Ferrum OOR Headphone Preamp Lab Report

Lab Report

Although the topology of the OOR is shared between the line/preamp output and headphone output, the latter does have its own output buffer that drops the (line) 22ohm source impedance down to a sub-1ohm level. This, in turn, ensures there's a minimal 0.14dB signal loss into a 32ohm headphone load with a similar resilience over the system (amp/'phone) frequency response. The OOR's native response is flat to within a tight +0.02dB from 1Hz-25kHz, rising slightly thereafter to +1.0dB/100kHz, while the A-wtd S/N is a robust 98.5dB (re. 0dBV) via the balanced line or headphone output, the latter showing no change whether unloaded or across 32ohm. Distortion is influenced by loading, however, from a minimum of 0.00014-0.00085% via the balanced line outs (20Hz-20kHz, re. 0dBV) to 0.00025-0.105% at 10mW/32ohm via the unbalanced headphone output [red trace, Graph 2 below].

Three levels of gain are offered via the front panel, the '–dB' setting equal to –4.25dB, '0dB' is +5.6dB and '+dB' is +15.7dB (all balanced in/balanced out, so you can knock 6dB off for a single-ended RCA connection). With an input headroom in excess of 12V, these gain settings have no impact on the maximum output of the OOR which is 24V (balanced XLR) and 12V (RCA and unloaded SE headphone out). Under load, the headphone buffer will sustain 242mW/600ohm and 2600mW/32ohm (2.6W) with sufficient current to support a little over 1W (1090mW) into very low 8ohm loads [see Graph 1]. There's a steady increase in distortion when driving lower loads from 0.00015%/10mW/600ohm to 0.0005%/10mW/32ohm and 0.002%/10mW/8ohm [black, orange, red traces, respectively], although these figures are orders of magnitude below that of a typical headphone. PM


Power output vs. THD into 600ohm (black), 32ohm (orange) and low 8ohm (red) headphone loads


Distortion versus frequency (preamp, black; headphone 0.6V unloaded, green; 10mW/32ohm, red)

Maximum output (<1% THD) 24V (XLR, pre)/ 12V (SE, headph)
Power output (<1% THD, 600/32/8ohm) 242mW / 2600mW / 1090mW
Output Impedance (20Hz-20kHz) 663-667mohm (22ohm, preamp)
A-wtd S/N ratio (re. 0dBV/32ohm) 98.5dB / 98.5dB
Distortion (20Hz-20kHz, 0dBV) 0.00014–0.00085% (preamp)
Distortion (20Hz-20kHz, 10mW/32ohm) 0.00025–0.105% (headphone)
Freq. resp. (20Hz-20kHz/100kHz) +0.02 to –0.02dB/+1.0dB
Power consumption 4W (1W standby)
Dimensions (WHD) / Weight 217x50x206mm / 1.8kg
Price £1795

Supplied by: Signature Audio Systems
07738 007776