‘OLLOT’ of value and performance makes the S4X headphones a sound investment for professionals and audio enthusiasts.
If you are an audio professional (e.g., musician, producer, engineer, etc.), you likely know of OLLO Audio or auditioned their headphones. If you have not, let this be your introduction!
The S4 are available in two variants: the open-back S4X reference headphones are preferred for mixing and mastering duties, and the closed-back S4R for recording.
I am immediately impressed by the thoughtfulness and responsibility of the packaging. I am even more impressed observing and holding the headphones. They are unique, handsome, well-made, and organic feeling.
OLLO intends to make a point and take a stand. What do they stand for? Values I enthusiastically support: scientific rigor, enthusiastic passion, user-centricity, courage, transparency, and environmental responsibility.
Given what I know about OLLO, I have high hopes that the S4X’s performance is equal to or greater than my first impressions.
The origin story begins with the current CEO and co-founder, Rok Gulič. Rok and his wife had their first children, twin sons. While trying to finish mixing an EP for his band in the late evening hours, Rok could not use speakers and began experiencing problems with mixing on headphones alone.
After creating the initial working model headphones and receiving broad positive feedback, a company was born – named after Rok’s twin sons Oliver and Lovro, hence OLLO (pronounced O-Lo).
Slovenian co-founders Rok Gulič and Mitja Sajovic began prototyping professional use reference headphones in 2016 and were recognized as a 2019 Slovenian Startup of The Year Finalist.
- Type: Circum-aural, open-back headphones
- Transducer driver: 50mm 2-layered coil Neodymium dynamic driver with 25u PET membrane
- Frequency range: 20Hz – 20kHz; variation in frequency response ~13.7dB SPL
- Impedance: 32 ohms (nominal)
- SPL/Sensitivity: 108dB / 1vrms@1Khz 30 ohms
- Cable length and type: 2m (6.6ft) 3.5mm to dual 2.5mm mini-jacks detachable braided silicone cable with OFC core
- Ear cups material: American walnut
- Ear cups dimensions: Outer diameter: 90 mm
- Earpads size:
- Outer diameter: 90 mm (3.5”)
- Inner diameter: 55mm (2.2”)
- Depth: 20mm (.8”)
- Earpads material: Acoustic foam, extra elasticity poly leather, and velour
- Headband material: Stainless steel and nautical artificial leather
- Weight (excluding cable): 350g (12.35oz)
- Warranty: 5-year limited warranty (earpads and cable are excluded due to the nature of exposure)
- Trial period: 30 days from the delivery date
Minimalist and functional, the headphones arrive in a cardboard box, well-protected with paper-based cushioning around the headphones. In addition, there is an entertaining and innovative small oval-shaped folding paper holder for the cable and adapter. A protective carry case is included.
I prefer to support companies advancing environmental stewardship with conscious material choice and less waste. OLLO Audio’s packaging and product are a model example of recyclable, environmentally friendly material choices and outstanding design.
In the box
- OLLO Audio S4X V1.1 reference headphones
- 2m long cable with 3.5mm to dual 2.5mm plugs (with 3.5mm to 6.35mm adapter)
- “OA” logo-embossed carry/protective bag
- Frequency response measurements, safety information, and certificate of authenticity
The S4X come with a 2m (6.6ft) long detachable braided silicone cable with an OFC core to ensure minimal signal distortion. The source plug type is a 3.5mm TRS gold-plated straight plug with a 6.35mm (1/4”) adapter.
The cable before the splitter is wrapped in black fabric and braided – after the splitter is red and non-wrapped. The dual 2.5mm plugs that attach to the ear cups are color-coded; black = left, red = right.
The plugs can go to either headphone cup, which are reversible for your preference. The jacks are offset from the bottom center of the cups. This choice of orientation is practical when playing an instrument (cables down behind your back) or when producing with a desktop interface (cables in front of you).
The well-constructed cable is attractive, flexible, and durable. My single biggest gripe with the S4X is not the cable’s fault. The microphonics are, for me, a touch irritating. This is not a problem if you do not move around much or touch the headphones while listening (especially the metal headband).
OLLO explains that the material choices and form factor that make the OLLO sound possible also result in some microphonics transmitted to the wood ear cups (from every surface of the S4X) that are difficult to eliminate.
I admit I am smitten with the handsome, distinct, and rugged look and feel of the S4X. Parts are made mainly in Europe and machined in Slovenia. All headphones are hand assembled by OLLO. The only exceptions are the drivers and ear pads made to OLLO’s specifications in China.
OLLO’s headphones are designed with the philosophy that form follows function. They are made for daily professional work applications and are built to last. If you wear out or break them, OLLO offers all spare parts that you can easily replace with standard home tools.
Materials used are primarily wood, stainless steel, and aluminum, with minimal plastics or artificial materials. One exception is the head strap, made of nautical faux leather for high durability.
A close look reveals that the ear cup backs are held in place with eight small screws. The stainless spring steel powder-coated metal headband and nautical leather strap are bound onto the cups with a fastener.
The cups have 360-degree rotation. The self-adjusting headband is almost perfect for me – it distributes weight equally and is so comfortable that I often forget it is there.
OLLO notes that the clamping force is set to achieve the most neutral frequency response reproduction. Wearing the OLLO feels like a comforting head hug.
Comfort and Isolation
The earpads are very comfortable and breathe relatively well. Clamp force is significant, not crushing – they are snug. The clamp may feel tighter for members of the BHC (Big Head Club), though the self-adjusting headband seems broadly accommodating.
For members of the BEC (Big Ear Club), the S4X’s ear pad space is also cozy for over-ear headphones. They will be fine for most, with an inner diameter of 55mm and a depth of 20mm.
Those with elephant-like ears may find them somewhat constricting. Though I prefer larger openings, it is not bothersome. Any change in the inner dimensions will require a full re-tuning of the S4X. Pads are removable and replaceable. Larger-diameter pads will be available in the upcoming S5X.
“We don’t have alternative pads – these were carefully created to deliver the sound we’re after. Bigger pads will create a bigger air pocket between your ears and the driver, causing acoustic coupling and impedance to change dramatically.
It’s a game of air volume, how well they seal, and materials absorption and reflections. All of these must come together for great sound.” – Rok Gulič, Co-Founder and CEO, OLLO Audio
The S4X’s 350g mass feels lighter due to the clamp and excellent balance and weight distribution. They are easy to wear for long listening sessions without any irritation.
As expected, you will hear quite a lot of what’s happening in your environment, and everyone will hear your music. The plus is that there is no claustrophobia and little heat buildup, as the ventilation is better than in closed-back designs.
OLLO has taken a unique approach to design and build, where form strictly follows function. The centerpiece is a 50mm 2-layered coil Neodymium dynamic driver with a 25u PET membrane, designed by OLLO and built to their specifications.
Hand-picked and left/right-matched transducers are chosen for every S4X. They feature a fully separated front and back of the speaker for minimal phase cancellation and a more direct sound signature.
Each pair is delivered with their own printed frequency response measurement charts so that the user will have a better understanding of their sound signature to have an edge in mixing and mastering for the best translation of their work.
The S4X have low impedance (32 ohms) and very high sensitivity (108dB) for compatibility with pro-level and laptop-integrated amplifiers. Industry-standard jack connectors make them a plug-and-play on just about any audio gear. A dongle DAC/amp is unnecessary for using the S4X on the go.
OLLO Audio S4X Sound
For evaluation, I listen to a great variety of music through numerous sources, including:
- Sony DVP-S7000 CD Player
- LG V20 & LG V60 phones (both have Quad DAC and headphone jacks)
- Lenovo IdeaPad 3
- Khadas Tone2 Pro
- Khadas Tea
- S.M.S.L SP200 Headphone Amplifier
- HIFIMAN EF400 Headphone Amplifier
- HELM Audio Bolt and DB12 AAAMP
When OLLO claims “flat out of the box” they are referring to the neutral sound signature that was developed in close collaboration with professional mixing and mastering engineers. They reverse-engineered what professionals consider flat and validated all data sets using state-of-the-art frequency, impulse, THD, and other measurement techniques.
The S4X are designed and built based on what audio professionals require from pro audio monitoring. The OLLO videos on how they approach sound signature are fascinating.
I have not seen this level of transparency from a headphone manufacturer (pun intended). OLLO’s website provides extensive frequency response graph comparisons of the S4X and more than 15 top-rated studio headphones.
Even though I am not a professional engineer or producer, the S4X meet my every expectation of a flat, neutral sound signature. Despite this, they are not clinical, sterile, or boring.
They lean toward a “warm” sound and will not irritate treble-sensitive people. There is a slight roll-off on high frequencies, which has now been addressed in the newest version.
To my ears, the S4X have an organic timbre quality and are perfectly well-balanced.
In all of my hours with the S4X, I have not experienced any listening fatigue from the sound. They are smooth and ouchless no matter the source. Linear and accurate are precisely what’s expected of a “reference” set – the S4X sound like the definition of reference.
In contrast, SIVGA’s SV021 Robin is about as far from flat as you will hear and still sound fantastic. They are “v-shaped,” meaning boosted in the bass and treble and recessed in the midrange. They are also a blast to listen to, with a high-energy sound signature.
In OLLO’s honor, our music journey will travel around Europe. There is no better place to begin than in Vienna, Austria, celebrating Wolfgang Amadeus Mozart’s birthday with a little night music. Happy 267th Birthday, Herr Mozart!
Serenade No. 13 in G Major, K 525, or ‘Eine kleine Nachtmusik’ as it is informally known, is Mozart’s most famous serenade (a light, breezy, entertaining piece of music). Translated into English, Eine Kleine Nachtmusik means ‘A Little Night Music.’
From Budapest, Hungary, the Liszt Ferenc Chamber Orchestra’s rendition is beautifully articulated through the S4X across all frequencies. The instruments sound natural, full, and distinct, while the sound is fully encompassing. Imaging and sense of space are excellent without the soundstage seeming artificially inflated.
The S4X, with the benefit of dynamic drivers, sound natural and warm, always appropriate to the source. There is no noticeable bleed into the midrange or masking of higher frequencies. Speed, resonance, and decay are moderate and tempered. They can reach down low with no unnatural boosting.
British bassist (half British, half Finnish roots), Charles Berthoud (pronounced BEAR-too), is a classically trained pianist and multi-instrumentalist. Charles is renowned as a versatile bass guitar virtuoso, earning a massive global YouTube following.
Let’s visit Charles in England to hear his 2023 re-release of The Last Ride. Through the SX4, bass guitar reproduction is exquisite – medium speed, smooth, and articulate. All of the bass’s granular detail, texture, and instrumental action are present. Starting at 0:57 is a fast layered section in which the S4X has no trouble keeping up.
In OLLO’s home country, let’s visit Nika Zorjan, a Slovenian singer and songwriter, to hear her new release, Zadnjič Kakor Prvič. As expected, Nika’s voice comes through naturally with warmth, clarity, and a robust and centered presence. This is especially apparent at the beginning of the song. Midrange frequencies carry well as the instrumentation fills in.
The S4X’s treble reproduction sounds accurate and detailed – enough to hear what’s happening in the mix easily. Roll-off on high frequencies is slightly noticeable and just sufficient to prevent listening fatigue. It’s best described as (you guessed it!) neutral and somewhat softened.
OLLO notes that while there is little perceived difference between the versions, the newest S4X v1.2 is just a tad more bright with a slight adjustment in this region. I imagine that this will help reveal slightly more detail in the mix.
Finally, let’s journey a few hours north to Austria to visit Bernth, an Austrian guitarist, composer, session musician, and YouTuber who graduated with honors from the Vienna Music Institute. His new release, Dopamine (plus the unplugged version), is a masterpiece of virtuosity.
The liveliness and encompassing reproduction through the S4X is almost perfectly balanced. What is surprising is the level of treble detail, without any sharpness. With high complexity and resonance in the recording, S4X transients are excellent. There is no evidence of sibilance or splashiness in any music tested.
Where to Buy
OLLO Audio’s mission for the S4X is to build perfect reference headphones for professional applications. They are created with the philosophy of ‘form follows function.’ Made more with hands than machines, they embody the craftsman spirit beautifully.
Their strategy relies on a vast amount of professional input for the sound signature, followed by applying rigorous science for the tuning and development of the physical product. I have not experienced any better expression of flat and neutral sound than the S4X. They are all-day comfortable, and the rugged build will help ensure a long working life.
If OLLO’s marketing budget were more significant, they would be far more prominent competitors to the big names in professional audio headphones. Regardless, their reputation and referral business among audio professionals is multiplying.
In my experience, this approach and consequent production have resulted in headphones that set the standard for ideal “reference” – čestitke OLLO, mission accomplished!
Please comment below with your thoughts, and as always – happy listening!