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Bellos Audio X4: Goldilocks 

My introduction to the Belonozkho brothers, Vlad and Bogdan, was relatively recent, with the privilege of reviewing the widely-admired Xenon 6, a product of their FIR Audio brand, just over a year ago. 

What I didn’t know was that, in parallel, the brothers had also started a second, smaller company, Bellos Audio, focused solely on finding new and better ways to create high-performance custom in-ear monitors for stage performers and engineers, without the usual complexity, lead times, and crazy sticker prices.

The result is the X-series, a family of IEMs that boasts several ‘firsts’ in terms of technology and design. But what I initially dismissed as strictly pro-market products turns out is much more than that; especially the subject of this review, the series flagship X4. 

There’s no question the blueprint for X4 – and the other X-series IEMs from what I gather – was motivated by stage and mixing room requirements: robustness, easy maintenance, streamlined production, custom fit. 

But, in the case of X4 at least, the tuning of this IEM is what made me stop and take notice. In fact, everything else I just mentioned is secondary to what I now consider to be one of the most pleasantly tuned and technically excellent and exceptionally comfortable IEMs I’ve heard in a while, especially at this price tier.

Introducing X4

Sporting a four-driver, four-way crossover configuration, the Bellos Audio X4 is a hybrid monitor in more ways that one. It uses a new 6mm dynamic driver for bass, twin balanced armature drivers for lows through highs, and a super tweeter balanced armature driver for ultra-highs. 

If you look closely at the nozzle opening, you’ll see the small, silver tweeter peeking through, and while this puts it very close to your ear, I’m told there’s little to no risk of wax and debris affecting the tweeter since the IEM itself doesn’t reach deep enough into the ear cavity. Still, I suggest X4 owners be fastidious when it comes to keeping the nozzle opening clean with the included brush tool.   

X4 features one of the more unique innovations I’ve seen in IEM design so far: a hybrid custom/universal fit, based on a 3D-printed custom shell with universal-like silicone eartips in place of the usual extended nozzle. 

Called In-Air Canals (these in-‘ear’ nozzles don’t actually go into your ears, hence ‘air canals’), it’s a counterintuitive design choice that works surprisingly well. So well, in fact, that I wonder why no-one’s ever thought of it before. 

The basic design of custom in-ears hasn’t really changed that much over the years, dominated by resin shells and their long, snakelike nozzles perfectly matched to the inside of our ears. It’s a solid design too, isolating stage performers – for whom custom in-ears were first devised – from external noise, and getting the sound delivered as close to their eardrums as possible. X4 doesn’t do away with this concept entirely, but tweaks it to very good effect. 

One of the downsides of long custom nozzles is the pressure they can often exert on the inside ear canals, which change size and shape when performers, especially vocalists, open their mouths to sing or talk on stage. This can make them moderately uncomfortable to use for extended periods, but even if comfort isn’t compromised, the sound can be. That’s because the change in canal shape can also break the seal, resulting in loss of bass, which throws off feedback accuracy for the performer. 

In-Air Canals eliminate the twin issues of in-use discomfort and bass loss, replacing the physical nozzle with a flexible silicone tip that not only seals at the canal entrance, but also keeps the seal intact, regardless of jaw movement. 

But it’s the unintended consequences of this cleverly unobtrusive tweak that make X4 a game-changer, in my opinion, even more so for audiophile enthusiasts.

First, this has to be one of, if not the most comfortable IEMs I’ve had the pleasure of using for longer sessions. The combination of the custom outer-ear mould with the ultrasoft, springy, shallow-fit tip, makes it literally disappear in my ears for hours of pain-free, adjustment-free use. 

Second, since the custom part of the shell provides part of the seal on its own, you just need to pick one of three different-sized tips to complete the seal, with minimal pressure on your inner ears. Isolation might not be quite as good as a physical resin nozzle, but the seal is almost impossible to break, at least in my experience. 

Third, and probably most importantly, because there’s no nozzle blocking your ear canal, there’s a much larger air volume inside your ears for the sound to travel through and resonate. You’d think this might adversely affect sound quality, but on the contrary, I feel it improves it, especially in unison with how Bellos tuned the X4. 

I’ll cover this in more detail in the sound impressions section, but to me the X4 sound is more open, airy and natural, but also more relaxed. Mind you, as enjoyable as this for my preferences, it might not be as agreeable for everyone, especially if you’re after a more lively, intense or tight-sounding IEM.

But I digress. X4, with its In-Ear Canals, has at the very least changed my opinion of traditional custom IEMs, ostensibly for the better. Time will tell if the concept catches on and becomes more widespread beyond this small Nashville-based company and its debut monitors.  

Packaging, design and fit 

Practical is how I’d describe X4’s packaging and accessory set. This is not a luxury presentation aimed at audiophile collectors; it’s a neat, attractive, compact package that’s a step above the cardboard-box-and-nothing-else approach of some pro gear, but not quite up there with the fancy all-frills decorum of consumer-oriented IEMs.  

What you get is a small box with a slide-out, silkscreened sleeve, containing a solid metal puck case with the IEMs, cable, replacement tips and cleaning tool inside. The case has your name (or whatever you asked for at checkout) etched onto the lid, a very nice touch, and something I’ve only previously experienced with FIR’s flagship customs.

X4’s stock cable is an 8-wire ‘Scorpion’ cable that’s supplied with some FIR IEMs (mine even has the FIR Audio branding), which is to say it’s a very respectable cable with a clean, clear sound profile. Although X4 uses the hardier RCX connector as standard, you can ask for a 2-pin connector instead, which makes aftermarket cable rolling much easier. As a bonus, you get the same Rigid 2-pin connector, rated for 1,000 connections, used in FIR’s Frontier Series IEMs, so there’s no compromise in quality. 

The three In-Air Canal tips supplied are compatible with Samsung’s Galaxy Buds 2/Pro TWS IEMs, so replacing them or getting spares is cheap and easy. It also means you can use aftermarket tips designed for the Samsung Buds, like Sedna’s AZLA Max, although I haven’t tried these myself to see how they fit and sound. 

Design and fit is where this IEM really stands out for me, however. The X4 shells are precision-3D printed using your custom ear moulds (or digital STL files in my case), and finished in a textured matte buff with a very pleasant skin feel. Keep in mind the matte finish isn’t quite as smooth and polished as that of the FIR customs, but has a more rustic ‘hammered’ look instead. 

X4 is available in three different colours (black, grey and army green), each with the silver Bellos Audio butterfly logo, so there’s less choice paralysis when it comes to picking one you like, at least compared to choosing from a gallery of custom faceplates and finishes.   

Speaking of faceplates, the X4 faceplates are attached with three countersunk steel screws, making them easy to remove for quick maintenance. Ease of serviceability is one of the design goals of the X-series, which makes sense for stage performers on the go, but also has practical benefits for audiophiles wanting a hardy, robust IEM that can take the punishment of everyday use in any location. 

Even the driver stack is pre-assembled and inserted into the housing before the QC phase, which makes any issues easily and consistently repairable over time. How often have you heard of IEM’s that ‘go bad’ and then take weeks if not months to fix. Not so with X4, which is quick swap-out. The robustness of the design makes me think these will take quite some beating before they fail, if they ever fail, which makes them even more useful as an everyday indoor/outdoor daily carry for me. 

Last but not least, comfort. I’ve already mentioned how comfortable I find these IEMs, so I won’t labour on that for too long here. Only to say that, once you’ve tried out a set of In-Air Canal customs, you might find your other IEMs suddenly fitting less than optimally, and missing that utterly unobtrusive feeling you get with these. 

Comfort extends to listening too, with a built-in Atom module effectively equalising in-ear pressure, which relieves ear fatigue for long listening sessions. The Atom module is rated at -17dB isolation (equivalent to FIR’s Gold Atom module), which trades sealed custom-like isolation for improved comfort.  

Continue to sound impressions…



Picture of Guy Lerner

Guy Lerner

An avid photographer and writer 'in real life', Guy's passion for music and technology created the perfect storm for his love of portable audio. When he's not playing with the latest and greatest head-fi gear, he prefers to spend time away from the hobby with his two (almost) grown kids and wife in the breathtaking city of Cape Town, and traveling around his native South Africa.


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