Naim CDX CD player Measurements

Sidebar 3: Measurements

I measured the CDX with the XPS, as that was how Michael preferred to use it. The maximum output level at 1kHz was 2.11V, just under 0.5dB above the CD standard of 2V. The analog output polarity was absolute-phase correct; ie, noninverting. The source impedance was extremely low: 3 ohms at 1kHz, 2 ohms at 20kHz. At 20Hz, however, it had risen to 680 ohms. The matching preamplifier input impedance had best have an input impedance of 10k ohms if the player's low bass is not to sound a little leaned-out.

The Naim's frequency response into 100k ohms (fig.1, top traces) was flat throughout most of the audio band, drooping a little in the top octave to -0.7dB at 20kHz. When the CDX/XPS played pre-emphasized discs (fig.1, bottom traces), the high-frequency output held up a little more, but the overall de-emphasis was still very small. The channel balance was excellent at better than 0.05dB, while separation was also superb, at better than 110dB below 1kHz (L-R) and 100dB (R-L). Above 1kHz, the crosstalk started to rise due to capacitive coupling between the channels, but was still below -80dB at 20kHz.

Fig.1 Naim CDX, frequency response at 0dBFS (top), and with de-emphasis (bottom). (Right channel dashed, 0.5dB/vertical div.)

Fig.2 shows a spectral analysis of the Naim's output while it decoded data representing a dithered 1kHz tone at -90dBFS. The noise floor is at the 16-bit level, suggesting superb linearity and noise performance. This is also seen in fig.3, the spectral analysis of the Naim's output while it decoded a special signal consisting of a DC offset of 1 LSB. A very slight peak can be seen at the AC mains frequency of 60Hz—this is still at -124dBFS—perhaps due to the fact that the XPS power supply was placed under the CDX chassis for these measurements.

Fig.2 Naim CDX, spectrum of dithered 1kHz tone at -90.31dBFS, with noise and spuriae, 16-bit data (right channel dashed).

Fig.3 Naim CDX, spectrum of -1 LSB with noise and spuriae; 16-bit data. (1/3-octave analysis, right channel dashed.)

Linearity error (fig.4) was very low down to below -110dBFS, while the Naim's reproduction of an undithered tone at -90.31dBFS, which should consist of three discrete voltage levels, was essentially perfect (fig.5).

Fig.4 Naim CDX, left-channel departure from linearity (2dB/vertical div.).

Fig.5 Naim CDX, waveform of undithered 1kHz sinewave at -90.31dBFS (16-bit data).

Into 100k ohms, the Naim offered very low distortion. (Fig.6 shows the spectrum of the player's output reproducing a full-scale 50Hz tone.) But when the load was dropped to a punishing 600 ohms—remember, the CDX's source impedance rises toward 600 ohms in the bass—a regular series of harmonics was produced (fig.7). But the fact that these are still relatively low in order and decrease with increasing harmonic order mitigates against their being audible. The demanding high-frequency twin-tone intermodulation test (fig.8) revealed very low levels of spurious products.

Fig.6 Naim CDX, spectrum, DC-1kHz, 50Hz at 0dBFS, 100k ohm load (linear frequency scale, 20dB/vertical div.).

Fig.7 Naim CDX, spectrum, DC-1kHz, 50Hz at 0dBFS, 600 ohm load (linear frequency scale, 20dB/vertical div.).

Fig.8 Naim CDX, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 0dBFS, 100k ohm load (linear frequency scale, 20dB/vertical div.).

I used the Miller Audio Research analyzer (which, I believe, Naim also uses) to examine the CDX's jitter. Fig.9 shows a high-resolution spectral analysis of the Naim's analog output while it played a special test signal consisting of a high-level 11kHz tone overlaid by the LSB toggling at 229Hz. The measured jitter level was a very low 155 picoseconds peak-peak, this increasing very slightly when the CDX was used without the XPS. The sidebands indicated with red numeric markers in fig.9 are spaced at 229Hz intervals, revealing them to be related to the signal data. All are below -120dBFS except for the lower fundamental sideband, which contributes 63.5ps to the total. The peak marked with a blue "4" is a spurious tone with an absolute frequency of 10,715Hz. I have no idea what it is due to. Just visible at the base of the central peak are two low-level, low-frequency sidebands at ±15.6Hz, each marked with a purple "1." These seem ubiquitous in players that use the HDCD chip.

Fig.9 Naim CDX, high-resolution jitter spectrum of analog output signal (11kHz at -6dBFS with LSB toggled at 229Hz). Center frequency of trace, 11kHz; frequency range, ±3.5kHz.

Like those of other Naim CD players we've measured, the CDX's word clock runs slightly fast, at a measured 227ppm. While this is still very low in absolute terms, I can't help but wonder if it contributes to the CDX's well-developed sense of pace.

When Naim's Julian Vereker drove racing cars, his philosophy was, "You can't win if you don't finish the race." Similarly, he appears to feel that a CD player can't make music if it can't retrieve the data from the disc. The Naim coped extremely well with disc errors, tracking up through the Pierre Verany test CD's track 33 (which has 1.5mm gaps in its data) without dropouts or muting.—John Atkinson

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