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Manufacturer: HEM Sp. z o.o.
Price (when reviewed): 1995 EUR (incl. VAT)

Contact: Al. Jerozolimskie 475
05-800 Pruszków | POLSKA



Provided for test by HEM


Images: Ferrum | Wojciech Pacuła

No 207

August 1, 2021


FERRUM is the newest Polish audio brand that has been established very recently. But the company behind it is HEM, one of the most experienced audio companies, that has been designing and manufacturing devices for the MYTEK brand for 20 years. The first Ferrum product was HYPSOS power supply. The second design is the OOR balanced headphone amplifier. We are the first audio magazine in the world to test it.

T THIS MOMENT, WHEN YOU’RE READING this test, the STATEMENT in High Fidelity award, awarded twice a year by the and "High Fidelity" magazines, has already been placed on a shelf at the HEM’s headquarters. We awarded it together with Dirk Sommer - the chief of HFS - and his team to the HYPSOS power supply by the Polish brand FERRUM (review LINK, HF | № 200 • December 16, 2020).

The brand, owned by HEM, formerly designing and manufacturing devices for MYTEK, is still at the start-up stage, and yet has managed to stir things up in the audio world - its large, full-page advertisements can even be found in the American magazine "Stereophile ". It turns out that innovation, combined with an exceptionally accurate artistic design and precise workmanship, result in a product that becomes a desirable point of reference for others.

By the way, let me also remind you that we published the first extensive interview with the owner of HEM, MARCIN HAMERLA and PAWEŁ GORGON, the head of their designer team, in which they talked about cooperation with the Mytek company - read HERE (HF | № 182 • June 1, 2019).


OOR IS THE SECOND IN BRAND’S HISTORY and (for now) the newest product of the Pruszków-based, HEM. It is a balanced, from input to output, solid-state headphone amplifier. The device chassis is exactly the same as the one of the Hypsos power supply, which saves money on a component that makes up a large part of the final price, and also allows the two products to be used together - more on this later.

The device offers two inputs - a balanced XLR and unbalanced RCA ones, "bypasses" that let the signal out, as well as two types of outputs - balanced with a 4-pin XLR socket and unbalanced one using a classic 6.35 mm TRS socket ("big jack"). On the front panel there is a comfortable to use volume knob - the volume is adjusted in analog domain, using a classic Alps resistance potentiometer, an input selector that in the middle position turns the device off, and a knob to adjust the gain in two steps - adding or reducing it (relative to 0 dB).

The amplifier is distinguished by its artistic design, thanks to the square Corten insert in which the nice-looking Ferrum logo, illuminated in white, has been laser-cut. The dimensions of the chassis are identical to those of the Brooklyn Bridge audio files player, manufactured by HEM for Mytek. The amplifier is powered by an external 24V DC switching power supply, but the Hypsos power supply can be used alternatively.


| A few simple words…

Founder, director

⸜ Max Matuszak – on the left – and Paweł Gorgoń, photo Marcin Zawadzki

THE MAIN DESIGNER OF OOR WAS MAX MATUSZAK. The signal amplifier circuit was supposed to be completely discrete, i.e. not to contain any integrated circuits (emphasis - ed.). Thanks to this approach, we were able to design a device that stands out on the market with its parameters.

There were three different versions of the project, and the schematic diagram was created using computer simulation software for electronic circuits. Then we made prototype boards, and then it was time to verify the operation of the systems in terms of compliance with the simulation results and listening tests that allowed us to select the best circuit. At this stage, we also eliminated a few bugs, which allowed us to move on to designing the first version of the OOR’s motherboard.

The power supply section is built similarly to the one of the HYPSOS. At the beginning we put a carefully designed pulse converter which works in spread spectrum mode. This stage has been decoupled by second order filters on both the input and output sides. Fast, low-noise LDO stabilizers are responsible for the final power supply filtration. To maximize the noise immunity, we used isolation of the signal ground using common-mode filters.

In order to ensure the best power supply stability, in addition to the standard DC socket OOR has also been equipped with a WEIPU socket that allows usage of the HYPSOS as external power supply via a Ferrum Power Link cable. This connection allows the internal HYPSOS controller to accurately control, using a four-wire measurement, the voltage inside the powered OOR.

⸜ HEM’s R&D team HEM (from the left): Arkadiusz Bochyński, Jarosław Jabłoński, Max Matuszak, Marcin Hamerla (sitting), Andrzej Dziwiński, Piotr Głudkowski, Paweł Gorgoń (sitting), photo Marcin Zawadzki

The signal path is completely symmetrical. The differential stage at the input allows you to connect both balanced and unbalanced signal source, with the latter being immediately balanced. The volume control is realized by a high-quality four-section ALPS potentiometer, which was manufactured to our order. The output transistors operate in a modified A / B class. To minimize harmonic distortions, the circuit maintains the transistors in the current source mode, which in the standard A / B class pass through a highly non-linear section of the characteristic until they are completely switched off.

The circuit uses precise bipolar compensation, thanks to which its frequency response in an open loop reaches as high as 60 kHz, and harmonic distortions are kept at a low level throughout the entire acoustic spectrum. Covering the whole with fast current feedback extends the frequency response to over 1 MHz at -3 dB and ensures accurate reproduction of a signal with steep slopes without introducing intermodulation distortions. MH


WE SHOULD ADD THAT THE NAMES OF Ferrum devices are - and will be - sourced from European languages. As Marcin Hamerla told me, "Ferrum is a European company from Pruszków", cooperating with many countries of the Old Continent and identification with the latter is important to us. The power supply took its name from ancient Greek, and it was also an abbreviation of HYbrid Power Supply. For the headphone amplifier, on the other hand we used a Dutch word - "oor", which means "ear".

The device features a steel chassis with an aluminum front and it sits on four tiny rubber feet - I'm sure you will want to replace them. The electronic circuit was assembled in the SMD technique (surface mount), together with bipolar transistors (two push-pull pairs per channel). All knobs are made of metal and the connectors are gold-plated. On the back there is a knob that controls the brightness of the "Fe" logo - it just so happens that the color of the backlight in the power supply and amplifier differs slightly in terms of the color temperature. The whole thing looks very solid and neat. The amplifier does not have a remote control.


⸤ HOW WE LISTENED The Oor amplifier was tested in the HIGH FIDELITY reference system, where I compared it to two tube devices: the Ayon Audio HE-3 headphone amplifier (SET with AA45 tubes, priced at PLN 17 900 - review HERE) and the Leben CS-600X integrated / headphone amplifier (push-pull using EL34 tubes, 27 900 PLN - review HERE). The music signal was delivered from the Ayon Audio CD-35 HF Edition SACD player. I used the Harmonix X-DC350M2R Improved-Version cable to power the Oor.

To make switching between amplifiers as fast as possible, I connected the Ferrum amplifier to the player's XLR output with Acoustic Revive Absolute cables and the reference amplifier I was using at any given moment using the same interconnect model but with RCA termination. The Oor worked in the optimal balanced mode, with the output also balanced, and the Leben and Ayon in the unbalanced mode, with unbalanced outputs.

I dedicated a separate listening session to the system consisting of the Oor amplifier powered by the Hypsos power supply. Both devices were placed on the top shelf of the Finite Elemente Master Reference Pagode Edition rack.

During the test, I used four very different pairs of headphones, namely:

  • HiFiMAN HE-1000 v2 – a planar magnetic design, impedance: 35 Ω, sensitivity: 90 dB/V SPL; review HERE,
  • Sennheiser HD800 – dynamic design, impedance: 300 Ω, sensitivity (measured): 103,77 dB/V SPL,
  • AKG K701 – dynamic design, impedance: 62 Ω, sensitivity (measured): 103,60 dB/V SPL; review HERE |PL|,
  • Beyerdynamic DT-990 Pro (first version, 600 Ω) – dynamic design, impedance: 600 Ω, sensitivity (measured): 97,56 dB/V SPL; review HERE |PL|.

The HiFiMAN and Sennheiser headphones featured after-market cables - the top model from the FORZA AUDIOWORKS, the Noir Hybrid HPC, with adapters for them also prepared by the same manufacturer, which allows me to test these headphones with all amplifier outputs.

Recordings used for the test | a selection

⸜ BILLIE HOLIDAY, Body and Soul, PolyGram/Mobile Fidelity UDCD 658, gold-CD (1957/1996).
⸜ DEAD CAN DANCE, Spiritchaser, 4AD/Beggars Japan WPCB-10078, „Audiophile Edition”, SACD/CD (1994/2008).
⸜ JOHN COLTRANE, Giant Steps, „Deluxe Edition 60 Years”, Atlantic/Warner Music Japan WPCR-18311/2, SD 1311, 2 x SHM-CD (1960/2008);
⸜ MARVIN GAYE, Let's Get It On, Tamla B0000935-36/Universal Music, Test Press SACD (1973/2003).
⸜ NEIL YOUNG, Le Noise, Reprise Records 525956-2, CD (2010).
⸜ PET SHOP BOYS, Hotspot, X2 Recordings/Sony Music labels SICX-148, CD (2020).
⸜ TAKEHIRO HONDA TRIO, This Is Honda, Trio Records PA-7005/Ultra-Vybe | Solid Records UVWA-0043 | CDSOL-1484, CD (1972/2020).
⸜ TAME IMPALA, Currents, Universal Music Australia/Hostess 4730676J, CD (2015);
⸜ THE BEATLES, The Capitol Albums Vol. 2: Sampler, Apple Records | Capitol Records DPRO-009463-59566-2-7, REFERENCE CD-R (2004).


I STARTED MY EXPERIENCE WITH THE OOR AMPLIFIER casually listening to the music while writing, processing photos, replying to e-mails and so on. Anyway, this is what I usually between "proper" listening sessions, browsing Tidal's resources, listening to new releases, to find out what is worth paying attention to and what I can let go of. The albums that I like the most I buy in physical form, usually on SACD and CD, and sometimes on LP.

This time, however, I immediately reached for physical albums, which I consider to be the best commercially available sound carriers, because I had a great respect for the Ferrum amplifier from the start. The reason was that the company's earlier product, the Hypsos power supply, proved to be a unique device, both technically and in terms of performance.

Was it a mistake? Should I have “cooled down” all emotions and then proceed with the listening session? After all, it is commonly believed that a critic, be it musical, culinary, artistic, or finally audio one - because that’s what specialist journalism related to evaluation is - should try to approach the assessed item as objectively and "coolly" as possible.

It’s just that a complete objectivity is impossible, and my experience shows that pursuing it at any cost can be downright harmful. Products related to any form of art require the human element in the evaluation, because they were created for people. The "technical" approach strips them of something that is associated with a more complete view, not a tunnel view of the product as a collection of components. So I got a bit emotional about the Oor test.

On the one hand, my expectations were high, and on the other, the chances that my Leben CS600 headphone tube amplifier would turn out to be a distant benchmark - just as strong. And it turned out that the difference in the class, despite the fact that the price differed significantly, was not that big as it could have been expected when looking at the price list - an interesting conclusion for me, both due to the way the signal is amplified in Polish amplifier - using transistors - its size and price. These are not the same amplifiers, they sound differently, but in some areas they are equivalent to each other.

The most important observation in relation to the Polish amplifier is that it sounds incredibly clear. It struck me right at the beginning, with the DEAD CAN DANCE’s Spiritchaser. Switching the headphones between the Oor and the Leben showed me that the tube amplifier from Japan adds some color to everything, turns it up and slightly warms it up. It is a wonderful sound, a wonderful presentation, and yet the Ferrum amplifier was closer to what was actually written on the disc, at least when it comes to the technical side of the recording.

This difference was best heard in the bass. Leben emphasizes and rounds this element a bit. The demanding planar headphones, such as the HiFiMAN HE-1000 v2, made it seem like the amp was just a tiny step away from distorting presentation. It never happened, even with the incredibly low bass opening in the Song Of The Stars, so without direct comparison to the Ferrum I wouldn't have paid any attention to it. However, when compared in A / B / A listening, with fast headphone switches between A and B, it showed the superiority of the Polish amplifier in this respect.

This device perfectly presents low sounds. There are no problems with their volume, saturation and timbre. Although the timbre seems slightly higher than in the CS600X, it is not because we get less bass, or because the midrange is lighter, but because the treble is more open and more precise. On the other hand, the midrange is not as saturated, so three-dimensional, as with both reference amplifiers. It's not bad, it is actually still very good, but one can hear these differences.

Despite the different tonal characteristics than that of the Leben, the Polish amplifier does not sound bright, lean or distinct. Listening to the Le Noise by NEIL YOUNG, an album digitally recorded in the RADAR hard drive system, I got really well saturated guitars, that were also very well differentiated. It was the same with the BILLIE HOLIDAY’s monophonic album Body and Soul, originally released in 1957. Her vocals were not downsized or withdrawn, I would even say that it was presented a bit closer than with the Leben.

| Our albums


Reprise Records 525956-2
Compact Disc (2010)

Le Noise IS THE 30TH ALBUM of the Canadian-American musician NEIL YOUNG. The album was released on September 28, 2010 by REPRISE RECORDS. Young asked DANIEL LENOIS to record this CD after watching the Black Dub project that Lanois had prepared with sound engineer Mark Howard. The album featured compositions played exclusively by Young, and while it was originally intended to be an acoustic album, it turned into something much more interesting.

After trying to record an acoustic version of the Hitchhiker song, written by the musician back in the 1970s, he decided to reach for an electric guitar, and finally recorded only two acoustic tracks. The basic tracks were recorded by Young, but Lanois and Howard added additional sounds to them, using the dub technique, ie by loading the sound over existing tracks; this method was developed for Black Dub.

The musician's unique guitar tone was achieved by processing the sound of the Gretsch White Falcon instrument with stereo pickups by two Fender Deluxe amps and processing it in the Eventide H3500 sub-harmonic tone generator (generated from the base sounds). Both the guitar and the vocals have long reverbs applied by Lexicon Prime Time and TC Electronics Fireworks.

As a result, an album with an amazing sound and aura was created. It has very low bass extension and great space. The effects in question were realized live, during the musician's "performance" in the studio - it can be seen on the film made at that time, available on DVD (and Blu-ray). The sound is excellent because it combines incredible power with intimacy - it is ultimately a solo performance of a musician with a guitar.

The recording was made in Daniel Lanois' home studio, spontaneously named Le Noise, in Los Angeles, California. The name of the studio is the distorted name of the producer La-noi. The recordings were made digitally using then new hard disk RADAR system. Musicians praised it for its very "analog" sound. This system was one of the first nonlinear recorders that allowed an easy editing - and was edited with the Pro Tools system.

The producer of the album was Daniel Lanois. Mark Howard is responsible for the recording and mixing, and Pretty Tony Mangurian & Florian Flo Ammon for the digital edition. Mastering was done - brilliantly! - by CHRIS BELLMAN in Bern Grundman Mastering. The album was never reissued. MH

LEBEN, THAT I TALKED ABOUT, like almost all other tube amplifiers, emphasizes recordings’ aura, slightly letting go of the sound definition. Aura - a sacred and once lost in music, impossible to make up for. This is why the system consisting of the Leben CS600Xi of the HiFiMAN HE-1000 v2 headphones was so addictive, so "enchanting" with the Young album. Oor approached it differently - it did not extinguish the aura, but it did not emphasize it either. Therefore, I had an excellent insight into the recording, without losing the musicality of the presentation. I paid for it only with a slight shallowing of the stage’s depth and its narrowing.

I do not know if you remember what JULEK SOJA, the owner of Soyaton, said during the 129th meeting of the Krakow Sonic Society when commenting on the re-edition of the album by Mobile Fidelity. As a reminder, he said that MoFi prepared the album in such a way as audiophiles imagined its sound, and not as it should sound (more HERE). Leben is a similar case - its warm, organic sound, although not entirely neutral and objective, is so pleasant that we forgive what it omits and smoothes out.

Oor is different. It sounds more like a studio device on the one hand, and on the other, like a high-end "home" audio device. It incredibly easy enters the recording, but without dividing everything into prime factors. Its sound is very, very coherent and smooth. It is also open - much more than that of the Leben. To some extent it resembles the sound of the headphone amplifier in the Mytek Brooklyn Bridge files player - and that's good, it came from the same team - but at the same time it is in a completely different place, as if the Mytek circuit was just an exercise.

Its resolution and differentiation ability are above average. Therefore, even though the treble and upper midrange are stronger in it than in the Leben, the sound is not too bright. Yes, the Leben presents it in an even smoother and warmer way, but ultimately it's about what we are looking for in music and what is our priority. For those for whom it is about information - Oor will be the only right choice. Even with a sound as dark and not fully open, as on the JOHN COLTRANE’s Giant Steps, the Polish amplifier kept moderation and did not try to force separation of all elements.

⸤ OOR +HYPSOS One of the options offered by devices powered using cheap external switching power supplies is a possibility of replacing them with higher quality ones. In the case of the Oor amplifier, the HYPSOS power supply seems to be the only right choice. Although during the test it did not have the settings for Oor loaded into its standard settings, choosing 24 V in the menu (this is the output voltage of the switched-mode power supply supplied with the amplifier) took only seconds and the system was ready to go.

Replacing the power supply significantly changes the sound of the tested device. First of all, it lowers it, bringing its timbre closer to the sound of the Leben CS600X, i.e. a fully tube amplifier (including its power supply section). This changes the tonal balance so much that we can talk about a more refined sound. Earlier it was not a particular problem, but listening to a tube amplifier on a daily basis, I got used to its characteristics. A quick switch to the solid-state Oor left a taste of lacking "something".

The Hypsos power supply made me realize what this "something" was - it was, it seems to me, not completely full, both in terms of volume and timbre differentiation, sound development by a Polish amplifier with a standard power supply. I am talking about a comparison to the more than twice as expensive amplifier from Japan and with regard to the (imagined) top of what can be achieved in audio at all. With the price of the Oor it won't matter, but it's worth knowing that it's not a perfect amplifier.

It get much closer to it with the Hypsos power supply. Saturation, tempering the upper midrange, but without losing the sound definition, are the most important advantages of this upgrade. Therefore, ultimately, I cannot imagine Oor without the Hypsos power supply by its side. At least if we are talking about the most expensive headphones. With good dynamic models, such as Sennheiser HD600, AKG K701, or the top models of Beyerdynamic, it won't matter so much. However, with expensive planar headphones it will be an important change.


FERRUM'S ACTIVITY started with a bang - the Hypsos power supply is one of the best devices of this type on the market and certainly one of the most technically and functionally interesting. Also in audio, however, the principle of a "second album" applies. In the review of the MELA KOTELUK album, posted on the portal, we read:

Explanation: the second album is always the hardest. The expectations after the debut are so huge that most artists are not able to meet them. Meanwhile, they had lost their aura of novelty. That is why so many "second albums" greatly inhibit the careers of many performers.

⸜ MICHAŁ MICHALAK, „Ta Mela to może fajna jest". Wywiad z Melą Koteluk, August 10th 2013; accessed: 23.06.2021.

On the other hand, in an interview for, Michał Stemplowski and Wojtek Kałuża from the J.D. Overdrive said that they didn't think about the "curse" and maybe that's why their second album is better than the previous one. And besides, "luck favors the brave" (more HERE; accessed: June 23, 2021). As they add, such a curse only threatens when "Your first album was a mega-ultra-spectacular success".

The case of Ferrum is different - although its first product WAS a great success (four Hypsos power supplies are working at, it turned out to be an introduction to the actual product, which is the Oor headphone amplifier. We get a well-made, nice-looking, professional-sounding product that can be used at home, in the company of a great signal source, as well as in a recording studio.

Its sound is clear, resolving and well-balanced tonally. But it is the Hypsos that gives it the final touch, making its presentation similar to what I know from the Leben CS600X amplifier. Oor would be closest to the Ayon Audio HE-3 amplifier, also a tube one. And although it is not that saturated and does not have almost unlimited resolution, specific to SET amplifiers, there is nothing to be ashamed of.

For the money it is - next to the Japanese, also semiconductor Aurorasound HEADA - the best headphone amplifier I know (the battery powered Bakoon HPA-21 was great as well). Viva la Hypsos!


Reference system 2021

1) Loudspeakers: HARBETH M40.1 |REVIEW|
2) Line preamplifier: AYON AUDIO Spheris III Linestage |REVIEW|
3) Super Audio CD Player: AYON AUDIO CD-35 HF Edition No. 01/50 |REVIEW|
4) Stands (loudspeakers): ACOUSTIC REVIVE (custom) |ABOUT|
5) Power amplifier: SOULUTION 710
6) Loudspeaker filter: SPEC REAL-SOUND PROCESSOR RSP-AZ9EX (prototype) |REVIEW|
7) Hi-Fi rack: FINITE ELEMENTE Pagode Edition |ABOUT|


Analog interconnect SACD Player - Line preamplifier: SILTECH Triple Crown (1 m) |ABOUT|
Analog interconnect Line preamplifier - Power amplifier: ACOUSTIC REVIVE RCA-1.0 Absolute-FM (1 m) |REVIEW|
Speaker cable: SILTECH Triple Crown (2.5 m) |ABOUT|

AC Power

Power cable | Mains Power Distribution Block - SACD Player: SILTECH Triple Crown
Power (2 m) |ARTICLE|
Power cable | Mains Power Distribution Block - Line preamplifier - ACOUSTIC REVIVE
Power Reference Triple-C (2 m) |REVIEW|
Power cable | Mains Power Distribution Block - Power amplifier - ACROLINK Mexcel 7N-PC9500 |ARTICLE|
Power cable | Power Receptacle - Mains Power Distribution Block: ACROLINK Mexcel 7N-PC9500 (2 m) |ARTICLE|
Power Receptacle: Acoustic Revive RTP-4eu ULTIMATE |REVIEW|
Anti-vibration platform under Acoustic Revive RTP-4eu ULTIMATE: Asura QUALITY RECOVERY SYSTEM Level 1 |REVIEW|
Power Supply Conditioner: Acoustic Revive RPC-1 |REVIEW|
Power Supply Conditioner: Acoustic Revive RAS-14 Triple-C |REVIEW|
Passive filter EMI/RFI: VERICTUM Block |REVIEW|


Speaker stands: ACOUSTIC REVIVE (custom)
Hi-Fi rack: FINITE ELEMENTE Pagode Edition |ABOUT|
Anti-vibration platforms: ACOUSTIC REVIVE RAF-48H |ARTICLE|

  • HARMONIX TU-666M "BeauTone" MILLION MAESTRO 20th Anniversary Edition |REVIEW|


Phono preamplifier: Phono cartridges: Tonearm (12"): Reed 3P |REVIEW|

Clamp: PATHE WINGS Titanium PW-Ti 770 | Limited Edition

Record mats:


Headphone amplifier: AYON AUDIO HA-3 |REVIEW|

Headphones: Headphone Cables: Forza AudioWorks NOIR HYBRID HPC