The NAD's digital inputs preserved absolute polarity at the balanced and unbalanced outputs but inverted polarity at the headphone output. With the volume control set to its maximum, the M66's output level with a full-scale 1kHz tone was 5.28V balanced, 2.64V unbalanced, and 7.93V headphone. The M66's DAC offers a well-managed gain architecture.
Fig.12 shows the M66's impulse response with network data sampled at 44.1kHz. The filter is a minimum-phase type with all the ringing following the single sample at 0dBFS. The magenta and red traces in fig.13 show the filter's ultrasonic rolloff with 44.1kHz white noise data at –4dBFS. They reach full stop-band attenuation just above half the sample rate (this indicated by the vertical green line), with the aliased image at 25kHz of a full-scale tone at 19.1kHz (cyan, blue) completely suppressed. (Peculiarly, when I repeated this test with TosLink data, the aliased image at 25kHz was suppressed by 70dB rather than completely—see fig.18—even though with optical data the white noise spectrum and impulse response were identical.) The frequency response with 44.1kHz, 96kHz, and 192kHz data is flat in the audioband, with a sharp rolloff just below half of each sample rate (fig.14).
An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS, dropped the M66's noisefloor by 33dB (fig.15). This implies a measured resolution of 22 bits, one of the highest I have found. When I played undithered data representing a tone at exactly –90.31dBFS (footnote 1), the waveform was symmetrical, with negligible DC offset. The three DC voltage levels described by the data were clearly defined (fig.16). With undithered 24-bit data (fig.17), the M66 output a superbly clean sinewave.
As it had with the analog inputs, the NAD's digital inputs produced very low levels of distortion. Fig.13 showed that the harmonics associated with the 19.1kHz tone all lay below –100dB. Intermodulation distortion with optical data representing an equal mix of 19 and 20kHz tones, each at –6dBFS, was also very low (fig.18).
Fig.19 shows the spectrum of the M66's output when it was fed 16-bit J-Test data via Roon. All the odd-order harmonics of the undithered low-frequency, LSB-level squarewave lie at the correct levels, though the central spike that represents the high-level tone at one-quarter the sample rate (Fs/4) is broadened at its base. This implies the presence of low-frequency random jitter, though at a low level. Repeating this analysis with 24-bit J-Test data via my network and with 16- and 24-bit S/PDIF J-Test data gave identical results (fig.20).
The appropriate word to describe the measured performance of the NAD M66's line, phono, and digital inputs is "superb." The company has some serious audio engineering talent in-house.—John Atkinson
Footnote 1: I have been asked why I use this particular undithered signal. In the twos-complement encoding used by 16-bit digital audio, –1 least significant bit (LSB) is represented by 1111 1111 1111 1111, digital zero by 0000 0000 0000 0000, and +1 LSB by 0000 0000 0000 0001. If the waveform is symmetrical, this indicates that changing all 16 bits in the digital word gives exactly the same change in the analog output level as changing just the LSB.
