Earlier this year, Finn Corvin Gallowsky and I introduced you to Ferrum's first-ever product: the Hypsos power supply, which provides voltages ranging from five to 30 volts and won us over with its flexibility and sonic impact on connected devices. The headphone amplifier OOR is Ferrum's second product.

By the way, our enthusiasm for the universally applicable power supply - it covers a very wide voltage and power range - led to the consequence that we conferred the Hypsos a Statement-in-High-Fidelity award in cooperation with our Polish colleagues. Until now we haven’t yet dispatched the award, because we hope to get the chance to hand it over personally later this year and to seize this opportunity to take a closer look at Ferrum's new premises. That is to say, that HEM and Ferrum boss Marcin Hamerla and his team, which now consists of more than 20 people, have moved to a larger company building in Pruszków near Warsaw since our last visit seven years ago. And this is where Ferrum's products are now primarily developed and manufactured.

The designer responsible for the OOR, Maksymilian Matuszak, explained in a Skype conference and by means of a technical white paper made available in advance to Hifistatement what makes the OOR stand out of the crowd according to his opinion: "First of all, the OOR has a unique discrete output stage. It is a hybrid Class A and AB concept. The special circuitry never allows either of the two output transistors, which are polarized similarly to a Class AB arrangement, to turn off. The current of both transistors is monitored by independent circuits that prevent the current from dropping below a certain threshold. In this way, distortion is much lower because the transistors do not go through the most non-linear part of their characteristic curve. This circuit is quite different from the well-known "Sliding Bias" and was therefore named "Gapless Current Transistor Bias" or "GCTB" for short by Ferrum.

This output stage is further linearized by the feedback loop of a discrete current feedback amplifier, which has a bandwidth of about 60 kilohertz at minus three decibels with open loop and almost one megahertz at minus three decibels with closed loop. The bandwidth of 60 kilohertz in the open feedback loop was achieved by a two-pole compensation method. This flattens the distortion in the audio spectrum as the open-loop gain drops off at higher frequencies compared to the typical Miller compensation. On the contrary, the large open-loop bandwidth mentioned does not exist in headphone amplifiers incorporating ICs.

In addition, the internal power supply is Hypsos' technological legacy. It features low-noise switching regulators with two-stage input and output filtering, followed by linear low-drop voltage regulators (LDO) with a fast transient response. For better channel separation, there are separate LDOs for both channels. Furthermore, the OOR features two DC inputs: a classic 2.5/5.5-millimeter DC jack plug and a 4-pin WEIPU plug, the same as on the Hypsos. To allow the use of the second option when purchasing a Hypsos, Ferrum includes with it a special power link cable with four-pin WEIPU connectors on both sides. When a normal cable is used, the "4-Terminal Sensing Design" (4TSD) feature in the Hypsos measures the voltage at the cable output, whereas with the FPL cable, the voltage is measured directly on the OOR’s main board. This allows the Hypsos to further stabilize the voltage in the current path.

There are eight independent main power rails in the OOR. For the amplifiers in the output stage, there are independent ±20 volt rails for both channels, based on low-noise, fast, linear low-drop voltage regulators. For the preamplifier stage, there are ±15-volt rails, also independent for both channels and based on linear LDOs. The ±20 volt rails of the LDOs are fed by switching converters that come up with two-stage filters at the input and output. The voltage at the output of the switching converters is approximately ±21.5 volts. Finally, there is an additional power rail just for the front LED, which is powered directly from the input power supply. The OOR features a six-layer circuit board. It has two layers just for the grounding surfaces. There are three groundings on the OOR: Signal ground, switching converter ground, and input power supply ground. These groundings and the power rails are separated by common mode chokes to further attenuate the high frequency common mode noise provenient from switching converters and the input power supply.

The input stage of the OOR is realized with carefully selected, fast operational amplifiers featuring a special external compensation. Within this section, the adjustment of the gain and by means of a four-channel ALPS potentiometer also the volume can be performed, as the entire signal path is completely designed as a balanced circuitry. It is containing neither transformers nor coils. Unbalanced input signals are converted into balanced signals by the input amplifier section. Before reaching the unbalanced RCA output, the signal is converted back to single-ended.

The output power amplifiers are – as aforementioned – of completely discrete design and operate using low-noise bipolar transistors. There are four of these amplifiers, two for each channel, as is required for a balanced output topology. These amplifiers are DC-coupled, and there is a DC servo that features a three-decibel cutoff frequency at about 0.1 hertz. The available output power into 32 ohms is four watts at the 6.35-millimeter jack output, and 5.5 watts at the balanced four-pin XLR output. At 600 ohms, 200 and 800 milliwatts are available, respectively. The output impedance here is less than 0.3 ohms, regardless of load, while at the RCA preamp outputs, it's 22 ohms, and at the XLR connectors, 44 ohms.

Since the OOR undoubtedly belongs to the group of stationary headphone amplifiers due to the mains power it requires, I connect it therefore to the balanced outputs of Chord Electronics' DAVE and in a first glance to the supplied external 24-volt switching power supply. The OOR spends the approximately 40-hour break-in period - partly powered by its own power supply, partly by the Hypsos - with the Audeze LCD-X that I received back from repair. Since I've been working with the incredibly well-sounding and opulently equipped Weiss DAC502 with its special equalization for the LCD-X, I am nevertheless aware that the magnetostat isn't exactly blessed with an ultra-linear frequency response without it. However, it really offers a lot of fun paired to the OOR on the rather randomly selected album Changing Places performed by the Tord Gustavson Trio. On "Deep As Love", I especially like the accurate differentiation of double bass, grand piano, drums and cymbals. The transient and dying-out oscillation of single tones or chords is immensely detailed to follow without distracting the listener from the musical content. The inner dynamics of individual sound events are rendered in an equally remarkable way. "Graceful Touch" begins with a quiet intro on the drums, and in the process you get a very credible impression of the imaginary recording space. Generally speaking, the instruments don't stick on a flat plane between the ears, and although the OOR doesn't come with a cross-feed circuitry, the sonic image doesn't seem to be factitiously widened out: I really wouldn’t mind to become a headphone aficionado!

This impressive sound experience is not only feasible in conjunction with the Audeze: With the SendyAudio Peacock, I enjoy the first songs of the album at least as intensively - but in a slightly less spectacular way, being tonally somewhat more balanced and coherent. The three-stage gain control was initially set to the middle position, but already in combination with the LCD-X I reduced the gain by ten decibels, in order to allow the potentiometer to be now used in the range between eleven and one o'clock instead of between eight and ten o'clock. All headphones used are operated in balanced mode, since the OOR is conceived balanced throughout as well.

The change from the SendyAudio Peacock to the significantly cheaper Aiva is easy to bear after having the level adjusted, yet no serious differences in quality come to light - at least that's my first impression. After a longer listening and some toggling between the two headphones, it is noticeable that the spatial image during the drum intro seems to be larger with the Peacock in play, at the same time giving the double bass a bit more volume. Even though the OOR incites the headphones to sonic excellence, it does not equalize the differences between them.

So I commit myself to the Peacock again and enjoy the airy and dynamically exciting playback of Shostakovich's Symphony No. 15 by fully immerging in warm timbres. Although I don't find fault with the sonic performance in the slightest, I connect the OOR to the Hypsos instead of the supplied external switching power supply, which of course lists the company's own headphone amplifier already in its shortlist. Therefore a few movements of the combined rotary and push control knob are sufficient to set the correct voltage and polarity for the OOR: Thanks to the much more elaborate power supply, even more air surrounds the instrument groups, the spatial information appears to be more precise, and the fortissimo passages gain a bit more of dramatic impact. In comparison, I rate the added sonic value offered by the Hypsos a bit higher than that when swapping from the SendyAudio Aiva to the Peacock.

Needless to say, that I give the initial two tracks of Changing Places also a comparative listen via the big SendyAudio on the SPL Phonitor x, whereby its matrix circuitry stays deactivated for the time being: Dynamically, the Phonitor is even a little more convincing, but the subtleties in the transient and dying-out oscillation of individual tones become clearer through the OOR. It is almost impossible for me to assign a hi-fi criterion to what fascinates me so much about the Ferrum’s performance. In all individual disciplines, the headphone amplifiers perform at a similar level, but there is something delicate, tingling about the way the OOR presents itself sonically, which the neutral, rational sounding SPL is lacking. As might be expected, these differences are all the more pronounced when the OOR is powered by the Hypsos. Even when listening to Symphony No. 15, the Polish duo stays my favourite: The musicians seem to be playing in a slightly larger, airier room. With the matrix circuit influencing the sound of the Phonitor, the imaging doesn't get any more spacious, but to my ears it sounds a touch more coherent. Nevertheless, when it's not exactly about large-scale orchestral music, I simply enjoy the Hypsos and OOR combo more.

Not to give the OOR an edge, but simply because I can't stop listening to one of my latest discoveries on Qobuz, Houston Person and Ron Carter's lyrical duo album Remember Love, I start off with "My One And Only Love" with the Peacock connected to the Phonitor, and then repeat it with Hypsos and OOR taking care of the amplification. Even with the SPL, you can get infatuated by the old familiar, but excitingly interpreted melody. However, the Ferrum combo does this a bit better thanks to the airier performance and an emotionally stronger approach. Yes, my previous enormous appreciation for OOR and his fellow mate still remains unchanged with this track.

Eventually, I relocate the two Ferrum reps into the living room, where they are required to demonstrate their mutual capabilities as a preamplifier unit in comparison with the Brinkmann Marconi. While listening to one of my test tracks twice through the Marconi, I am once again intrigued how much better this system sounds since a Hypsos powers the Auralic Aries Femto. I am even more staggered when the OOR takes over the preamplification in conjunction with a second Hypsos. Now the system in the living room catches up yet another step closer to the one in the study: The midrange sounds much more differentiated and open, while the playback has improved dynamically a bit as well. Perhaps I should have the Marconi somewhat dusted off by the manufacturer after almost 20 years of continuous operation. In any case, the OOR and its partner cut an excellent figure as a preamplifier.