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Overall Score: 78/100

OLLO Audio S4 Review – Crafted with passion

Disclaimer: OLLO HPA S4 were sent to us as a review sample, in exchange for our honest opinion. We thank team OLLO Audio for this opportunity!

My video review:

Researching headphone manufacturers from Europe I stumbled upon OLLO Audio, a fresh Slovenian company that is making hand-made headphones. As of right now they have only two dynamic driver models: HPA S4 (S4 from now on) – the open back one and HPA S4R (S4R from now on) – the same headphone but with a closed-back design and if you would like to have a one-of-a-kind S4R, OLLO can put a custom laser-engraved logo on them.

My curiosity levels, as usual skyrocketed, I told myself I need to hear those. One month later and I had the open-back S4 on my desk.

I am quite impressed by the Slovenian tinkering skills, I already made a review for a boutique headphone amp manufacturer from the same country called Erzetich Audio and I already know that music is somehow passing though their veins, this is surely not a coincidence.

OLLO Audio is describing their S4 and S4R as studio monitoring headphones that should have a linear performance from the lowest bass to the upper treble and is ensuring that no DSP or EQ corrections are needed when listening to them. I will of course test all those claims, will measure them myself and will compare them to other candidates.

Unboxing Experience

OLLO S4 came in a small card-board box, upon opening the box a fresh smell of wood and paper hit me straight away. It was clear to me that OLLO is using only environmentally-friendly materials. The box is free of any styrofoam or any type of plastics, S4 came wrapped in a wide sheet of paper, the cable was wrapped in paper as well, you are greeted with a nice message from the company.

What was really unexpected is that every single pair of S4 or S4R is coming with its own measurement chart, even very expensive headphones nowadays do not include measurements in the box and that is sad. Way to go OLLO, I really like your approach!

Build Quality and appearance

Design wise I find them a little bit unusual and they will surely stand out from the crowd. I find them on the heavier side since the ear-cups are entirely made from thick walnut wood. The headphone structure is made out of stainless steel and the headband is made our faux-leather. Their adjusting mechanism is exactly like that of Meze 99 Classics and Empyrean and this kind of system is the easiest to use. Just put them on your head and it will automatically adjust to your head size. The head-pressure is a bit higher than normal, as a comparison think of fresh out-of-the-box Sennheiser HD650 or HD660S, well S4 is almost like that, but clamps your heard just a little bit more.

The ear-pads are made out of faux-leather as well with a red colored acoustic foam that was put there to absorb excessive treble energy.

The ear cups are just moderately big, so S4 and S4R can even be used as a portable headphone or traveling around since it is not that big or hard to drive.

The exact ear-cup diameter is 90mm, inside them a 50mm dynamic driver is mounted by hand that was tuned for a wide frequency range of 20 Hz to 22 kHz with a flat response design, will see about that really soon.

I have normal sized ears and yet for me S4 can be barely considered over-ear since there is close to no space inside the cup. Folks with rabbit sized ears could have a problem with them as they might transform into On-Ear headphones, add a higher clamping force and not a single carrot can save your listening session.

S4 is of course using a detachable cable that looks very DIY to me (in a good way), to the ear-cups it will attach using 2.5mm plugs and it is terminated with a 3.5mm jack, inside the package there is also a 3.5mm (1/8”) to 6.35mm (1/4”) adapter. The cable is free of any microphonics and has a sturdy mesh on the outside that should resist few good years.

In terms of build quality S4 is just screaming high-quality, as every single piece of it is made out of stainless steel, walnut wood and faux-leather and the best part is that every single part of S4 is replaceable with home tools. When I am holing S4 in my hands I feel them really sturdy, think Beyerdynamic sturdy, I know dropping them occasionally should not damage anything inside since I already stepped on the cable and the headphone flew and hit the floor pretty hard – they worked flawlessly after this incident, nonetheless kids please don’t try that at home.

There is also a very nice faux-leather pouch, carrying them to work and back should not be a problem, with or without the pouch S4 is made to last a life-time and I already feel the love and soul OLLO Audio put into making these.

Technology Inside OLLO S4

Creating S4 and S4R OLLO Audio used matched pairs of 50mm dynamic drivers. The impedance is a low 32 Ohm, but no sensitivity numbers were given. From my own experience S4 are not that easy to drive and definitely needs a bit more power than the 32 Ohm Meze 99 Classics or the 32 Ohm Sennheiser Momentum 2.0 Over-ear. Power requirements are about 4 db lower than the Sennheiser HD660S, doing my math I presume a sensitivity of around ~99 to 100 dB per 1mW of power, it means it can be driver by portable sources as well.

I really like that OLLO choose this 100% symmetrical design, any ear-cup you want can be your left or right earcup, the only L and R markings are on the cable itself. The same symmetrical design approach I’ve seen on the full-sized Meze headphones, I personally dig it.

I think it is time to have a long listening session.

Sound Performance

I. The Good

To fill some of my void I listened to Staid – The Singles, the last four live tracks from the recording are special to me and a very good example of how a live recording should sound.

From the first notes of Everything Changes I knew I am listening to an airy presentation with a good layering. I am impressed by the very clear cymbals, by the hit of the drum sticks and by the voice that flew far away fading just at the right time. I really like that nothing from the mix is fighting for the prime-time, nothing particularly stands from the crowd, yet everything is at its place never outdone or overshadowed. Overall, HPA S4 sounded transparent on this track without a hint of veil or muddiness that is present on the Momentum M2.0 and even on Meze 99 Classics that costs a little bit more.

Switching the pace to a faster S.O.A.D – Toxicity I was pleased to find a really good timing of the driver. The “mashing effect” and an unpleasant resonance of the Momemtum M2.0 on faster music is nowhere to be found, S4 has speedy transients and overall quite good information retrieval.

The more I listen to acoustic music on the S4 the more I realize they have just a small emphasis on the upper bass and over the midrange area as more often I feel this slight warmish tint with an uplifting musical performance.

OLLO Audio is supplying a frequency measurement but is applying a 1/6 smoothing to the chart. I on the other hand I measured their raw performance, their waterfall and later on I applied a 1/12 smoothing that I feel is representing their true nature.


I used a Matrix Audio Element X as the source and Bechmark HPA4 as amplification, MiniDSP E.A.R.S as the measurement rig for the OLLO S4.

I used the headphone compensation for flat EQ (HEQ) for this particular EARS serial number, I ran multiple measurements tests and here are my final results. The first chart is the RAW measurement.

![](/uploads/OLLO S4 RAW.png)

There is a very small deviation of about 1 to 2 dB between the drivers up to 200Hz, that is more than OK, all-in-all the right and left driver is matched and even on the treble area the matching is quite impressive. As I presumed the upper-bass and the midrange is slightly boosted for this natural and life-like presentation. Our own brain is boosting the midrange so we can hear it clearly, so I think OLLO S4 are quite flat with a small drop in the sub-bass area and a very short but nasty drop at 9 kHz.

![](/uploads/OLLO S4 SMoothing.png)

Applying a 1/12 smoothing is looking very similar with the measurements of the OLLO Audio and if I am applying a 1/6 smoothing, they both look identical. I don’t know what OLLO used but I presume an MiniDSP EARS measuring rig as well, they just look impressively close to each other.

Waterfall plot shows a longer decay of the sub and mid-bass, for a dynamic driver that is normal behavior, closed-back dynamic designs are looking much worse in this department. As it can be seen the biggest emphasis is in the midrange department that will infuse just a little bit of joy and naturalness while listening to music. Of course, the 9kHz dip is also visible, a very short 10dB deviation in terms of frequency range.

![](/uploads/OLLO S4 Waterfall.png)

All in all, OLLO S4 are measuring much better than my own Sennheiser Momentum 2.0 and Meze 99 Classics and about on the same level with $500 Sennheiser HD660S. I think they are already showing great potential of how they might sound. Measurements can’t really tell how clean, detailed, transparent or how wide they might sound, so from this point on my subjective opinions will follow.

S4 have a seductive and inviting character, somehow calling for another track. Musical sounding some might say, as a result S4 can be listened for a very long time without listening fatigue.

As another result, dryness is never present, S4 choose to be full sounding with some meat to the bone. Listening to simple orchestral or acoustic music with lots of musical instruments around me it is clear as the blue sky that S4 was just made for this kind of music. Everything sounds clear, very outlined, having a sharp texture. The voice in the background sounds really smooth. Even Jazz and classical is a delight on S4. If acoustic, classic, jazz or folk is your thing then S4 should be a good musical companion.

II. The Bad

S4 have a pretty good left to right separation but I don’t consider them really open-wide sounding. Soundstage size is almost on the same level with HD600/650/660S family of headphones and will not Wow you with out-of-your-head experiences. S4 has a small to medium size soundstage and on some aggressive rock can be even up-front sounding. Some might like this kind of performance, some might not.

Depth performance is directly tied to the soundstage and airiness, so again depth is pretty good but it isn’t impressively deep reaching and the notes are decaying sometimes faster than how I’d want to.

Mid-bass is impressively deep, layered, sounds pretty clear too but the sub-bass area is just moderately good. This is really normal behavior for open-back dynamic driver headphones, not a single one is offering a better sub-bass performance. Anyway, it should be mentioned, the 20 Hz hits on The Prodigy – Invisible Sun are not tickling my ears like how much more expensive Sennheiser HD820 or Hifiman Arya are doing. I hear a weaker energy in this area and a weaker slam as well.

Although there is just a very short dip at 9 kHz – the high treble region, some tracks that are emphasizing exactly this area will not sound as impressive and as biting, sharp or outlined. It is like a tiny portion of your treble energy is missing. I am air guitaring around and I am expecting those triangles, cymbals, hi-hats and tambourines to hit me with layers of information but that is not really happening. Again, this happens only on some tracks and it was done for a relaxed music listening.

III. The Great!

Oh man, people know that at around $500 is hard dethroning the legendary Sennheiser HD600/650/660S, as anybody who is asking me for a musical headphone in that price range more often exactly those headphones will be my recommendation.

However, I will add another headphone to that list as S4 is having some additional traits that HD6xx is just not doing as good.

First of all are the leading-edges of the notes, the outline of every note. On Sennheisers you hear them but not impressively clean or outlined, a little bit fuzzy somehow. S4 is having those clearer, more outlined, sharper start and end of a musical note if you will. As a result, the overall presentation is a bit clearer on OLLO S4, sounds are sharper and every note is really more outlined. Switching back and forth between them this is the first thing that will hit you.

Another cause and effect are of course the timing and transient response as every note will hit you a tad harder with a bigger force. S4 out of Benchmark S4 are punchy sounding and have the right amount of slam, speed and impact. At this price point they perform admirably in this department. Did I hear a better slam? Hell yes. At this price point? Hell no!

To counter balance the sub-bass deficiency OLLO gently boosted the mid-bass that is doing its magic on the right music. For the past few weeks I am quite impressed by the Infected Mushroom – Guitarmass track that is impressive in terms of sub-bass, mid-bass, layering, slam and speed of delivery. All of those points are impressive on S4, except for the sub-bass area that loses a bit of oomph and slam. OLLO S4 will sound incredibly good with electronica and rock tunes, actually I listened mostly to this type of music on S4, can’t get more of it.

Powering them with Benchmark HPA4 I have about 45 dB of headroom left, on the mighty KECES S3 I am left with about 60 dB of headroom. Even portable sources are performing admirable with them.

The low power FiiO M6 sounded fine (but not mighty fine), after some proper evaluation, portable sources are losing a bit of driver control, speed and impact but overall enjoyment level remains intact. People are driving their HD660S with portable sources so S4 should not pose a problem to those folks.


Meze 99 Classics (€309) VS OLLO S4 (€290)

Both have the same symmetrical design, both have walnut ear-cups and stainless-steel headphone structure, Meze have bigger ear-cups and a lower pressure on my head. 99 Classics is a closed design and S4 is open-back.

In terms of sonics Meze have a bigger emphasis on mid-bass and midrange in general, so it will sound warmer, fatter and even juicier. However, the tonal balance is not ideal since a flat FR is out of the question. Meze are also a bit easier to drive and can be powered even by smartphones.

S4 has a better grip, faster speed, impact and the slam is tighter, not as loose sounding as the Meze. S4 also has a better outline of the notes, every sound is just sharper on it. For pure musical enjoyment and relaxation Meze is doing that better. S4 is flatter but also truer to the recording, with better diaphragm control.

Hard to tell which one is better; they both have their magic moment.

Sennheiser Momentum M2.0 (€300 at first, €200 right now) VS OLLO S4 (€290)

Build quality wise both are fine, Momentum have leather headband and ear-pads, S4 has them from faux-leather, Momentum are not as heavy and even easier to carry due to their folding mechanism. In terms of looks I like them both. Momentum 2.0 is a closed-back design, S4 is open-back.

When it comes to sonics Momentum has a big emphasis on midrange and mid-bass and sacrifices a lot of treble energy. Momentum is warm, full bodied sounding. They are never tiresome in the long run and are sounding quite engaging and fun. Their biggest problem is the soundstage that is very in your head, on-stage presentation. They are also loose sounding, especially with fast electronica.

S4 is better on almost all key aspects: they offer a higher detail retrieval, a better pin-point imaging. The sound is more transparent, bigger as well. S4 don’t have an impressive stage, still they sound grander and wider spread. S4 is of course more controlled, has a better grip over the drivers and hits harder as well.

S4 is in a higher league and that was felt from the start

Sennheiser HD660S (€499) VS OLLO (€290)

HD660s are made entirely from hard plastic with a metal structure and mesh, however uses much bigger ear-pads so the conform level is higher as well. HD6xx have their legendary status already, so no introduction is needed.

In terms of frequency response both are very close and I can’t fault any of the two. Both are quite engaging from a powerful headphone amp. S4 is easier to drive, HD660S can be used only with powerful mobile solutions (a powerful DAP, portable headphone amp and so on).

HD660S are sounding airier and just a hair wider and deeper and that is apparent mostly with high-quality live recordings. With HD660S you can follow a note from its inception to its decay, S4 is doing that faster. As a side effect the notes have a bigger void space between them, so overall HD660S in an airier sounding headphone.

On the other hand, S4 is a hair faster, slams harder and you can clearly hear the leading-edge, the outline of the note that is sharper. It is like boosting the resolution of in image. In terms of detail retrieval both are on the same level, on the right system both will reveal small nuances and micro-details.

Sometimes S4 has a little bit more treble energy, sometimes HD660S tickles my ears with more treble detail, so it’s a tie on the treble area.

Overall, I am splitting hairs since both are great sounding headphones, HD660s are just wider and deeper and S4 has a better driver control and a better slam. Pick your poison.


If a narrower soundstage and smaller ear-cushions will not put you off, OLLO S4 will reward you with a flat frequency response, just a tad towards a natural and musical presentation with a very good grip and control over the headphone drivers. I enjoyed my time with them starting with classical music and finishing with some metal tunes, everything sounded fine to me without obvious flaws.

The headphone construction is built to last and just in case something fails, don’t worry too much as any part is replaceable with home tools and you will be back to music listening in no time.

For a first attempt, OLLO Audio did a remarkable job and at that price is hard complaining about anything at all. The best part of this incredible package is the price, they were €349 at full price, but you can get them right now at a discounted price of €290 from here. For this environmentally friendly, build to last construction and linear sound-performance I think it is a no-brainer purchase.


  • Amazing build quality, hand-built with lots of care for the environment
  • Great tonal balance! Almost flat frequency response
  • Good speed, impact and slam
  • A high level of detail retrieval
  • Good transparency, low distortion levels
  • A sharp outline of the notes, good texture as well
  • A very attractive price point, Recommended!


  • Up-front sounding
  • There is a dip at 9 kHz but at a short frequency range


  • DACs: Matrix Audio Element X, Matrix Audio X-Sabre Pro, KECES S3, Burson Swing

  • Headphone amps: Benchmark HPA4, Aune S7 PRO, Erzetich Bacillus, Headamp Gilmore Lite Mk2

  • Full-sized headphones: OLLO S4, Hifiman Arya, Quad ERA-1, Sennheiser HD660S, Momentum M2.0, Meze 99 Classics

  • Loudspeakers: KEF LS50W

  • Interconnects: QED Reference XLR, Aune AL3 XLR

  • Power Cables: Isotek EVO3 Premier

  • Balanced Power Conditioner: PLiXiR Elite BAC 400, KECES BP-600

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