inclusion of true bypass: if someone trips over the power cable on stage and pulls it from the back of the unit, you’ll still have audio passing from input to output.

In fact, I can only come up with one small gripe with the IMC, which is the choice of a mini‑jack for the headphone output. A headphone output is important on stage for cueing up before you let things loose over the PA, but mini‑jacks are flimsy and don’t always stand the test of time, and many professional headphone models don’t come with mini‑jack connectors, meaning you’ll have to use an adaptor. Some people might also find the lack of any metering offputting. While I rather like using my ears to find the right settings, anyone trying to set up a gig in a noisy environment might appreciate some visual feedback for confirmation that the compressor is active. That said, one of the buttons will flash to indicate overload, so that’s something.


One could argue that a lot of that lovely signal processing — all that extra clarity, glue and detail — might be lost on a typical PA system. The IMC is not cheap, and there are portable boxes costing a fraction of the price that could be considered alternatives. I’m thinking of the Elektron Analog Heat, for example, or the OTO Boum. But good as they are, neither of those boxes could be considered ‘mastering grade’ — I have a Boum in the studio and love it, but it’s not usually the right choice for two‑track pre‑masters. The IMC, on the other hand, can take your pre‑master and give it precisely the spit and polish that it deserves; it really is a ‘make everything sound better’ box.

Another alternative would be one or two pieces of rack equipment or some 500‑series modules. But while the number of compressors, EQs, and overdrive units available in these formats is dizzying, you’d lose the IMC’s main selling point: its convenient portability. In the end, the IMC represents something you can’t get anywhere else. So if you’re in the market for a portable, beautiful‑sounding, instant mastering chain, the DOCtron IMC is both an excellent solution and potentially the only one. It commands a premium price, as you’d expect a product of this quality, if not size, to do. But offsetting that to some extent is the fact that you will end up using it in the studio too — I’ve become very fond of its sound during the review period.  


The DOCtron IMC might just be the only portable box capable of high‑grade mastering. It won’t do it all, and it won’t replace traditional mastering equipment, but for anyone playing live, it provides a unique tool for instant ‘record‑ready’ polish.


€3326.72 including VAT.

€2,772.27 (about $2900).