Naim Audio ND 555/555PS DR Network-Attached DAC Lab Report

Lab Report

Although it would have been insightful to compare the test results for this flagship ND 555 with those for the NDX 2 [HFN Sep '18] and see just what gains the uprated PSU, mechanical isolation and analogue stage may have wrought, any such distinctions are entirely clouded by Naim's choice of alternate DAC chips: the NDX 2 employs the Burr-Brown PCM1792A while the ND 555 is fitted with the PCM1704U. Although both units benefit from Naim's '40-bit' 16x upsampling/jitter-suppressing DSP, the key measurable advantage is realised in the ND 555 as a near-total elimination of correlated jitter [see Graph 2, below]. Otherwise, and like the NDX 2, the fixed analogue output offers a maximum 2.23V from a low 17ohm source impedance (increasing to 170ohm at 20Hz), while the A-wtd S/N ratio is reduced by about 1dB to 108.1dB. Although distortion at spot frequencies and levels is not dissimilar to that of the NDX 2 – with 0.0008-0.006% at 1kHz and 0.002-0.0009% at 20kHz over the top 30dB of its dynamic range – the pattern of THD vs. level is distinctive to the PCM1704 DAC [see Graph 1, below].


Distortion versus 48kHz/24-bit digital signal level over a 120dB dynamic range via S/PDIF/USB-A/network (1kHz, black; 20kHz, blue)


High resolution 24-bit jitter spectrum via S/PDIF/USB/network (48kHz, black; 96kHz, red)

Naim's combined digital/analogue filter acts earlier than typical, providing reduced attenuation of stopband artefacts immediately adjacent to the top-end of 48kHz music media (just –43.6dB at 26kHz re. 22kHz) and restricting the response of both 96kHz and 192kHz digital audio to a –3dB point of 27kHz. Lower 44.1kHz/48kHz sample rates feature a (subjectively insignificant) –0.65dB roll-off at 20kHz while Naim's dual-mono analogue stage layout ensures that channel separation is >100dB through bass and midrange frequencies. PM

Maximum output level / Impedance 2.23Vrms / 17-170ohm
A-wtd S/N ratio (S/PDIF / USB) 108.1dB / 108.0dB
Distortion (1kHz, 0dBFs/–30dBFs) 0.00078% / 0.0063%
Distortion & Noise (20kHz, 0dBFs/–30dBFs) 0.0022% / 0.0009%
Freq. resp. (20Hz-20kHz/30kHz) +0.0 to –0.65dB/–15.9dB
Digital jitter (48kHz / 96kHz) <5psec / <10psec
Resolution @ –110dB (S/PDIF / USB) ±0.2dB / ±0.6dB
Power consumption 20W (1W standby)
Dimensions (WHD) / Weight (ND 555) 432x87x314mm / 14kg
Prices £12,999 (DAC)/£6599 (PSU)
Naim Audio Ltd
Supplied by: Naim Audio Ltd
01722 426600