Ollo Audio S4X

Sound Section.

Prelude – Concept of Reference, Studio monitors and Flat Frequency Response Calibration.

As most of you may know, musicians, producers and audio engineers like to use studio monitors with a frequency response as close to flat in their studios. The main problem one faces when using studio monitors in a room is generally the problematic acoustics of the room heavily influencing the studio monitor’s sound. So, professional studios measure their rooms, design, construct and acoustically treat their monitoring rooms to sound as flat and neutral as possible. There are a lot of techniques and solutions to solve room acoustic problems, like basic construction of the room, use of absorbers, diffusors, bass traps, etc. But even after all that, studio monitors might still not sound perfectly flat, which can be because of the studio monitor’s own frequency response or leftover influence of the room even after all the treatment.

That’s where calibration softwares like Sonarworks Reference 4, IK Multimedia ARC System 3 and DIRAC Live come in. They come with measurement microphones and software which helps you measure your studio monitors at various points in the room and then show you the frequency response of how your studio monitors sound in your room. They then enable you to compensate the measured FR of your studio monitors for proper flat frequency response. Check them out if you want to know more about them.

I personally use Sonarworks Reference 4 Studio Edition in my production room to calibrate my studio monitors for absolute flat response. The Headphone Edition of the software has calibration profiles for a ton of popular headphones and Sonarworks have their own target curve that they use for calibration, which is actually quite close to the Harman Target but with a lower bass shelf and a tiny bit more upper treble. So, loosely their idea of flat response in headphones is basically a slightly modified Harman Target curve, which is also their idea of how flat studio monitors in a treated room are perceived and sound to our human ears. Reference 4 works really well for my use and my flat response compensated studio monitors sound better than they ever did before in my room. I achieve my mixes which translate even better, way quicker now.

Diffuse Field (DF) and Harman Target (HT) are two popular reference target curves used by manufacturers around the world for tuning or as compensation in their graph measurements. I personally quite like both for proper reference tuning; Harman Target a bit more for the bass shelf, which makes mixing bass easier for me personally. So, if I’m looking for a proper ‘reference class headphone’, I’d want it tuned close to or in the ballpark of one of these curves to properly translate the music in true reference style.

Ollo S4X & UAD

Ollo Audio S4X as a reference headphone.

Now coming back to the S4X, it isn’t tuned to follow any of the theoretical target curves like Diffuse Field or Harman Target per se, but is designed and tuned to measure as close to a flat line as possible on their G.R.A.S. 60318-1 setup, which as I explained earlier, does not show the ear/pinna gain peak because of it using an ERP coupler instead of a DRP coupler. So loosely, a headphone measuring close to flat on an ERP setup will be closer to the Diffuse Field than the Harman Target (which has a 7-8dB bass shelf at 100Hz).

S4X does measure very close to a flat line but with a minor low-shelf of around 4dBs around 600Hz and one prominent lower treble dip in the 6-9kHz range. I don’t know why they have a dip at 3kHz in their graphs because S4X in fact has very good ear/pinna gain, quite in line with the Harman Target style upper-mids peak and gain.

Sound Analysis – By now you all might eagerly be wanting to know if S4X really does sound ‘brutally honest and neutral’ as Ollo Audio markets it to be, or if it is ‘musical’ enough to enjoy music, or if it really can work as a ‘true reference’ grade headphone or not. The short answer is – Yes, it does and can, but it can do much more than just work as a reference headphone for professionals.

S4X, I would say is a ‘musical reference’, neutral and an honest headphone that first and foremost sounds tonally correct; has good instrument tonality, timbre and definition, and is mostly linear throughout its frequency response with good end to end extension.

Contrary to the belief that I see quite a lot on the internet, overly bright and thin sounding headphones aren’t what true monitoring or reference headphones are supposed to sound like if you really want them to mimic or work as a substitute for good flat response studio monitors. I mean, you can use headphones like Beyerdynamic DT770 Pro (which are quite bright with peaky treble and not the best upper midrange) to monitor if you really have to but mixing or cross referencing on such headphones can be a pain, especially if you can’t get used to its overly bright nature. Prominent treble boosting gives an illusion of more clarity, resolution and details at the cost of coming off as treble peaky, fatiguing and even intolerable for some. But frankly, that shouldn’t be the way to go about tuning a proper reference headphone.

Ideally, an honest reference headphone should in fact bring out all the details and clarity but by maintaining good balance and without over-emphasising any particular frequency range, and particularly maintaining the correct tonality and timbre of instruments. This is what the S4X basically does. I personally want reference headphones to sound as close to or at least in the ballpark of my studio monitors and not something that sounds way off or different from proper reference sound. Hence, I pick up the S4X not only to use as a reference headphone for my production work but also when I want to listen to or cross-reference my favourite/new records because of its natural tonality, transparency, balance and musicality without over-emphasising any frequency band for the same.

Let’s dive in deeper…

Ollo S4X & PRS

Bass – S4X’s sub-bass is boosted by around 3dBs above flat which kinda puts it midway between DF-neutral and Harman Target-neutral. Mid-bass is a little more prominent than sub-bass and is around 3-4dBs above neutral. I quite like the overall bass presentation because it is tight, sharp and translates the bass in songs with good neutral presence and sub-bass extension without any of them sounding too boosted or prominent. The bass character and timbre is very clean and clear which enables you to focus on the characteristic traits of different types of bass models and tones played in different songs nicely. Because of it being an open-back, the sub-bass rumble in Hans Zimmer’s ‘Why so Serious’ at 3:27 is heard more cleanly than rumbly. I personally would’ve loved if the sub-bass was boosted by at least 2-3dB more, more in line with the Harman Target curve because S4X would’ve then been able to portray sub-bass with a bit more rumble and that would’ve been quite cool for an open back headphone. But it does deserve praise for the stock tuning because a lot of other open-back headphones in this range actually have a sub-bass roll-off instead. The mid-bass dominant bass playing in Dua Lipa’s ‘Pretty Please’ comes across very nicely, with good attack, definition and punch. It is never too prominent or overpowering but is technically very slightly north of neutral than how I hear it on my monitors or DF/HT-neutral headphones/IEMs. But overall, because of S4X’s relatively neutral bass character, I actually found it quite easy to use when it came to balancing the bass in the mix while doing my rough mixes. I was never inclined to over-mix bass in my songs and bass in my rough mixes was translating quite well to other speakers without me feeling the need for a Sonarworks compensation for the S4X in order to do the same.

Mids – Mids are the name of the game here. S4X is very natural sounding, majorly thanks to how well done the midrange is. Midrange is revealing and very well layered. It has very good separation of instruments and brings out micro-details pretty well. Lower midrange is mostly linear but has very slight emphasis above neutral in the 250-600Hz (around 2dB above neutral) which adds a tiny bit of body and warmth. So, deeper vocals and snare body sound very good, which also enables you to focus and work on them a bit more precisely when working on your mixes.

But upper-midrange definitely takes the MVP title. Upper-mids have a nice accurate revealing forward presentation with good ear/pinna gain at around 3kHz, which I hear to be almost in line with Harman Target curve if I’m hearing it right, even though Ollo’s own graph shows a dip here. S4X’s upper-mids are revealing but very natural, enabling good strong definition, tonality and timbre of instruments. It is very well layered and micro-detailed without coming off as dry or too clinical. Good big rock guitars and acoustic guitars have the right tone, presence and definition. Vocal are positioned exactly how they are intended to be heard in the mix. S4X does not highlight vocals or brings them unnecessarily forward, just like what a reference headphone is supposed to do. Frankly, my own preferences in the ear/pinna gain is 2dBs or so lesser than what the Harman Target or S4X has. So, if S4X had a tiny bit less ear/pinna gain, I would’ve been even more happy personally but the stock gain is nowhere a downer by any means.

Treble – S4X’s treble can be classified as clear, detailed, layered yet smooth without any harshness or sibilance. It has good resolution and detail retrieval without being overly bright. There is a perceivable dip in 6-9kHz of lower treble which is responsible for keeping sibilance and harshness in check but can take out absolutely neutral representation of cymbals and hi-hats and the harmonic content of other instruments in this range. But to be honest, it’s not missing as prominently as I expect it to in songs, though fixing this dip will definitely make S4X’s treble response even better. It has a nice natural upper treble extension which I feel is quite in line with Harman Target’s upper treble response. This along with easier lower treble allows for a comfortable listen without fatiguing the ears as much over time. My particular S4X unit’s graph shows a minor dip around 5kHz too but I’m not really hearing it. Maybe it’s so minor that it isn’t as perceivable or maybe it’s an artefact. I cross-checked with S4X’s graph on Ollo’s website and that one didn’t have this 5kHz dip either. So, I guess we can just ignore this. With a test tone test, I could clearly hear the upper treble extending till 18.8kHz. So, either the headphone rolls off past that or my hearing does. I think it could very well be the latter. Haha. Last time I checked, I could hear till 19.2kH but nevertheless, S4X does most of the treble response in line with my preferences and I don’t have any major complaints besides that dip in lower treble, which if corrected would definitely perfect the S4X even further.


S4X is rated 32Ω but sensitivity figures are not listed on the website. It is fairly easy to drive through smartphones, laptops and SE output of DAPs, though needs a tiny bit more juice than other 30-40Ω rated closed-back headphones like AKG K371 (32Ω) and Audio Technica M50X (38Ω).

Soundstage and Imaging.

S4X being an open-back headphone, has soundstage a bit wider than most average closed back headphones and even open-backs like HD6XX but isn’t exceptionally wide as such. It is in fact wide enough, with good depth, to enable an out of head experience mostly but generally follows the mix of the song precisely. Imaging is very well done and you can pinpoint every instrument precisely, which adds on to its capabilities as a reference headphone. Again, in general brighter treble helps give an illusion of an airier, more open and wider soundstage, but the real skill is tuning a headphone to do the same without overly boosting the treble. This is where I think S4X does good, staying true to its reference nature and yet giving a decently good out of head experience for the most part for its price.


Comparisons include some cool competing reference tuned headphones around or below S4X’s asking price of €400, regardless of the type of headphone or driver technology.

AKG K371 ($149) – Price wise, this is not a fair competition but K371 is AKG’s relatively new headphone that is tuned to the Harman Target curve plus I heard Rok himself praising its tuning and value in one of the videos, so I thought it’ll be a fun comparison to see if it really competes with S4X or not. K371 is a closed-back headphone with a 50mm dynamic driver with titanium coated diaphragms. So, right off the bat, K371 can be used for monitoring purposes but S4X cannot if you are recording and monitoring in the same room. Fit wise, K371 is lighter and covers the whole ear better but the fit isn’t as snug as S4X. S4X does have a tighter clamping force than K371 but nevertheless remains comfortable for hours. S4X headband tensioner exerts a bit more force than K371’s too but overall, S4X gives me a more reassuring fit than K371. Build quality wise, S4X definitely has the upper hand with its boutique and premium design and build quality.

Sound wise, even though K371 traces the Harman Target almost perfectly, S4X does have the upper hand in a lot of things. K371 has around 4dBs more sub-bass and more neutral mid-bass presentation compared to S4X. So, you can feel the sub-bass rumble and punch better in K371 but mid-bass shines more in S4X. S4X does have the upper hand in bass precision, sharpness and speed. K371 does lower-midrange more linearly neutral throughout whereas S4X has a bit more fullness in the 250-600Hz range. Both have similar upper-midrange presentation but S4X has better resolution, clarity and instrument definition. K371 does not have a lower treble dip in the 6-9kHz range like the S4X, so it does lower treble more evenly but S4X has better upper treble clarity, resolution and extension. S4X is also more open sounding owing to its open-back design and also has a bigger soundstage in comparison.

Ollo S4X vs K371

Sennheiser HD6XX with fresh pads ($220) –
HD650/HD6XX have been long time reference sweethearts for professionals as well as audiophiles, so I think a comparison between the two is a must. Both are open-back dynamic driver headphones and even though HD6XX has an impedance rating of 300Ω, it is fairly easy to drive through DAPs but is also known to scale better with good power. Even though HD6XX’s build is not bad for its price, S4X’s boutique design and build quality seems more premium and attractive. Fit wise, HD6XX is fairly comfortable because of large ear pads that go over the ear properly but the headband can have a stronger clamping force for people with bigger heads. On the other hand, S4X’s auto headband tensioner mechanism and clamping force is more comfortable but the ear pads are smaller and do not engulf the whole ear comfortably. S4X has a much better looking and feeling cable compared to HD6XX. S4X is much easier to drive considering its lower impedance rating of 32Ω. Sound wise, running both through the 4.4mm output of Hiby R6 2020 in low gain, S4X has better sub-bass extension whereas HD6XX rolls of around 40Hz. S4X has a bit more mid-bass and lower midrange body in the 250-600Hz whereas HD6XX is more linear and neutral there. Both do upper-midrange very well but S4X has a bit more note weight and definition owing to a bit more ear gain whereas HD6XX is a bit easier and comfortable sounding. When it comes to lower treble, HD6XX has a minor dip in the 6-10kHz region too but is not as dipped as the S4X and as a result, hi-hats and cymbals are a bit more prominent and better portrayed in the HD6XX than S4X. Both have good upper treble extension but HD6XX becomes more laid back as the ear pads start wearing out but is a bit more prominent in upper treble with fresh ear pads. S4X comes across slightly more even and warmer in upper treble presentation in comparison. Soundstage wise, S4X is slightly wider and deeper as well as has better imaging and layering across its soundstage. 

Ollo S4X vs HD6XX

HifiMan Sundara pre-2020 revision (~$350) – Sundara is another popular headphone under €400 that is tuned close to reference. It is an open-back planar magnetic headphone which has excellent fit and finish. Sundara is very light and has a very comfortable fit. Its metal headband is click adjustable and the leather hand band is there for top head support and comfort only. This mechanism is the exact opposite of S4X’s mechanism. Sundara fits me much better than S4X owing to bigger ear pads and a very nice and easy clamping force. Sundara too has an impedance rating of 32Ω but needs a bit more juice than S4X to shine. Sound wise, S4X has a relatively neutral tuning whereas I’d label Sundara as more neutral bright. Even though I’m not treble sensitive and love well done sparkly treble, Sundara has some treble spiciness which can come off quite bright with some songs. Going deeper, S4X has better sub-bass presentation and extension whereas Sundara rolls-off around 50-60Hz. S4X has a better and nicer mid-bass presentation with a nice subtle boost whereas Sundara’s mid-bass is closer to neutral but a little held back in comparison. S4X does rest of the lower-midrange quite linearly and neutrally up till 2kHz whereas Sundara is very linear and neutral till 1kHz but then has a perceivable dip in the 1-2.5kHz range which takes away from absolute natural tonality and timbre of instruments, which S4X does better. Both have a forward upper-midrange presentation but S4X has slightly better tonality and timbre of instruments as well as better instrument note definition. Sundara is a bit more boosted in the 4-5kHz region which can come off as shouty at times with some songs. Sundara is bit more neutral and accurate in the lower treble. Both have good upper treble extension but Sundara is brighter in its upper treble presentation. Sundara, because of its brighter character portrays a wider soundstage but at the cost of it coming off as bright whereas S4X keeps things more organic and natural and enables a decently wide soundstage, which isn’t as wide as Sundara but definitely not bad at all in isolation. 

Ollo S4X vs Sundara


So there you have it! Ollo Audio S4X in my opinion is not only a well tuned Reference headphone but also oozes of very good boutique brand design, build and quality. Ollo Audio differentiate themselves from other headphone manufacturers by offering a headphone that is upgradable to future models for a fraction of the cost, completely serviceable at home with easily available and well-priced spare parts that can be purchased from their online store as well as having tight QC by carefully hand selecting matched driver pairs for proper channel balance and FR consistency. Sound wise, even though S4X doesn’t quite follow any of the established target curves, it comes rather close to Harman Target style of reference tuning. The small tuning changes that take it slightly away from absolute reference neutrality are rather easy to overlook because the rest of it is so well done. In my opinion, if you like me, like and appreciate natural tonality of instruments and want your music to be played very honestly without much added sugar or spice, S4X fits in very well as a musical headphone too. I’ve been using the S4X for the past month and a half, not only for my production work but also to cross-refer and listen to my favourite records and to be honest, I’m definitely not complaining having paid almost full price of €399 for it in the form of import taxes. Plus the industry and class leading 5-year warranty goes on to show how confident Ollo are in their product. So after all this, if you think the S4X is for you, definitely give it a shot! I definitely highly recommend it, be it for a musician, artist or producer/engineer looking for a reference headphone to mix on or an audiophile looking for a reference headphone to enjoy his music collection with. Also, definitely subscribe to and keep Ollo Audio as a brand to watch out for as I see them going places with products like these. Happy listening!

Gear used for testing and review.

  • DAPs – Hiby R6 2020 | iBasso DX160
  • Audio Interfaces – Universal Audio Apollo | Focusrite Clarett 8PreX
  • Laptop – Apple Macbook Pro 15″
  • Phone – OnePlus 7 Pro

Reference Songs list.

  • Foo Fighters – The Pretender, Best of you, Everlong & Sonic Highway album
  • Coldplay – Paradise, Up in flames & Everglow + Everyday Life Album
  • Biffy Clyro – A Celebration of Endings & Ellipsis albums
  • Ed Sheeran – Thinking out loud, Bloodstream & Galway Girl
  • Dave Matthews Band – Come Tomorrow album
  • Dua Lipa – Future Nostalgia album
  • Chainsmokers – Somebody, Sickboy, This Feeling & Closer
  • John Mayer – Slow dancing in a burning room, Stop this Train, Say & A Face to Call Home
  • Gavin James – Always & Hearts on fire
  • Switchfoot – Meant to live & Dare you to move
  • Porcupine Tree – Sound of Muzak, Blackest Eyes & .3
  • Our Lady Peace – Do You Like It & Innocent
  • Linkin Park – Papercut, Somewhere I belong & Talking to myself
  • Maroon 5 – She will be loved, Payphone & Lost stars
  • Lifehouse – All in all & Come back down
  • Breaking Benjamin – Diary of Jane
  • Karnivool – Simple boy & Goliath
  • Dead Letter Circus – Real you
  • I Am Giant – Purple heart, City limits & Transmission
  • Muse – Panic station
  • James Bay – Hold back the river

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