8W headphone amplifier with discrete power stage

The Ferrum OOR is an all-analogue headphone amplifier with a fully-balanced signal path and a proprietary discrete power stage. It delivers up to 8W into 60Ω through its balanced output, with three gain settings to accommodate headphones with different sensitivity requirements. XLR and RCA inputs, balanced and single-ended headphone outputs.

An all-analogue signal path

The OOR keeps the audio signal entirely analogue, without signal processing. Its signal path remains fully-balanced whether you connect a source through the XLR or RCA inputs. This lets you use either connection while retaining the amplifier’s balanced internal architecture.

Discrete amplification with controlled transistor bias

Ferrum’s proprietary discrete power stage uses circuitry that keeps both output transistors conducting. Maintaining this bias helps reduce distortion associated with the transistors switching off and on. The design supports the amplifier’s low-distortion performance and its ability to respond to changing headphone demands.

Up to 8W of balanced output power

The balanced 4-pin XLR output delivers 8W into 60Ω and 1.6W into 300Ω. The single-ended 1/4-inch (6.35mm) output provides 2W into 60Ω and 400mW into 300Ω. A headphone output impedance below 0.3Ω helps limit the amplifier’s influence on a headphone’s frequency response as its impedance changes.

Three gain settings for different headphones

A front-panel gain control lets you match the amplifier’s voltage gain to your headphones. Balanced operation offers -4dB, +6dB, and +16dB settings, while single-ended operation offers -10dB, 0dB, and +10dB. Lower gain gives sensitive headphones a more useful volume adjustment range, while higher gain accommodates headphones that require a larger voltage swing.

Designed to pair with HYPSOS

The OOR operates with its supplied AC/DC power adapter and also supports the Ferrum HYPSOS power supply. The proprietary Ferrum Power Link connection allows HYPSOS to perform its 4TSD voltage-sensing measurements deeper within OOR, helping it monitor the voltage delivered to the amplifier. HYPSOS is an optional addition.

Direct controls in a compact chassis

The front panel provides a gain knob, volume knob, and input selector for choosing between XLR and RCA sources. Corten steel and anodized aluminum give the chassis its distinctive finish. Measuring 8.6 inches wide (21.7cm), OOR fits comfortably alongside a DAC on a desk or equipment shelf.

Product highlights

• All-analogue headphone amplification without signal processing
  
• Fully-balanced internal signal path with XLR or RCA sources
  
• Proprietary discrete power amplifier technology
  
• Output transistor bias circuitry designed to reduce distortion
  
• Up to 8W into 60Ω through the balanced headphone output
  
• Three selectable gain settings
  
• Balanced 4-pin XLR and single-ended 1/4-inch (6.35mm) headphone outputs
  
• Headphone output impedance below 0.3Ω
  
• Compatible with the optional Ferrum HYPSOS power supply
  
• Corten steel and anodized aluminum chassis

Specs

• Amplifier architecture: Fully-balanced, proprietary discrete power amplifier
  
• Signal path: All analogue
  
• Analogue inputs: XLR and RCA
  
• Headphone outputs: Balanced 4-pin XLR and single-ended 1/4-inch (6.35mm)
  
• Balanced gain settings: -4dB, +6dB, +16dB
  
• Single-ended gain settings: -10dB, 0dB, +10dB
  
• Balanced output power: 8W into 60Ω; 1.6W into 300Ω
  
• Single-ended output power: 2W into 60Ω; 400mW into 300Ω
  
• Frequency response: 20Hz–100kHz, 0.1dB
  
• THD: Below 0.0003% / -110dB at 100mW into 50Ω through the balanced output
  
• Input impedance: 94kΩ
  
• Headphone output impedance: Below 0.3Ω
  
• Single-ended preamp output impedance: 22Ω
  
• Balanced preamp output impedance: 44Ω
  
• Power connections: Center-positive 0.10-inch (2.5mm) DC connector and proprietary 4-pin DC connector
  
• Idle power consumption: Below 15W
  
• Power adapter: 100/240V AC to 22–30V DC
  
• Dimensions (W × D × H): 8.6 × 8.1 × 2.0 inches (21.7 × 20.6 × 5cm)
  
• Weight: 3.97lb (1.8kg)

What’s included

• Ferrum OOR headphone amplifier
  
• AC/DC power adapter