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IGS Audio Zen

Dual-channel Zener Compressor/Limiter By JG Harding
Published July 2022

IGS Audio Zen

Diode‑based compressors can be delightful but also rather noisy. True to its name, the Zen promises a more peaceful approach...

Polish company IGS, the vehicle of ‘mad scientist’ designer Igor Sobczyk, are fast becoming one of the most interesting hardware manufacturers out there. Their range includes Igor’s interpretation of various classic bits of studio outboard gear, including several types of compressor, as well as more original designs such as their Tilt N Bands equaliser. I have one of their Tubecore Mastering Edition compressors, and it’s well built, sounds great and finds its way onto plenty of my work. Based on my experience of that and the other IGS toys I’ve tried over the years, I trust this company to do a good job.

IGS have offered their V8 diode‑bridge compressors for a while now, but more recently they released the Zen. Whereas the V8s seem to find their inspiration in the classic Neve diode‑bridge designs, the Zen instead takes the Abbey Road EMI TG12413 limiter as its starting point. The TG12413 is one of three designs famously developed for Abbey Road that employ Zener‑diode limiters; the other two are the RS168 limiter and the TG12345 console channel. Its two modes (compress and limit) were reportedly developed to offer engineers a choice between the Fairchild 660 (limiter) and Altec 436 (compressor) behaviours with which they were familiar. Sadly, I’m not based at Abbey Road (I do have a scar on my arm from one of their REDD.37s but that’s a story for another day — for now let’s just say the definition of ‘portable’ has changed over the years!). However, I do own a TG12413 clone, based on the original circuit diagrams but with some custom modifications, and given my previous experiences with IGS I was really keen to see how this and the Zen would compare.

Diode In Your Arms Tonight

Before I dive into the pros and cons of the Zen itself, some general context about analogue compressors might be useful. There are various ways to design the gain‑reduction stage of an analogue compressor and each imposes certain characteristics on the source, in terms not only the gain reduction itself but also of noise levels and harmonic distortion. VCA, FET, diode‑bridge, valve (or Vari Mu, to borrow Manley’s trademark term) and optical are probably the most popular types, and the order in which I just listed them is a good rule of thumb when it comes to judging how fast they’re able to clamp down on a transient (the list runs from the fastest acting to the slowest). Diode‑based limiters and compressors sit somewhere in the middle: while they can be set up to act more slowly, they generally sound pretty fast to my ears, even if they can’t act quite as quickly as VCA or FET types can.

In terms of colourful harmonic distortion, VCA and optical circuits tend to sound pretty neutral/clean — there are characterful VCA and optical devices, but that’s usually down to things like transformers, amp stages or dedicated distortion circuits. FET‑based designs tend inherently to be more colourful. Again, a diode‑bridge compressor sits somewhere in the middle.

Compared with other approaches, then, it’s fair to say that a diode‑based compressor‑limiter is capable of reasonably speedy dynamic control while adding a pleasant touch of colour. One factor that affects the behaviour and sonic character of a diode‑bridge compressor is the type of diode used, and Zener‑based designs have a distinct sound.

A potential problem with almost all diode‑bridge designs has been noise. The audio signal must be attenuated so it hits the desired section of the diode’s non‑linear ‘transfer curve’ and must be boosted back up again, along with the noise floor. This can mean more noise than you’d like if pushing the compressor hard and applying makeup gain.

Zen & The Art Of Noise

The Zen is a dual‑channel compressor, with separate controls for each channel and a Link switch for stereo operation. Eight carefully matched diodes are used for the compression (they must always be closely matched for this design to work well) and there are 14 bipolar transistors, along with custom input and output transformers. The chunky knobs are all stepped switches, with clear labels — this will be welcome news for anyone looking to use this unit for mastering or other applications in which precision setting and/or recall are important. There’s an internal linear power supply and, on the front, a pair of nice, illuminated gain‑reduction meters. In short, the Zen ticks all of the ‘high‑quality esoteric hardware’ boxes!

The Zen is a dual‑channel device, with a separate set of controls and meter for each, but the channels can be linked for stereo operation.The Zen is a dual‑channel device, with a separate set of controls and meter for each, but the channels can be linked for stereo operation.The EMI TG12413 might have provided the inspiration, but IGS have added a fair few features to make the Zen better suited to use in a modern studio. Not only are there 12‑position attack and release time knobs, but these are joined by a switch to multiply the selected times by four. There are also four high‑pass sidechain filter frequency options. This is essentially a fixed‑threshold design, so you ‘push’ the level of the incoming signal for more compression and adjust the output level accordingly; the large 24‑position Elma‑switch input and output controls make this pretty easy.

As with the TG12413, there are two compression modes to choose from, Comp and Limit, and in both the ratio changes with the level. Comp seems to have a soft‑knee characteristic that starts with a ratio of around 2:1 but goes up pretty quickly to roughly 8:1. Limit appears to average about 12:1, again gradually ‘flattening off’ as input level increases. There’s also a third mode, called Amp, in which the gain reduction is effectively turned off, so that the unit acts as a line amp. In this mode, it’s capable of applying up to 36dB of gain.

The sound of the Zen’s compression is a rare combination: the Zener sound is pretty versatile, with a nice balance of character and cleanliness. On the one hand, I could describe it as being pretty much tonally neutral. On the other, it adds enough thickness and vibe to make it worth patching it in for the flavour alone. When dropping this compressor in on a vocal track, for example, it added a low‑mid richness, while leaving the upper frequencies sounding clear and present.

Importantly, the Zen is remarkably quiet in operation for a diode‑based design.

Importantly, the Zen is remarkably quiet in operation for a diode‑based design: it’s much less noisy than my TG clone, which, at high compression levels, can need gating. Not being an electronics engineer, I can’t tell you precisely how they’ve achieved that — but they have, and it’s really impressive!

The Comp and Limit modes are actually a little closer in character than I expected. Obviously, you do hear a difference when switching to Limit, with transients being more noticeably hammered down, but with some settings the two sounded very similar to my ears. While there are no other ratios on offer, you can select longer attack and release times and play with the x4 extender switches — not the same thing but it does grant access to gentler compression.

Sidechain high‑pass filters, which reduce a compressor’s sensitivity to the low end, are common enough, but IGS also use this switch to put the Zen into Amp mode, to enable its use as a characterful line amplifier.Sidechain high‑pass filters, which reduce a compressor’s sensitivity to the low end, are common enough, but IGS also use this switch to put the Zen into Amp mode, to enable its use as a characterful line amplifier.A sidechain high‑pass filter is very useful for any compressor that’ll be used to really hammer a source, since it reduces the sensitivity to low frequencies, which tend to be higher in level; this prevents over‑compression and the pumping effect that LF‑rich sounds like bass guitar/synth and kick drums often trigger. Most of the time I set the Zen’s filter to 120Hz, which I find is a great starting‑point setting for drums, vocals and many other tasks. The 240Hz setting was very useful too, really helping the Zen shine with certain voices, big jumbo acoustic guitars, and other devices that have plenty of content in that range. I also found some use for the two lower settings, especially for bass guitar; a 90Hz sidechain filter allows you to limit more in response to the instrument’s mids and higher frequencies.

The Amp setting, which as I said allows the unit to act as a line amp, is accessed using the high‑pass switches, and it’s a nice feature. Often known as a soft‑bypass mode, the Zen isn’t compressing when this is engaged, but as you crank the input levels the amount of harmonic distortion from the amplifier circuitry and output transformers increases. It’s a pleasing effect, though obviously you might need to turn down the output, or the input gain on your interface, to avoid clipping your A‑D converters. (I use the TC Electronic Level Pilot as a simple and inexpensive way to attenuate a balanced stereo line signal.)

Some tracks really suit more of a ‘limiter’ or heavier compression sound, and when this is called for it’s there with bells on. Hammering a drum bus with the Zen is a joy: you get all kinds of pumping and swing and it sounds phenomenal. And if you’re a fan of mix‑bus ‘glue’ of the SSL VCA variety in its higher ratio modes but crave something a little less ‘snappy’, the Zen is a great option too. It allows you to get things pumping should you want to but you can also carefully balance your attack and release, input level and sidechain filter controls to keep the Zen riding some continuous gain reduction too. While the Zen can perform most tasks required of a good hardware compressor in a modern studio, though, there’s an argument that it would be even better if it were joined in your studio by a second unit that is designed primarily to be slow‑acting and offer low ratios. If you’re looking for a very transparent and benign soft, riding compressor, you can create a facsimile using longer attack and release settings, but it’d probably be better to pick something intended for that task, rather than attempt to ‘hack’ the Zen. IGS’s own V8 diode‑bridge compressor and Tubecore are both adept at this sort of subtle ‘mix riding’, for example.

I do wonder if there might be scope to add a lower ratio setting to a future version of the Zen, so as to make it more versatile. Another feature that I feel is ‘missing’ on pretty much every TG12413 clone I’ve seen is a wet/dry mix control. It’s something I do a lot using my own clone with Studio One’s Pipeline plug‑in: hit the compressor hard and set the mix to between 50 and 70 percent, to achieve the desired balance of the limited sound and the dynamics of the untreated signal. I prefer to have that control on the hardware’s front panel, though, and think a mix knob would be a great addition here.

The Zen features an internal linear power supply, which draws mains power through an IEC inlet on the rear, and can be switched to work with US or European voltages.The Zen features an internal linear power supply, which draws mains power through an IEC inlet on the rear, and can be switched to work with US or European voltages.

Diode To Joy?

Reviewing equipment of this quality is always a pleasure, and you can tell real love, care and attention has been put into every aspect of this design. It can sound wonderful on a range of sources. For vocals, once you get the hang of setting your attack and release correctly, you’ll find it imparts a certain warmth while remaining somehow ‘true to the source’ and keeping dynamics nicely controlled. This was true both of voiceover and singing: both just sounded nicer with the Zen in place. It can sound lovely on all the other usual suspects too: bass, guitars, keyboards and so on. As a drum‑bus compressor the Zen is great, even if as a mix‑bus compressor it’s more of a mixed bag — its suitability in that role really depends on the genre and intended result.

Comparing the Zen directly with my TG12413 clone, they imparted a similar character but the IGS unit sounded a lot cleaner in the top end and had massively lower noise, even when really cranking things, and this makes it much more useful in a modern production context: the Zen is an all‑round improvement on my ‘straight clone’ unit.

There’s no compressor that will do everything but, while a subtler ratio option and wet/dry knob would be nice additions, this one can do most of it. And although professional tools of this quality inevitably cost a good chunk, for two channels of this sort of compressor the Zen is very competitively priced. It also looks great, is well built, has a wonderfully ‘knobby’ feel and can be great fun and inspirational to use. In fact, if I were cursed by the gear‑acquisition genie to choose only one hardware compressor to use for the rest of my life, I could do a lot worse than pick the Zen.  


Chandler’s Abbey Road‑endorsed EMI TG12413 Zener Limiter is the most obvious competitor. It’s the original ‘amped up’ TG12413 device with some useful extra settings, but it costs considerably more than the Zen. It’s possible to source custom‑made or DIY clones, too, for rather less, the price depending on the builder and the options you choose. In software, the official Chandler Zener Limiter by Softube is a lauded emulation of the Zener limiter available for UAD and native platforms. Acustica Audio’s Coffee and Waves’ Abbey Road Collection are also worth taking for a test drive. Other (non‑Zener) diode‑bridge compressors include Rupert Neve Designs’ 5452 and 535, Buzz Audio’s DBC‑M, Heritage Audio’s Successor and Audio Scape’s D‑Comp Limiter, as well as IGS’s own V8 models.


  • Versatile, top‑quality compression and limiting.
  • Great build quality, with switched settings.
  • Pleasing combination of clarity and great character.
  • Solves the ‘noisy diode’ issue.
  • THD line‑amp mode a nice bonus.


  • An additional, subtler ratio setting and wet/dry mix knob would be nice.


The Zen updates a classic EMI design nicely. It’s versatile, cleanish yet characterful, versatile and fun enough to provide some inspiration. Like all IGS gear, it’s attractively priced given the quality on offer.


£2499 including VAT.

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