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.

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

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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

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