Headphones

All inputs are eventually routed via Naim's choice of PCM1791 DAC, including the analogue RCAs which run through a PCM1803A ADC at 48kHz/24-bit. This limits the frequency response to –3dB/22kHz while all other 88.2kHz+ digital inputs are limited to –3dB/27kHz courtesy of Naim's custom IIR digital filter solution. The DAC/preamp path offers a modest 104dB A-wtd S/N ratio but excellent ±0.1dB linearity over a 100dB dynamic range, low distortion that falls to 0.0002-0.001% over the top 30dB of its range [20Hz-20kHz, Graph 2] and exceptionally low <15psec jitter with all input sample rates.

Note that the ADC clips with analogue inputs above 2.9V and that the maximum balanced line output is 4.75V via analogue or full-scale digital inputs. Gain is a higher +12.5dB via the headphone output and so its maximum output is 7.1V with a 0dBFs digital input (clipping at volume '92') or 7.8V via the analogue input. The maximum single-ended power outputs [see Graph 1] are 89mW/600ohm (voltage limit), 1190mW/32ohm and 2225mW/8ohm (current limit), all perfectly in line with Naim's rated 1500mW/16ohm. The output impedance is not vanishingly low at 5.2ohm, representing a signal loss of some 1.2dB/32ohm, but this and any possible response variations, will be minimal into higher, 600ohm headphone loads. Moreover, the –93dBV (22µV) unwtd residual noise suggests the Uniti Atom will serve high sensitivity 'phones with usefully quiet backgrounds. Finally, headphone distortion actually reduces under load here from 0.006% at 0dBV/unloaded to 0.005%/30mW/32ohm through the midrange, albeit with a slight increase from 0.0075% to 0.01%, respectively, at 20kHz. PM