The Ferrum OOR is a reference-class headphone amplifier engineered with the same obsessive attention to detail that earned Ferrum its EISA award pedigree. Built around a unique discrete output stage, a fully balanced signal path, and a proprietary internal power supply derived from the acclaimed HYPSOS, OOR raises the bar for what intimate, analogue listening through your favourite headphones can sound like.
A Unique Output Stage
OOR's output stage is unlike anything found in a typical headphone amplifier. Rather than a conventional Class A or Class AB design, Ferrum's engineers developed a proprietary topology that sits between the two. Independent monitoring circuits prevent either output transistor from ever fully switching off, keeping both away from the most non-linear region of their characteristics. The result is dramatically lower distortion than a standard Class AB design, without the heat and inefficiency of full Class A operation.
This output stage is further linearised by a discrete current feedback amplifier with approximately 60 kHz of open-loop bandwidth (-3 dB) and almost 1 MHz of closed-loop bandwidth (-3 dB). This wide open-loop bandwidth — achieved through a two-pole compensation scheme — keeps distortion unusually flat across the entire audio spectrum, a characteristic simply not achievable with IC-based headphone amplifiers.
Fully Balanced Throughout
The entire signal path in OOR is fully balanced from input to output, with no transformers or inductors in the signal path. The input stage — an internal preamplifier handling gain and volume — converts single-ended signals to balanced internally, so even RCA sources benefit from fully balanced amplification. The signal is only converted back to single-ended at the unbalanced outputs themselves, preserving maximum signal integrity throughout.
Internal Power Supply
OOR's internal power supply is the technological heir of HYPSOS. Low-noise switching regulators with double-stage input and output filtering feed fast transient response linear LDOs — with separate LDOs for each channel for improved channel separation. Eight independent power rails in total ensure that analogue and digital sections never interfere with one another.
OOR also features two DC power inputs: a standard 5.5/2.5 mm DC jack and a proprietary 4-pin connector compatible with Ferrum's HYPSOS. When connected via the dedicated FPL (Ferrum Power Link) cable, HYPSOS measures voltage directly on OOR's motherboard rather than at the cable output — allowing it to stabilise voltage even further along the power path for a meaningful uplift in performance.
Key Features
Unique discrete output stage
A proprietary topology between Class A and Class AB, with independent transistor current monitoring for dramatically reduced distortion across the audio spectrum.
Discrete current feedback amplifier
~60 kHz open-loop bandwidth and ~1 MHz closed-loop bandwidth via two-pole compensation — delivering unusually flat distortion performance not found in IC amplifiers.
Fully balanced signal path
No transformers or inductors in the signal path. Single-ended inputs are converted to balanced internally, with conversion back to single-ended only at the RCA outputs.
HYPSOS-derived internal power supply
Eight independent power rails with separate LDOs per channel, low-noise switching regulators, and double-stage filtering for a supremely quiet noise floor.
Made for HYPSOS
Connect via the proprietary FPL cable and HYPSOS performs 4T voltage sensing directly on OOR's motherboard, squeezing every last drop of performance from both components.
Industrial design
Designed by a specialist industrial design studio in Warsaw, with custom-designed volume knobs and gain switches refined through extensive prototyping for optimum ergonomics and visual cohesion. The signature lacquered corten steel logo ties OOR into the wider Ferrum ecosystem.
Why Choose OOR?
- Proprietary discrete output stage — unlike any IC-based headphone amplifier on the market
- ~60 kHz open-loop bandwidth for flat distortion across the full audio spectrum
- Fully balanced signal path from input to output, with no transformers or inductors
- 8 W balanced output power into 60 Ω — drives virtually any headphone with ease
- Eight independent internal power rails with separate LDOs per channel
- Two DC power inputs — compatible with included supply or HYPSOS for elevated performance
- Six-layer PCB with two dedicated ground planes and common mode chokes for EMI attenuation
- EISA award-winning pedigree — engineered by the team behind HYPSOS and WANDLA
About Ferrum
Designed and engineered by Ferrum / HEM in Warsaw, Poland. Ferrum was founded in early 2020 with a clear mandate: combine sustainability, durability, and quality into compact, attractive packages — focused entirely on delivering the best possible audio experience at an affordable price. Built on over 20 years of digital audio expertise and a history of collaboration with Mytek Digital on world-class DAC technology, Ferrum brings unmatched engineering pedigree to every product. OOR raised the bar for intimate, analogue headphone listening — and it remains one of the finest headphone amplifiers available at any price.
Specifications
| Operation | Fully balanced, proprietary discrete power amp technology |
| Gain | Balanced: -4 dB, +6 dB, +16 dB Single ended: -10 dB, 0 dB, +10 dB |
| Inputs | XLR, RCA, 2.5 mm DC connector centre positive, proprietary 4-pin DC connector |
| Outputs | Balanced 4-pin XLR, 6.35 mm jack |
| Frequency response | 20 Hz – 100 kHz (+/- 0.1 dB) |
| Output power (single ended) | 400 mW into 300 Ω; 2 W into 60 Ω |
| Output power (balanced) | 1,600 mW into 300 Ω; 8 W into 60 Ω |
| Input impedance | 94 kΩ |
| Output impedance (single ended) | 22 Ω on pre-amp |
| Output impedance (balanced) | 44 Ω on pre-amp |
| Output impedance (headphones) | < 0.3 Ω |
| Power consumption | Idle < 15 W |
| Power adapter | 100–240 VAC to 22–30 VDC |
| Dimensions | 21.7 cm × 20.6 cm × 5 cm / 8.6″ × 8.1″ × 2.0″ (W × D × H) |