Luxman P-750u headphone amplifier Lab Report

Equipped with its three-stage Darlington output, the P-750u is rated by Luxman as if it were one of the brand’s specialty tube amps – 2x4W/8ohm – destined to drive a pair of high sensitivity horn loudspeakers. In practice this headphone amp really could raise as big a sound as the typical 300B tube amp for it delivers an even heftier 2x7.3W/8ohm (or 2.6W into our standard 25ohm headphone load) and all at very low distortion [see Graph 1]. The LECUA volume circuit [see sidebar] ensures there’s very little variation in distortion, S/N, output or response between the three sensitivity settings with ‘Low’, ‘Mid’ and ‘High’ offering gains of +2.9dB, +8.9dB and +15.1dB, respectively (balanced in/single-ended out). Power output, for example, is 2635mW/25ohm in ‘Low’ sensitivity mode and 2665mW/25ohm in ‘High’, while the A-wtd S/N ratio is 95.3dB (re. 0dBV). Also, while the P-750u will drive the toughest headphone loads, it also enjoys sufficiently low residual noise (–96.8dBV or 14.5µV) that the most sensitive of ’phones will remain free of hiss and hum.


Continuous power output versus distortion in ‘High Sensitivity’ mode (black, into 25ohm headphone load; red, into low impedance 8ohm load)


Distortion versus frequency from 5Hz-40kHz (black, 1V unloaded; red, 10mW into 25ohm load)

Furthermore, the P-750u’s output impedance remains so usefully low at ~500mohm that there will be little variation in system response with the varying load of different headphones – reaching down to –0.5dB points of 2Hz-32kHz whether loaded or unloaded. The same is true of distortion, already 10x lower than Luxman’s specification at 0.0005-0.009% from 20Hz-20kHz at 10mW/25ohm or 0dBV/47kohm [see Graph 2]. As further testament to the efficacy of the LECUA volume regime (and the dual-mono layout), channel balance is true to within 0.05dB over its top 70dB dynamic range and channel separation is >90dB re. 0dBV (20Hz-20kHz). PM

Maximum output (<1% THD into 47kohm): 8000mV
Maximum power output (<1% THD): 2.67W/25ohm / 7.33W/8ohm
Output Impedance (20Hz-20kHz): 400-500mohm
A-wtd S/N ratio (re. 0dBV): 95.3dB
Distortion (20Hz-20kHz, re. 10mW/0dBV): 0.0005-0.009%/0.0005-0.009%
Frequency resp. (20Hz-20kHz/100kHz): +0.0dB to –0.21dB/–4.2dB
Stereo separation (20Hz-20kHz): 71-102dB
Power consumption: 27W
Dimensions (WHD) / Weight: 440x92x400mm / 13.3kg
Price: £4000

Luxman Corp.
Supplied by: IAG (International Audio Group), UK
01480 447700