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SL4000 MK2 – Stereo mixbus compressor


SL4000 MK2 mix buss compressor. Nekotronics hand built, flag ship compressor. 

Built to order. Current lead time is 8-10 weeks

SKU: SLMK2 Category:
Nekotronics SL4000 MK2 compressor Blue knobs white buttons orange buttons

Nekotronics SL4000 MK2 compressor

The SL4000 MK2 is Nekotronics flagship compressor. Built to order, to your spec and customisations.

Based on the classic VCA console compressor and rack version. The SL4000 MK2 builds on this design with mods, tweaks and improvements, whilst staying faithful to the originals sound and character. The end result is flexible compressor thats happy on the mixbuss, mastering, drum buss and even some creative tracking uses.

The Sl4000 MK2 has found home in the studios of bedroom producers levelling up all the way up Grammy award winning producers and even a few producer legends. They have also been toured around the world in arenas and even has a small place in history when one was used to record the first government backed, covid free gig in the UK by The Blossoms head Live engineer.

The latest MK2 variant of the compressor is the result of years of feedback from engineers and producers using the original MK1 and famous units. I took the most requested features and built them into the compressor, pushing it past the original design and updating it for the modern user.

Quick spec info

11 attack settings (stepped control)
6 release settings (stepped control)
6 ratio settings 1.5:1, 2:1, 4:1, 6:1, 8:1, 10:1
4 HPF settings and external sidechain input
Wet dry mix control
Turbo mod
Transformer mod with choice of Carnhill neve or edcor transformers switchable in or out
Smash mod (distortion overdrive mode that can be manipulated, think smashing drum overheads or room sound)
Silver anodised & machined aluminium case
Improved power supply
Improved internal design
Choice of colours on the knob caps and buttons

Ordering & lead time

This item is made to order at a clients request. I do not often have stock ready to go. Only small numbers are allowed for each batch. As each item is handmade it can take some time before it is ready. During the build process I will provide updates and photos of the progress.

The most current lead time for orders is listed at the top of this page. If you have extra requirements or an urgent deadline please contact me to discuss.

To order please pick your colour and transformer options and check out as usual. I will contact you via email to confirm a serial number for the compressor. If you have specified mix and match colour options i will confirm them.

If you would like to discuss any further customisations or check on current delivery times please use the contact page to send me an email.

Tony Visconti (David Bowie, T Rex) with an early SL4000 MK2 in 2020

Tony Visconti Nekotronics SL4000 MK2 compressor

Backlit meter

VU meters are cool and backlit ones are even better. The meter on the compressor shows the amount of compression being applied. Fun to watch endlessly.

Attack and release controls

11 attack settings & 6 release settings on stepped switches allow for instant recall of settings and a wide breadth of control. Auto relesse is also available

Wet dry Mix

The wet dry mix control allows you to blend between the uncompressed dry signal and the compressed wet signal. Instant parreall compression.

Nekotronics SL4000 MK2 compressor Blue knobs blue red buttons

6 ratio settings

Sporting the classic 2, 4 and 10:1 ratio settings but adding 6 and 8:1 for more flexibility. 1.5:1 ratio is available for mastering purposes

High pass filter & external input

4 high pass filter (HPF) settings to sculpt the compression reaction to bass. A key feature for modern music. External input (1/4″ jack on rear) allows for sidechaining effects.

Turbo mode

Toggle Turbo mode on or off from the front panel. Turbo mode switches between a stereo or mono (summed) sidechain. The changes the way compression is calculated and how the compressor sounds and reacts. Deep subject and more info in the manual

Transformers are


Well kind of! The compressor sports a pair transformers in the output section. Transformers add a certain something to sound that is quite often hard to pinpoint and describe. However my ears very much enjoy it!

Available as an option in around 2016 the transformer mod has been very popular. So popular some other manufacturers have borrowed the idea. It aims to bring some of that transformer magic tone into the compressor signal path and your mixes.

The transformers can be switched in or out of the signal path (bypassed) from the front panel “Iron” button. They are also available when the compression is bypassed meaning you can use them for other sonic shaping duties.

When ordering you can choose between Carnhill (of Neve fame) or USA made Edcor transformers. Each transformer offers sonic qualities of their own. The Carnhills are large core and have great big bass response with subtle sparkle and presanse to the high end. The big bottom end can suit modern bass heavy music. The edcors have a more coloured sound than the Carnhills but have thicker mids & high end to my ears. It’s a tough call but pick the ones that appeal to your ears and production style.

Other options are available via special request. Please contact me to discuss.

It’s time to


Theres a kind of hidden mod called the smash!!! mod.

Move the release switch all the way to the last setting and the compressor is transformed into a distortion/overdrive box.

No compression happens in smash mode. Instead the threshold, ratio and high pass filter control the sound and tone of the smash mod. Theres a wide range of sounds available. From slight chirpy distortion to crunchy drum room mics smashing.

You can use the wet dry mix control to blend the dry signal with the distorted smash signal and add a ton of charcter to the signal.

My favourite use of this mod is to crunch up and smash drum overheads and drum loops.

Customise your compressors look

Customise your knobs

Knobs are available in a rainbow of colours, appealing to all tastes and styles.

Customise your buttons

Buttons are available in 4 different colours, white, grey, red and black.

Nekotronics SL4000 MK2 compressor knob and button choices

In more detail

Power supply

The improvements begin with the power supply, which is a superior for audio, linear style. It provides clean, low noise power with each power line separated. All the non audio parts of the circuit, leds, switching, relays, are separated to keep the “noisy” parts away from the audio goodness.


On the control front the MK2 has 6 ratio settings, including the originals famous 3 (2,4 & 10:1) and adds 1.5:1, 6:1, 8:1. The 1.5:1 was much requested by mastering engineers. Theres 11 attack settings and 6 release settings including the auto release. Smash some drums or gently compresses a mix, theres plenty of scope for control. The ratio, attack, release and high pass filter are all stepped switches for easy recall.

High Pass filter & external input

The high pass filter takes care of the bass, allowing more bass heavy music to benefit from VCA compression. It allows the bass to move through the sidechain without causing pumping effects. The classic buss compressor used to suffer somewhat with over compression and pumping on bass heavy material. This problem has increased with more bass heavy modern electronic music. The high pass filter solves this issue. The frequencies for the high pass filter are 60, 90, 130 and 200 Hz. It can also be completely bypassed. Theres also an external sidechain input on TRS 1/4″ jack on the back, allowing for sidechaining effects and tying the kick drum to the bass. Can also be used for ducking effect.

Turbo mod

The turbo mod allows you to switch the sidechain between stereo (2 sidechain VCA’s turbo mode) or mono (non turbo 1 sidechain VCA). This is a useful feature as changes the way the compressor behaves. The non turbo mode compresses the center of the stereo field and bass more, useful for drums. The Turbo mode allows more open and gentle compression which can be suited to the mixbuss or subtler compression. This feature is switchable form the front panel via the illuminated button.

Wet dry mix mod

A much requested feature is the wet dry mix knob. This allows you the blend between the dry uncompressed and the wet compressed signal. Giving you instant parallel compression and allowing control of some of the original dynamics back.

Nekotronics SL4000 inside

Hand built to order with care and passion

Hand built

Each compressor is fully hand built and tested in the UK. Nekotronics is passionate about quality workmanship and attension to detail


High quality components such as Wima poly caps, Panasonic FC “gold” elec caps, 1% tolerence resistors, THAT VCA’s, Alpha pots and lorlin switches.


Every compressor has a 24 hour monitored burn in to ensure all parts are working and stress tested. Compressors have final tests performed multiple times before shipping.

Weight 6 kg
Dimensions 60 × 35 × 37 cm
Transformer Mod

Carnhill, Edcor, Sowter

knob cap colour

Blue, Red, Orange, Black, White, Grey, Green, Yellow, Purple, Pink / Lupin, Mix & match

Button colour

Orange, Blue, Red, Mix & match