AmplifiersHeadphone Amps
Overall Score: 93/100

A Detailed Review for the FUNtastic Ferrum Audio OOR and Hypsos PSU

My Video Review:

I still remember as if it was yesterday when HeadWize (the first headphone enthusiast forum) started offering a simple battery powered headphone amplifier DIY kit that I’ve purchased out of pure curiosity. I’ve soldered it myself, I’ve put the cMoy in a card board box and wrapped it with tin foil, as an aluminum case was out of my reach in distant student years. I felt so accomplished, as if I’ve done something spectacular, having learned the unknown. A tiny circuit board, a single dual op-amp, a couple of film resistors, two capacitors, a volume knob and a 9V battery was everything a headphone enthusiast needed to unlock the sleeping beauty behind his headphones. I was trying to push a pair of Sennheiser HD580 much further, unlocking their majestic midrange and at that time, it was the only option a headphone junkie could get. Commercial headphone amplifiers didn’t exist and people as Pow Chu Moy (Rest in Peace), Jan Meier, Ti Kan, Kevin Gilmore and Tyll Hertsens started paving the ground for future designs, unleashing two decades of ground breaking innovations, creating a beautiful hobby for all of us. As years passed, I’ve tried plenty of small and big headphone amplifiers, but the ones that were unlocking my imagination were all-discrete units, unshackled by the limitations put in place by tiny operational amplifiers (op-amps).

Was it pure coincidence or the thunder god heard my prayers I was murmuring before bed time last year? Without a single doubt, 2021 was The Absolute Best year for headphone enthusiasts, as Flux Labs Acoustics released the kraken with their mighty Volot, Burson Audio outdid themselves with their Soloist 3X Grand Tourer, Enleum changed their hide (Bakoon Int.) and shooked the world with a brand-new AMP-23R current driven amplifier. But there was a fourth player that entered the arena, as if their confidence was over 9000. Ferrum Audio tested waters with an intelligent power supply unit that got its own name and a few months later, they unveiled their top dog headphone amplifier. OOR might look as the smallest unit of the bunch, but you should never judge a book by its cover, as it’s one of the most powerful units out there. Offering some whooping 10 Watts per channel in 32 Ohms, 8 Watts in 60 Ohms and up to 1.6 Watts in 300 Ohms, OOR can drive pretty much anything, declaring war to most TOTL headphone amplifiers. Compromises? What compromises, when everything was crafted with discrete components, biased into Class-A/B operation in a fully balanced quad-mono design, all driven by the smartest power supply out there. OOR and Hypsos are their A-team that never prepared for a plan B, a dream team for headphone enthusiasts that are head-banging with a plethora of IEMs, portable over-ears and all kinds of desktop headphones.

Would the Ferrum OOR and Hypsos be remembered as a legendary duo or as an unsuccessful coup? In typical (overkill) fashion, I will be going deep into the rabbit hole, I won’t go easy on them and in the latest chapters of this review I’ll be comparing this duo with a couple of world-class headphone amplifiers, including a clash with my own unit. Be sure to check that out and until that happens, let’s unbox them and see what’s inside.

Unboxing Experience

They came in two separate boxes that were bigger compared to the usual ones, suggesting that I’m dealing with something interesting. Inside the packages there was an unhealthy amount of hardened foam as an extra protection measure. Both units were sitting in a carved spot completely surrounded by foam, so you can be sure that they’ll arrive safe and sound to your front door.

Hypsos is coming with a single umbilical cord, it could be either the Ferrum Power Link (specifically tailored for OOR) or it could be terminated with a DC plug having a diameter of 2.1/2.5mm. By default, they have a length of 50 cm and if you’ll need a longer one, Ferrum is providing them separately right here. These cables are thick, I find them well-made, terminated with sturdy metallic plugs. You’ll need them to connect the Hypsos to the OOR or to other components that can accept a DC voltage in between 5 and 30 Volts, with a total power consumption up to 80W. There is a power cable and a Quick Setup Guide and that’s all you’ll find in there. OOR can also be used as an active preamplifier, but it isn’t coming with a remote control which is a bit of a bummer. In the OOR package, besides the usual stuff, you’re also getting a Meanwell switching mode power supply that provides 24V and 2.5A. Both units aren’t coming with user manuals and in case you’ll need one, you can download OOR’s manual from here and Hypsos’s manual from here.

Design & Build Quality

While they look like simple matte-black metallic bricks, they stole my attention with their gorgeous look, with a big volume wheel, neat looking display and advanced UI. This is what I’m calling a polished combo from any point of view, they look serious and pleasing at the same time. I really dig their lit Ferrum logo carved on their face plates. You can increase or decrease the light output and if those are bothering you in late night listening sessions, you can disable them completely. On top you can spot a bigger area drilled with holes that will move hot air away from their electronics, prolonging their life. Both units are getting warm after few hours of use and I’m recommending leaving them in a well-ventilated area. You can stack them on top of each another, I used them this way and I had no problems with heat dissipation for more than two weeks. With all that said, I don’t recommend putting another unit like a headphone amplifier or an integrated amplifier on top of the OOR. With audio gear of this caliber, I would put them on a rack, maybe on different levels for a nicer heat dissipation.

Their industrial, yet modern look is attracting a lot of eyes, especially after seeing Hypsos screen in action. It would be cool if Hypsos could be updated with a digital VU-meter in the future, à la Audiobyte HydraVox + HydraZap that is so mesmerizing to look at. Their front plate has a thickness of about 4mm and their case feels thick enough, so you can be sure that external wireless interference wouldn’t pose a problem.

I find their front panels quite sexy, there is only an LCD screen in the middle of the Hypsos and a knob that works as a menu navigator. With OOR you’re getting a big volume knob, input and gain switches, SE (6.35mm) and BAL (4-pin XLR) headphone sockets and that’s basically it. You won’t find a single button, nor screws on their face plates.

The switches have a firm click and aren’t wobbling at all, OOR’s volume knob has a decent amount of resistance, so you’ll not accidentally turn it all the way up. Compared to their competitors, Ferrum stack offers a smaller footprint, so you can easily integrate them into a tidy looking desktop setup. With a net weight of 1.8 kg for OOR and 2.8 kg for the Hypsos, I find them lightweight and easy to maneuver.

I appreciate putting taller rubber feet under them, that will absorb shocks and tiny vibrations coming from their electronics, making them work at optimum parameters. Their rubber feet are bolted and if you fancy nicer looking shoes, you can always swap them with Viablue QTC spikes that are matching with their matte-black cases. That would look classy and really badass.

There are plenty of tiny details that you can observe after closely inspecting them, like a fine grain texture around their logo, everything looks perfectly aligned to me and there isn’t a lot more to add about their sleek ‘n’ durable cases.

Controls & Connectivity

The star of the show is that sharp looking LCD screen that adds a lot of value to this combo. If you’ll be using it only with the OOR, then you can control the incoming voltage to the OOR from 22V to 30V, that will be slightly altering the voicing of the amplifier. Hypsos is showing in real time useful information as how much power and current is being delivered to your headphones and the working voltage of the OOR. I never saw anything like it, this is really one of the smartest power supplies and I get it why it deserved a name.

With a simple press on the Hypsos rotary knob you’ll power it on and a second later OOR’s logo will start glowing, suggesting that they are ready to play some tunes for you. When OOR is powered, with its huge volume know you can change the volume of its headphone out and of its preamp output. OOR employs a high-performance headphone amplifier and a dedicated line stage, it’s a fully balanced one and you can spot a 4-pin XLR and a 6.35mm headphone jack on its front plate.

On Hypsos back, from left to right you’ll be greeted with a DC output that can offer you back a wide voltage range starting with 5V and finishing with 30 Volts, up to 6 Amperes of current. There’s a trigger I/O socket that will help you power on multiple audio components at once, a micro-USB port – used for future firmware updates and lastly there an On/Off switch and an AC socket.

Taking a glance on OOR’s back-panel, from left to right there’s a pair of XLR and RCA analog inputs and the same number of analog outputs, suggesting that OOR can work as a fully-fledged line amplifier in between your source and power amplifier. Next comes a bypass rotary switch that allows you to bypass OOR’s internal volume pot, making it work as a power amplifier. Be warned that in this case you should use your DAC in preamp mode and lower its volume first, otherwise it would blast its full power output, that could be lethal for your headphone drivers. Below it, there is another pot that will control the brightness of the Ferrum logo, you can also turn it off if you please. Lastly, there’s a Ferrum Power Link 4-pole DC input that connects directly to the Hypsos and an additional 2.5/5.5mm DC input that can be used with 3rd party power supplies or with its own Meanwell switching mode power supply.

Technology Inside the OOR and HYPSOS


When you are giving a name to a power supply, you know that’s not your regular power supply, it’s something else and after closely inspecting it, I started realizing how smart it was designed.

First thing to know is that Hypsos is a hybrid power supply, combining a switching and a linear and regulated power supply in the same chassis, getting advantages from both techniques and that is a low ripple and noise, as well as a fast transient response and high efficiency. There’s a huge EMI filter that will clean up the power lines, it doubles as a switching mode PS. A power relay from Schrack follows and then a custom-made O-core transformer made by HEM – their parent company from Poland. After AC power was cleaned up and regulated, an overkill filtering stage follows with the help of several aluminum electrolytic capacitors.

If you’ll be feeding third party devices with Hypsos, an advanced UI lets you choose from a long list of compatible devices. If you can spot your device in that list, just select it and confirm by pressing the dial. Once those settings have been approved, a short timer will appear on its screen after which the voltage and current output will be set in stone. There are already tens, if not hundreds of devices to choose from and many others will be added later via firmware updates.

If your device can’t be found in that list, you can request it to be added in a near future via a firmware update or you can select Manual Configuration, after which you should select the appropriate polarity of your device and the voltage output. You can select between 5V and 30V. Hypsos maximum power output is 80W and if you would like to learn more about it, I strongly recommend checking out its user manual right here.

Last but not least, Hypsos makes your current audio setup (including the OOR) perform at its maximum potential, providing a detailed and transparent, yet unfatiguing sound to the end user.


The most important thing to know is that OOR is an all-discrete, fully balanced Class-A/B headphone amplifier that could power any headphones you can think of. You won’t find cheap op-amps in there, nor questionable noise shaping techniques that could alter the essence of the music. No matter if you’re using its XLR or RCA inputs, the signal path remains balanced, meaning that you can use even single ended sources and you’d still be getting maximum performance out of the OOR.

Inside it you can spot a beautiful layout and about 80% of its internal space is occupied by four quad-mono amplifier modules. Two transistors per module and hundreds of diodes and resistors are combining together an amplifier module. Its output transistors are always being fed, exactly how Class-A amplifiers are working, resulting in an immaculate transient response and dynamic range.

OOR uses a balanced ALPS Blue Velvet volume potentiometer, but since this isn’t a stepped volume pot – several Nexem relays were added in there for a perfect channel balance. You can use ultra-sensitive IEMs at low gain and low volume and you’d still be getting an excellent channel balance. OOR is a wide bandwidth amplifier, covering a much wider frequency range of 20 Hz to 100 kHz, it outputs some whooping 8 Watts per channel in 60 Ohms and 1.6 Watts per channel in 300 Ohms, making it one of the most powerful headphone amplifiers ever made. Starting with dynamic headphones and finishing with AMT and Planar-magnetic ones, OOR will drive them all with a lot of power to spare.

Output impedance stays at just 0.3 Ohms, fully preserving its damping factor even for low impedance headphones. It outputs a very low impedance via its RCA and XLR outputs on its back, suggesting that OOR could easily replace a dedicated preamplifier in a stereo setup.

If you’re craving for ultra-low distortion, zero listening fatigue and for huge dynamic swings, then I strongly recommend adding the Hypsos power supply, that would elevate its performance to a whole new level, regardless of the headphones that are being used. There’s an excellent performance out of the box, but it will excel above and beyond when used together with the Hypsos.

Test Equipment

Ferrum OOR and HYPSOS are making together a high-end headphone amplifier stack and that is precisely why I decided testing it with every single headphone I have at my disposal. Firstly, I will be testing its noise-floor via ultra-sensitive IEMs, I’ll move to portable over-ear headphones and lastly all sorts of desktop headphones will be hopping on my head, ranging from affordable dynamic headphones to high-end planar-magnetics. In some of the latest parts of this review I’ll be driving the most difficult load – the notorious Hifiman Susvara.

A high-end amplifier, would need a high-end DAC to shine at its best and that’s why it was connected mostly to a Rockna Wavelight, Audiobyte HydraVox, Gold Note DS-10 Plus & PSU-10 EVO, to a SMSL VMV D2 and later on to a Singxer SDA-6 PRO.

There aren’t many headphone amplifiers that could brawl for a few rounds with it, nonetheless I’ll be comparing it with my daily driver Benchmark HPA4 and then with a giant of an amplifier that is called Flux Lab Acoustics Volot.

Alright everybody, my body is begging for some music, so let’s hit some eardrums!

Sound Performance

I. Preliminary Impressions

The Ferrum stack arrived at my front door just before Christmas and since I was giving a helping hand to Santa and to his elves, I left it play for about a week with a playlist on repeat. I didn’t stack them, as working non-stop could raise their temperatures higher and I don’t like gambling with electronics. Just a day before the burn-in period would pass the finish line, a close friend came over and passed me his Rockna Wavelight DAC, as he went skiing across Europe, lucky him!

I on the other hand, for three days straight went to bed at around ~4 A.M. as for me, this was one of the yummiest experiences I had via headphones. With up to 10 Watts in 32 Ohms and 8 Watts in 60 Ohms, the little Ferrum stack didn’t had the slightest problem driving my whole headphone collection with flying colors, including that nasty Hifiman Susvara. I know that Wavelight was bringing a lot of magic to the surface, but the final benefactor was the Ferrum stack, that infused so much joy and listening pleasure, completely removing the tiniest traces of glare and listening fatigue. If on Benchmark HPA4, the Susvara and the rest of the Hifiman flock were almost unlistenable due to its dead neutral approach to music reproduction, OOR felt considerably more alive, fleshed out and natural sounding to me. Ferrum Audio team wasn’t joking around when they described their creation as the only amplifier with a soul, as it was indeed one of the most joyful, highly dynamic and bold sounding headphone amplifiers.

Having a fully discrete circuitry and being powered by an over-engineered power supply, the Ferrum stack never limited dynamics, never added glare or brightness up top, never pressed the brakes and didn’t limit the air travel in any way. On the contrary, it felt more organic to all-discrete amplifiers that passed through my hands and instead of going with a dead-neutral approach, it was always showing me its personality and charisma. It was like one of those friends that constantly asks you outside for a drink or two, that always tells fresh jokes and has the biggest mouth with ladies. To me, OOR felt by a hair more life-like sounding to the Flux Labs Acoustics and Burson Audio creations and that felt so obvious, that a comparison wasn’t necessary.

This stack wouldn’t impress you with over-sharpness, with over-detailed leading edges or with an on-stage presentation. Instead, OOR will be throwing a well-spread soundstage around of the listener, expand it in all possible directions and place every note on individual shelves. OOR is sounding like music to me, with nothing standing in its way, with no particular highlights to mention and yet everything is located exactly at their right place. It knows how to accelerate those dynamic swings and you should prepare a pillow, as OOR clearly knows how to land some punches when it comes to transients. It was textured and dense sounding, music was never hollow inside, like what I’m normally getting from nested feedback amplifiers. It had more meat on the bone, a lot of boldness and a lot of substance, that sincerely a lot of modern amplifiers are missing nowadays.

In my view, the key words with the Ferrum stack are: absolute control of the drivers, an effortless presentation, gobs of bass heft and midrange density, while gently slashing glare and treble ringing for good. With this combo you’ll be getting a pleasing and an easy-going presentation. After tens of hours of music listening, I forgot about system matching, as even delta-sigma modulation DACs offered outstanding results. Be it SMSL SU-9N, VMV D2 or Singxer SDA-6 PRO, it paired well with all of them, while constantly trying to show the beautiful side of my music.

That’s right folks! OOR is indeed altering a little the final outcome, but who cares, when I can’t take my headphones off for a few hours straight. Ever seen red eyes in any of my latest videos? That’s because I’m going to sleep at 4 A.M. and the ones to blame is the nasty OOR and its soulmate Hypsos PSU finger-pointing on them

II. To HYPSOS or to not HYPSOS

When you’re getting the OOR, you are also getting a Meanwell switching mode power supply and while it wouldn’t induce a higher noise floor into its analog amplifier stage, something just wasn’t clicking into place, I felt it less impactful and less dynamic sounding. I’ve got a similar effect with the Gold Note DS-10 Plus, when PSU-10 EVO wasn’t powering it.

An external power supply for your equipment isn’t a new concept, as there are brands that are doing this for quite some time. Remember SBooster, Farad, SOtM, Naim NAPSC/SuperCap and Plixir Elite BDC, all of them are making linear and regulated power supplies for your digital or analog sources and some of them could even work with low powered headphone amplifiers, that use a DC input instead of an AC input.

Hypsos uses a very similar concept, but it’s not your usual transformer-based power supply, it’s something else entirely. I’m calling it a hybrid PS, as there is both a transformer and a switching mode PS, getting the advantages of both. Hypsos is certainly not your average PSU, it’s a very smart one as it adjusts transformer voltages automatically, it’s protecting your equipment from over-voltage, over-current, short circuits and there’s also an EMI filter for a better noise attenuation.

While OOR is already a clean sounding unit, with defined leading edges and full-bodied presentation, after adding the Hypsos, the void spaces between the notes became bigger and blacker, the textures and leading edges became clearer, but the biggest change can be summarized with a single word: Dynamics! On its own, OOR is a snappy and engaging sounding unit, always putting a smile on my face, but with the Hypsos in place, everything became livelier sounding. I had more low-end, a denser midrange and even less digital glare. It added muscle mass, it added a stronger punch down low, it added deeper bass notes, all coming in an effortless and easy way towards the listener. Interestingly enough, the biggest change was felt with harder to drive headphones, as higher dynamics and a better layering were much easier to spot. Soundstage improved a little bit, but the engagement factor was the star of the show, going considerably higher.

I’ve immediately felt a higher dynamic peaks and a better sense of space. If you ask me, Hypsos is a must-have upgrade, especially if you’re driving open back planar-magnetics. Simply put, I can’t go back listening to the OOR without its complementary Hypsos power supply. If you’re planning on getting the OOR, then Hypsos looks like a logical upgrade down the road. With it you’ll be killing the noise for good, while unleashing higher dynamics with anything connected to it.

III. Noise Floor & IEM Compatibility

First of all, let me tell you that you shouldn’t get the OOR for the sole purpose of driving sensitive IEMs, this would be insanity and the pure definition of overkill. If you do have a substantial collection of portable and desktop headphones, including IEMs, then be my guest, as OOR could take that spot and drive pretty much everything.

I power my desktop headphone setup with a passive power conditioner from Plixir, it’s called BAC400. It wasn’t put in place to clean up my power lines, I just wanted to preserve dynamics, I wanted my music to sound in daylight exactly as it sounds at 4 A.M. in the morning when nobody messes up the power lines. I’m glad to report that it didn’t matter if Hypsos was powered from the BAC400 or directly from the wall, as I didn’t experience an increase or decrease of the noise floor,

Usually, when stereo or headphone amplifiers are offering a heftier power output, an unwanted guest will be paying you a visit – the noise floor. I’ve experienced that myself in my stereo and headphone setup and there are plenty of examples, but it seems that Ferrum Audio guys are being driven by a different principle. Their hybrid power supply cleans up the mess, while never limiting dynamics with either IEMs or hard to drive planar magnetics.

I was prepared for gremlins paying me a visit, increasing the noise output as I am raising my volume levels. Most of fully-discrete designs were doing that, but luckily OOR’s low-gain was a chef’s kiss with my IEMs. Be it a 113 dB per 1mW sensitivity IEM (FiiO FA9) or a 115 dB per 1mW sensitivity desktop headphone (Apos Caspian), OOR’s low gain sounded as a quackless duck. There was an absolute silence in between passages and no matter how high I was raising its volume; noise floor didn’t increase even for a tiny bit. When I was unplugging and plugging back a pair of IEMs with the music on pause, OOR performed as if it was turned off completely.

Naturally, its mid-gain position raised the noise floor by a tiny bit and its background didn’t feel black as night anymore. My portable over-ears were still noiseless and didn’t pick up any of that residual noise. High-gain was the only setting that was elevating the noise floor with IEMs, but still, my desktop headphones couldn’t pick it up.

The lesson is really simple: use low-gain with IEMs, mid and high gain with everything else and you’ll be safe & sound, as gremlins wouldn’t come uninvited anymore.

I redid my tests in a stereo setup, using the OOR as a dedicated preamplifier. Benchmark HPA4 went out and OOR took its place in between my source and two AHB2 monoblocks. KEF’s Reference 3 sounded exactly as I know them to be, their transparency and speedy presentation remained intact. OOR added some of that all-discrete warmth, a heavier and more impactful bass, at the cost of being less grippy sounding to the HPA4. Those tweeters could pick up immediately an increase of the noise floor, luckily that didn’t happen and except for a more vivid and fleshed out presentation, OOR performed more or less the same. My biggest issue was it lacking a remote control, as I needed to run back and forth a few times. Maybe OOR was designed to be used as a preamp for active nearfield speakers, than like a component staying in the living room.

IV. Power Output

On one hand, OOR isn’t the most powerful headphone amplifier that ever walked the earth, I believe Flux Labs Acoustics Volot still holds that title. With some crazy numbers as 16 Watts per channel, that thing was driving my headphones as if IEMs were connected to it, whilst offering a huge headroom even for the notorious Hifiman Susvara. However, it should be mentioned that power is never made the same, the way it is being delivered and its cleanness/distortion are considerably more important. I am still giggling when I see people driving their Hifiman Susvara with an unrefined and plain bad sounding Emotiva BasX A-100 – that is name changing their Susvara into Sundara.

Ferrum Audio team functions by different principles, some of those are old but great sounding (all-discrete circuitry), some are new and unconventional (hybrid PSU), but together these are working as Ying and Yang, putting a higher importance to power’s quality. Ferrum is specifically mentioning its output in 60 Ohms, as if that’s a crypted message, suggesting that OOR was designed to power the mighty Hifiman Susvara. If you would check my Susvara review, there’s chapter where I’ve ranked 22 headphone amplifiers and OOR + Hypsos together with the Flux Volot came on top of that list. As far as headphone amplifiers are being concerned, the Ferrum stack was by far the most dynamic, explosive and engaging sounding and with units like this, I would think again about ever driving the Susvara with integrated or power amplifiers. There was always a huge power reserve remaining on tap, even with high dynamic range tracks. Be them from Chesky, Stockfisch, Linn, Reference Recordings or Deutsche Grammophon, OOR never felt out of breath, always trying to impress with a higher dynamic range. It was easily reaching those peaks, be them in the sub-bass or top-octave, there wasn’t a track that didn’t sound great on the OOR while driving the Hifiman Susvara.

I clearly remember hearing it for the first time and rising an eyebrow, it returned Susvara to life, it unlocked those sleeping dynamics, it pushed and pulled those drivers as if it was driving an ordinary headphone. I’ve had an extraordinary moment when lightning-fast electronica tunes appeared on my playlist. Not only did it awake those 20 Hz notes so bravely, but there was an absolute control of the drivers, decaying the notes in the same manner, never letting them linger for a microsecond longer.

It’s unnecessary mentioning its performance with the rest of my headphones, as be it Kennerton Rognir, Audeze LCD-4, Hifiman HE1000SE or Erzetich Phobos V2021, it drove them all on mid-gain, never going past 12 O’clock position, providing a substantial headroom for all of them.

Out of pure curiosity, I’ve tried it in the power amp mode, controlling the volume with a Rockna Wavelight or SMSL VMV D2 and sincerely, it performed more or less the same. Sometimes it felt as cleaner sounding, sometimes it sounded the same or worse and unless you’re not using a TOTL active preamplifier, I don’t see the reason of using its power amplifier mode.

V. Transient Response

Alright people, I don’t like hyping things that don’t deserve your attention. This chapter alone is the closest to my heart…toes and head, as without some proper transients, I wouldn’t tap my feet and headbang like a madman. This part alone is extremely important to me, as I can’t stand mellow and gentle sounding amplifiers. An amplifier that knows to accelerate and decelerate in an instant will be good with every musical genre, but the ones that are mellow and slow, can’t possibly impress an electronica and rock addict as myself. It’s the same with cars, slow ones are just like that and faster ones can go the way you like it. Mediocre analogy? Maybe, but you are getting my point.

With amplifiers, you are either getting fast transients or you aren’t. Usually, op-amp based amplifiers are quite narrow minded in here, due to their limited current output. It’s in their nature and spec sheet, but with discrete components…sky is the limit and that is precisely why, OOR is a BMF (in Tarantino’s terms) when it comes to transients. Its power output was impressive, but more impressive was its mean and thumpy attitude.

When an amplifier is running out of steam, the first sign will be sending your headphones into clipping territory or shutting down completely. The second sign would be lacking dynamics when music is trying to reach its crescendo. Sign no.3 is lacking sub-bass punch and sustain with bass intensive music. OOR driven by Hypsos didn’t have tiny glimpses of those signs, it completely bypassed them and everything that remained was a speedy, impactful and a powerful presentation.

I’ve took my big guns and put the most demanding tracks, if it can deal with Infected Mushroom, The Prodigy, Igorrr and Noisia, then it can deal with everything…and here I was grinning with a glass of smoked whisky, gently nodding my head that everything seemed at its right place. OOR just single handedly outperformed any other all-discrete amplifier I’ve tried in terms of dynamics and that includes the Volot. It pains me writing this, but OOR was more visceral sounding, arriving much closer to what I’m usually getting from two AHB2 power amplifiers put in bridged mode and connected to the Susvara. Bass notes started rumbling, they were going deep, I felt them textured and heavyweight, as if an invisible air mass wanted to make its presence felt and steal your attention every fricking second. While its midrange delivery was spectacular and juicy sounding, its bass output stole the show for me, as the rest of my amplifiers weren’t as engaging and alive sounding.

OOR is a highly customizable amplifier, you can change its nominal voltage, you can decrease or increase it and depending on your headphones, you’ll be making it smoother or more impactful. For example, with low-impedance headphones, I certainly liked its 22V setting more and with higher impedance ones, 30V setting sounded better to me. You aren’t getting the usual sound signature that is set in stone, but a personalized sound that could be tailored to your needs. There is no point in mentioning op-amp driven amplifiers as nested feedback (NFCA) or feed-forward amplifiers (THX), as OOR felt as a scarier beast altogether, being as technical and fast, but considerably more enjoyable and organic sounding at all times.

VI. Soundstage & Depth

If you look closely, a lot of these chapters are interconnected in between. For example, you can’t get a well-spread soundstage if transients aren’t being pushed with a higher force and you can’t get a sensation of depth, walking around your tunes without an exemplary noise rejection. Ferrum team didn’t compromise its creation and as a direct result, it offered a transparent and airy presentation that will impress jazz, blues and classical lovers even from the first seconds. With OOR, music flies far and wide and nothing is stopping it from hitting some imaginary walls. Live music sounded impressive on all axes and crowded music didn’t feel closed-in or claustrophobic sounding to me. Musical notes felt separated by void spaces, never interplaying or bumping into each other.

Searching for tiny details or focusing on the musicians playing in the background wasn’t difficult at all and I perceived that as a small boost in terms of depth. Sincerely, I’m yet to hear a closed-in sounding all-discrete amplifier and as far as solid-state amplifiers go, you can never go wrong with such a working principle. Transistor-based amplifiers like this one are delivering the maximum amplitude to your drivers, passing the same current at any load and listening volume, tensioning the magnets of your headphones for an instant power delivery. That is precisely why, units like OOR will move air much easier and if your music has plenty of that, you can feel the music expanding, nicely decompressing the song so you could see the full picture in front of you. I would always have at least an all-discrete amplifier sitting on my desk for this reason alone. No matter from angle I’m looking at, OOR revealed itself as an airy, wide and deep sounding unit.

While it wouldn’t outperform the soundstage king that for me is Flux Labs Acoustic Volot, it easily outplayed the likes of Flux Labs FA-10, Burson Soloist 3X, Musician Andromeda and Benchmark HPA4. When Kennerton Rognir hopped on my head and the newest Leprous – Aphelion (Qobuz / Tidal) album started playing, I’ve immediately felt a dreamy opening sequence and a classic-rock vibe, it drifted towards a hypnotic ambiance, delivering a clear image that played tricks with my imagination. The percussion work and vocal performance sounded deeper, as if I was sitting farther away from all musicians. When suspense was building up, I was getting a sense of grandeur and epicness. If you listen closely, you’ll observe that every track feels like a single, as if there isn’t a common theme or a leitmotiv. All dramatic moments were sending shivers through my spine and that eerie feeling was perfectly executed by the OOR.

The perception of a bigger stage compared to my own amplifiers wasn’t that big, due to the nature of headphone listening itself, but still, the whole experience felt more holographic, music wasn’t squeezed inside my head anymore, I felt at peace and more relaxed as it pushed all that energy around me. Even tiny IEMs started sounding like semi-open desktop headphones. OOR impressed with its depth and it was uncommon hearing deep reaching sounds, well spread across my listening spot even with a pair of 7Hz Timeless in-ears. While their drivers were sitting centimeters away from my eardrums, it fully preserved its airy and clean nature, expanding wide open and uncluttering my music.

If you love walking through your music collection and picking sounds from thin air, then OOR wouldn’t disappoint a single bit.

VII. Resolution & Transparency

I am reminding you that Ferrum didn’t use a single op-amp in its signal path. None are being found anywhere, as everything was crafted from discrete components biased into a modified Class-A/B operation. It isn’t limited by anything, meaning that OOR will be showing you everything that is lingering in the background, without going to the clinical or dry side. It was clean, yet organic and believable sounding at all times. There were plenty of sounds hovering around and nothing can really hide from its gaze. I’m not shocked by the amount of detail coming out of it, as its competition performed equally impressive. I find the Flux Volot and Burson Soloist 3X on the same level and I’m happy that the newest generation of amplifiers are putting a bigger accent on dynamic range and THD numbers.

Its detail retrieval didn’t feel artificial or overdone, as NFCA and THX-AAA amplifiers are doing, instead OOR focused on their innards. You aren’t getting an empty shell anymore, but a full-bodied representation of your music.

When it comes to resolution, the ugly truth is that besides the Benchmark HPA4, everything else was close, but not exactly on the same level. Even the mighty Volot and Burson Soloist 3X were transparent and clean sounding, but they weren’t offering the last bits of resolution that I so crave about. I know my music well; I know the mastering errors and the micro-details that are hiding in my mediocre-mastered music. Besides HPA4, all others would hide a bit of truth, beautify my music in some way or apply a smoothing filter that would hide away part of said errors and I don’t want that. I’m happy to report that OOR is doing this on a much smaller scale. I find it brutally honest and thanks to its fullness and textured approach, I am feeling those details less itchy and bothering.

Micro-details can sometimes be tiresome to experience, especially when there’s a lot of glare, but luckily OOR doesn’t have any of that. Thanks to its natural textures, you can listen to nasty trebles, cymbals, snare drum hits and bells all day long without tiring you down. Transparency felt on a high level too, as I could easily walk through my music and analyze it without putting a hard task in front of me. OOR felt as clean sounding, never being a burden with linear or bright sounding headphones.

OOR wouldn’t steal your attention with over-sharpness or powerful leading edges, as it cares for the act of music listening more than anything else.

VIII. Frequency Response

A. Bass

While I was taken away by its warm and smooth midrange delivery, OOR was constantly moving the spotlights towards its powerful bass delivery. I don’t want to sound overenthusiastic about it, but it’s bass output and everything that is tied to it was magnificent from day one. OOR possesses a wild and unrestrained bass performance. I find it punchy, but also effortless, sustaining it for longer periods of time if the track was asking for it. It appears on my radar and starts pulsing, attracting my full attention. Low end felt muscular and hefty, easily reaching the lowest pits of 20 Hz and regardless of the music I was listening to, it just delivered an enjoyable and fun listening experience. If you feel that your Soloist 3X is extended down low, wait until you hear the OOR after a few minutes of warm up. I was getting a bad boy vibe from it and it wasn’t shy showing off those traits. For some it might appear as slightly overdone and elevated in here, but in my humble opinion, this is exactly how bass should be delivered. It was never aggressive or all over the place and you can forget about muddiness or distortion, as OOR is free of that nonsense. Quantity and quality wise, OOR was impressive, up there with some of the nicest headphone amplifiers.

B. Midrange

Creamy, juicy, full-bodied and warmer than usual were my first thoughts. Its beautiful timbre starts with a fuller midrange delivery and oh boy, what a beautiful midrange! OOR isn’t a straight as a line amplifier, it never felt that way, instead the whole midrange region took a step forward, revealing itself as meatier and denser sounding to op-amp based designs. Instead of being on the same level with the rest of the FR, OOR carried more emotions, it felt heavier in here, sweeter and smoother at the same time. I just couldn’t get enough out of my rock, jazz and blues tunes, due to its pleasing and soothing midrange delivery. If you’re getting a vibe that I’m describing a slightly elevated midrange, then that is correct. It worked well with instrumental and acoustic music and don’t get me started with lifeless sounding headphones, as it will be infusing some of its own medicine. My tunes became livelier and I wasn’t listening to a simple recording anymore, music was happening in between my left and right ear, appearing from a black void of nothingness. When Ferrum team described it as an amplifier with a soul, they weren’t joking around.

C. Treble

Its treble performance resembles the sound of most discrete amplifiers that passed through my hands. There is an easiness and a harsh free presentation that is missing with op-amp based amplifiers. You can feel the cymbals body, you can feel a nice impact of the snare drum hits, but you won’t feel a fake treble ringing. There is plenty of detail and transparency, but there was never sharpness and this is another trait I find unique with such designs. I’m into all kinds of music, including treble-intensive and with some particular gear, I refuse listening to such genres, as I wouldn’t enjoy my time. With OOR, I could listen to prog rock all day long, hearing loud and clear those metallic notes, sans the issues associated with harsh sounding amplifiers. I can’t call it rolled-off in here, as there’s still plenty of presence and detail, it just isn’t overdone and never attracts a lot of attention. This is an outstanding treble performance that I wish I could borrow and place inside my own amplifiers. Sometimes it feels over-powerful down low, but it’s a much calmer animal when trebles are coming to play. I believe it wouldn’t impress treble heads that much, but it will music lovers that care for the act of music listening. Sometimes it felt less extended up top, but that could work as a plus, as I know plenty of individuals that can’t stand bright trebles and it seems that Ferrum Audio team did a marvelous job in recreating such moments.

Overall, I would describe it as a little warmer and meatier sounding, while wiping clean listening fatigue out of existence. It remained true to my recordings, infusing just a little of its own medicine and sometimes sugar coating the whole affair.

IX. Comparisons

I wouldn’t compare the OOR with a lot of units, as it’s more powerful and considerably more expensive and that’s why I decided to focus only on two particular units. I know that Susvara users are asking themselves which amp would be better tailored to their needs, so I decided comparing it with the Flux Volot and with my own Benchmark HPA4, using three of the hardest to drive headphones I have at my disposal, so let’s get this party started.

A. Ferrum OOR + Hypsos ($3190) VS Flux Lab Acoustics Volot ($2549)

Volot is the pure definition of overkill in every possible way, starting with dimensions, weight and power output, it’s really one of the biggest and most powerful amplifiers out there. Its sheer size might put off several headphiles, as it occupies around 1/3 of my table and mind you, I’m using an oversized PC desk. Since we’re trying to compare apples to apples in here, its 16 Watts per 32 Ohms would equal to ~8.5 Watts in 60 Ohms, so it’s just slightly more powerful to the OOR that nets you 8 juicy Watts. Build quality wise, both units are equally impressive, but I find the OOR stack more beautiful to look at and it occupies way less space. When it comes to features, Volot is a pure headphone amplifier at heart, providing 4 pairs of analog inputs, while OOR is more versatile as it can work as a dedicated preamplifier. Flux Labs Acoustic team will be offering at a later date a newer Volot that would swap two pairs of analog inputs with two pairs of analog outputs, but it’s line amplifier circuit will cost you an additional ~$500.

On the inside things are getting a lot more interesting, at least for me. Volot is an all-discrete amplifier that is biased into Class-A operation, hence its bigger size, hotter case and higher power consumption. Its power supply design, regulation stage and capacitance are some of the most impressive things I’ve seen on a headphone amplifier, but Hypsos is equally impressive and I find them more or less on the same level.

Before I’ll tell you about their sound performance, I’ll remind you that both units won the best headphone amplifier award for 2021 and if you dying to know which units won the second and the third place, then please check out our Best Of 2021 video right here.

With Hifiman Susvara on my head, right off the bat I’ve felt two different design schools. While Volot is an all-discrete Class-A amplifier, their creators tuned it in such a way that it wouldn’t sound like one. While their FA-10 (or FNC-10) was oozing warmth and a higher midrange density, Volot was tuned as a more revealing and linear type of amplifier. Volot isn’t elevating a particular frequency region, nor does it drag your attention anywhere. The final result is a neutral sounding headphone amplifier that completely discards treble glare and listening fatigue, while carrying a reference tuning. Volot is an incredibly clean and detailed sounding amplifier, while controlling headphone with an iron grip. Probably the biggest difference lays in its sound staging capabilities, which are still unmatched by any solid-state amplifier. Volot sounds big, spacious and very layered as if its mods semi-open headphones into fully open-back ones. There is a sensation of air traveling in between the notes and an impressive layering, as if their designers cleverly hid two vacuum tubes in there. There is just a little bit of warmth, but it always tries to stay true to the recordings, never messing, adding or subtracting anything from the mix. The only thing that I slightly dislike about it, is a limited kick in the sub-bass that happens only with Hifiman Susvara. Dynamics were great and dandy, but it was not the most visceral and thumping sounding amplifier.

OOR on the other hand is all about blackness and dynamics. It’s more like a rock star, always pushing those dynamics higher, infusing a meaner attitude down low and a higher density in the midrange. It’s not the most linear amplifier, but it doesn’t care for that. There is definitely a little bit more bass definition and oomph and its midrange section felt meatier and livelier sounding to me. As a whole, it felt more engaging sounding, I knew that it was lying to me a little, but those lies were sweet and pleasing. We all know by now that Susvara is a difficult headphone to tame and while Volot was crafted as a do-it-all amplifier, never trying to mess the character of a headphone, OOR was better tailored for treble intensive and linear headphones like Susvara for example. Maybe it’s my system or my listening habits, but OOR felt as the closest sounding to TOTL integrated or power amplifiers whilst driving the Susvara. I sounds in such a way, that I decided moving the Benchmark AHB2 duo back into my living room, as I am getting similar dynamic swings from the OOR. I’m beyond impressed.

If you use IEMs with your desktop amplifiers, then OOR offers a slightly blacker background on its low gain position. Volot was extremely good in here, but OOR feels blacker and completely dead silent as if it’s powered off. With a higher power output, noise levels are also plunging a little higher, so this is a normal behavior.

All in all, I believe both units are offering a similar performance, but they have a different tonality and philosophy behind them. OOR + Hypsos wants to be fun and rebel, while Volot wants to be precise and correct in its power delivery. Being all-discrete units and offering a lot of power on tap, you’d be always getting an outstanding control of the drivers, while sounding effortless and huge on all axes. If you are getting listening fatigue or treble glare with your op-amp based headphone amplifier, then any of these two would be wiping that for good, never to be spotted again. It really comes down to personal preference and to the rest of your setup, but since I’m into all kinds of musical genres starting with classical and finishing with metal, then I guess I will be using them both.

B. Ferrum OOR + Hypsos ($3190) VS Benchmark Media HPA4 ($3000)

I find it interesting that HPA4 has absolutely the same footprint and dimensions to the Ferrum stack, as if Ferrum were paying homage to it. HPA4 is my trusty headphone amplifier that I’m using for more than two years, as there aren’t that many amplifiers that are offering me back two pairs of XLR inputs and additional pairs of RCA inputs. When I’m comparing multiple DACs, you can guess which amp makes my life much easier.

When it comes to user experience, nothing comes even remotely close to the HPA4. This is bar none the most advanced preamp and headphone amplifier that I’ve tried. Tapping on its huge screen unlocks an advanced graphical user interface where you can enable or disable any input or output, you can individually increase or decrease the gain of your source or of its internal line amplifier. It’s incredibly smart, but also honest and linear sounding at all times. For me, it’s the definition of a wire with gain headamp/preamp. At first, HPA4 looked as an expensive unit, but after finishing my HPA4 review and playing with it for more than a month, I decided purchasing a brand new HPA4 and that was the best decision I’ve made in 2019.

OOR together with Hypsos are also customizable, Hypsos adds an unmeasurable value to this package, that intelligent voltage adjustment is so addicting that I’m lowering or rising it when I’m swapping headphones, trying to find the best match. Build quality wise, HPA4 is slightly more impressive as it uses thicker milled aluminum panels and it never attracts a lot of attention due to its rudimentary industrial design. OOR and Hypsos on the other hand are looking more like polished gems to me with their bright lights and playful color scheme.

When it comes to features and I/O, HPA4 is ahead of its game, offering every possible input and output, plus an advanced GUI. You can tweak and tailor it to your needs, it can accept tiny or huge voltages. Its huge display and advanced UI are reminding of a modern smartphone, as every single feature can be enabled, tweaked or disabled by simple taps on its screen.

When it comes to sheer performance numbers, HPA4 will impress immediately with the lowest noise floor and distortion and with a super wide bandwidth of 0.01 Hz to over 500 kHz. However, when it comes to power output, HPA4 offers you back only 6 Watts in 16 Ohms and around 1.6 Watts in 60 Ohms and that’s 5 times less power output compared to the OOR and it really shows when Hifiman Susvara hops on my noggin. OOR on the other hand will impress with a fully discrete amp stage and with an over-engineered power supply. From an engineering standpoint, OOR and Hypsos are combining the old school with the new school, on top of that it’s still one of the most powerful headphone amplifiers out there. A maxed out HPA4 was equal to the 12 O’clock volume position on OOR, leaving a much bigger headroom for difficult loads as Susvara, HE6SE, LCD-5 and AB-1266 TC.

When it comes to important things as music reproduction, their masters had different philosophies behind their creations. I find them poles apart when it comes to tonality. HPA4 is all about killing the noise for good, about unleashing the highest levels of detail retrieval and transparency, while passing through your tracks like a Koenigsegg Jesko flies on Nürburgring. HPA4 is a lightning-fast amplifier and no matter how big, heavy or more expensive its competitors were, it was still winning the pole position.

On the other hand, the final kicks that are landing on my eardrums were heavily affected by HPA4’s power output. It could pound like Thor’s hammer with several headphones like Audeze LCD-4 and Kennerton Rognir, but it’s gentle and inoffensive with harder tasks as Susvara, seriously lacking nerve and dynamics. OOR is by a hair slower, but it always pounds relentlessly like putting you in between the hammer and the anvil. It’s bass output, kick and control needs to be heard to be believed. Not a single headphone calmed its mean attitude, not even the Hifiman Susvara and if you are dreaming of driving every single headphone, without getting into integrated and power amplifier territory, then OOR is so far the best headphone amplifier I’ve put my hands on.

If HPA4 always tries to disappear and never mess with the signal coming from the source, OOR was always infusing a higher energy down low and in the midrange, while removing glare, grain and digitus from my tunes. I could finish a few albums with OOR and I would still look enthralled and ready for more action, while HPA4 can tire me down after an album or two. HPA4 focuses on technicalities more than anything, like looking at your music through a magnifying glass, while OOR wants to relax you, show the beauty of your music from a different angle, boosting the textures and their weight, adding charisma and a fuller bodied tonality.

It’s clear as the blue sky that there is nothing in common between the two, neither is wrong or right, just different and I understand why you would choose one over the other. At the end of the day, after writing a few pages and filming a few video B-Rolls, I would always grab a glass of whisky, power on the Ferrum stack, lean back, have a quality time in the company of some the best musicians and smile as soon as play button is being pressed.


Horns high, a cigar in its mouth, strong spirit in its glass and smelling bad attitude from afar is how Ferrum OOR and Hypsos appeared to me. Dynamics and fun factor are its second names and this is really what makes them so unique sounding together. If you’re already saving for the OOR, then please consider adding the Hypsos down the road, as it’s The One that made it muscular and visceral sounding in the first place. It got a fantastic set of features, great connectivity, an amazing industrial design, a high-quality craftsmanship and it’s hard finding a needle in the haystack, when your body is filled with goose bumps. Although it has an eye watering price tag, no matter from what angle I’m looking, it just delivered every single time I pressed play and if you’re looking for a no-compromise solid-state amplifier, then you definitely came to the right place.

There is a lot of power to spare, you can drive anything out there and its noise floor was always in check. I didn’t spot compromises on their PCB boards, nor when music started playing and if you hunting for one of the best amps of the personal audio world, then please add the OOR and Hypsos to your short must-audition list. I’m extremely happy that headphone amplifier makers finally acknowledged our needs in driving the most sensitive and the hardest loads with a single component and this is where OOR and Hypsos are fitting in like a glove.

Considering its immaculate technicalities, its feature set and unique approach to music reproduction, it’s my pleasure awarding it our prestigious Gold Award. This is the second headphone amplifier to receive such an award and the only thing remaining to do is congratulating the Ferrum Audio team!

As a no-compromise combo you should prepare a pretty penny, as OOR will set you off €1995, HYPSOS another €1195 and you can get them directly from their web-store right here.

If you get the OOR or the full stack, please come back and leave a comment below. I’m curious to know how it works with your headphone collection.


  • One of the most gorgeous looking stacks I laid my eyes on
  • Rock-solid build quality, small footprint that works great in tight spaces
  • Amazing I/O options, neat looking display
  • Hypsos is probably the smartest power supply I’ve seen in a long time
  • Over-powerful, tightly controlled and effortless sounding at all times
  • Sounds unrestrained, spacious and layered
  • Excellent imaging and depth, offers a precise location of the notes
  • Fuller-bodied, lusher and warmer sounding to usual all-discrete amplifiers
  • Tilts toward a fun and rhythmic musical experience
  • A Dynamics and Transients monster! There isn’t a headphone it couldn’t push to its limits.
  • An extended frequency response at both ends
  • Noiseless performance via Low-gain, equality impressive in a stereo setup
  • Lacks any forms or brightness/treble glare
  • Accurate and detailed sounding, without adding over-sharpness and strong leading edges
  • A statement all-discrete headphone amplifier from any point of view


  • Sounds the best with Hypsos
  • Lacking a remote control to be used as a preamp in a stereo setup


  • DACs: Rockna Wavelight, Audiobyte HydraVox & HydraZap, Gold Note DS-10 Plus & PSU-10 Evo, SMSL VMV D2, SU-9N, Singxer SDA-6 PRO
  • DAPs: FiiO M17, M11 Plus, Shanling M8, Hiby RS6
  • Headphone Amps: Ferrum Audio OOR + Hypsos, Flux Lab Acoustics Volot, Benchmark HPA4, Burson Soloist 3X Grand Tourer, Musician Andromeda, Gustard H16 & others
  • Preamps: Benchmark HPA4, Topping PRE90
  • Power Amps: Benchmark AHB2 in bridged mode (x2), KECES S300, SMSL SA400
  • Loudspeakers: KEF Reference 3, Natural Sound NS-17
  • IEMs: FiiO FA9, FH9, FH7, 7Hz Timeless, Meze Rai Penta, Rai Solo, LittleDot Cu KIS, Hiby Crystal 6 & others
  • Portable headphones: Meze 99 Classics, Sennheiser Momentum 2
  • Full-sized headphones: Hifiman Susvara, HE1000SE, Arya Stealth, Audeze LCD-4, Erzetich Phobos V2021, Erzetich Mania, Kennerton Rognir, Magni, Gjallarhorn, Vali, M12S, Sendy Audio Peacock & AIVA, Apos Caspian & others
  • Interconnects: QED Reference (x2), Topping TCX1 (x2)
  • Speaker cables: Kimber PR8, Audioquest Type4
  • Power Cables: Isotek EVO3 Premier (x3)
  • Balanced Isolation Power Conditioners: PLiXiR Elite BAC1500, Elite BAC400

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