Headphones

While the Nirvana gets full marks for build and finish quality, Auris Audio could do better with its specification – the ambiguous 'output power of 6.5W pure Class A' trotted out by every Internet blogger without clarification. So here it comes... In practice the Nirvana's output power varies with frequency, with transformer tap selection, with load and, most importantly, with distortion. Selecting its preferred 32ohm tap and driving an 'average' 25ohm load from its single-ended (6.35mm) output, power increases along with distortion to reach 205mW at 1% THD to 705mW at 2% THD and up to 3.16W at 5% THD [see back trace, Graph 1 below]. Into a low 8ohm load – we have reviewed high-end earphones with lower impedance – the trend is maintained [see red trace, Graph 1] but at lower power still: 42mW at 1% THD, 179mW at 2% and 760mW at 5% THD. Using the 600ohm tap into a 25ohm load yields just 3.5mW at 1% distortion, but into a high impedance load up to 24.1V is possible at the same 1% THD. Versus frequency, distortion is highest through the bass – 1.1%/20Hz vs. 0.45%/1kHz at 10mW/25ohm [see Graph 2].

The practical source impedance(s) of the 32/80/150/300 and 600ohm taps are 24/55/130/165 and 245ohm, respectively, and the best-case response is flat to within 15Hz-24kHz (±1dB). The sub-bass and extreme treble are sensibly rolled-off. Residual noise can be a real issue for headphone preamps and the Nirvana is not the quietest at –69dBV (unweighted), leading to an A-wtd S/N of 81dB (re. 10mW/25ohm). Overall gain is a sensible +14dB (XLR in/jack out), so if the distortion is not subjectively troublesome then lower sensitivity 'phones might be a better bet here. Finally, in case you were wondering, '0dB' on those front meters equals 2.85V into a high (line-level) load. PM