This is the most powerful and, arguably, the most capable 'analogue-only' headphone amp we've measured so far. Rated at 1400mW (1.4W) into 32ohm, the HA-501 achieved 2215mW (2.2W) into our 25ohm test load with a maximum output voltage capability in excess of 10V into 47kohm. The full +8.5dB gain is more than sufficient and there's enough headroom to accommodate inputs up to 6V via its balanced XLR connections. The MUSES 8920 op-amp(s) used to drive the headphone output are very robust and levels of feedback are not excessive so any increase in distortion versus level is gradual (0.001%/1mW; 0.003%/100mW; 0.025%/1W) under load rather than abrupt at clipping [see Graph 1, below]. Moreover, distortion versus frequency has been engineered to remain very consistent under load (0.0015-0.0018%, 20Hz-20kHz at 40mW into 25ohm), presenting a uniform 'colour' [see shaded red trace, Graph 2, below].
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