Sidebar 3: Measurements
Like the Hegel Music Systems Mohican and Bryston BCD-3, which we reviewed in the May and August issues, the Naim CX5 XS has no digital inputs of any kind. This limited my measurement of its technical behavior to using 16-bit test files burned to a CD-R. I tested the Naim with my Audio Precision SYS2722 system (see the January 2008 "As We See It").
The Naim's error correction was adequate—no interruptions were apparent in the player's output until the single gaps in the data spiral on the Pierre Verany Digital Test CD reached 0.5mm in length, when there were occasional glitches. (The Compact Disc standard, the so-called "Red Book," requires that a player cope with gaps of up to 0.2mm.) The CD5 XS had problems playing some of my older CD-Rs, on which I had burned test-signal files, there being dropouts in its output. The maximum output level from its unbalanced outputs was 2.08V, and the outputs preserved absolute polarity (ie, were non-inverting). The output impedance was a very low 1.5 ohms at 20 and 1kHz, rising to 391 ohms at 20Hz, presumably due to the presence of a series capacitor in the signal path.
Fig.1 shows the CD5 XS's impulse response; it's typical of a linear-phase reconstruction filter, with symmetrical ringing either side of the single sample at 0dBFS. Tested with white noise sampled at 44.1kHz, the CD5 XS's output rolled off quickly above 20kHz (fig.2, red and magenta traces), and had almost reached full attenuation by the Nyquist frequency, 22.05kHz (fig.2, vertical green line). The aliased image at 25kHz of a full-scale 19.1kHz tone (blue, cyan) is suppressed by 85dB. The distortion harmonics of that tone are visible at 38.2 and 57.3kHz, at respectively –70dB (0.03%) and –90dBFS (0.003%). The blue and red traces in fig.3 show the Naim's audioband response taken with spot tones; it is flat to 15kHz, and reveals excellent channel matching. The cyan and magenta traces in this graph show the response with preemphasized data. Unlike the Hegel and Bryston players, the deemphasized response is virtually identical to that with regular data.
The spectrum of a full-scale 50Hz tone into 600 ohms (fig.7) indicates that the subjectively innocuous second harmonic is the highest in level, at –64dB (0.06%). Into the kinder 100k ohm load, this harmonic and the third dropped to below –90dB (0.003%). When tested for intermodulation distortion with an equal mix of 19 and 20kHz tones, the resultant spectrum (fig.8) looks hashy in the audioband, but this is an artifact of the 16-bit encoding. Actual intermodulation products are very low in level even into 600 ohms, the second-order difference product at 1kHz lying close to –100dB (0.001%).
Finally, when I tested the Naim CD5 XS with 16-bit J-Test data, the two channels differed in how they handled the signal. In the right channel (fig.9, red trace), the spectral spike that represents the high-level tone at exactly one-quarter the sample rate is well defined, and the odd-order harmonics of the LSB-level, low-frequency squarewave are close to the correct levels (sloping green line). However, the left channel (blue trace) features significant smearing of the 11.025kHz spike, which I assume is due to random low-frequency jitter. This smearing can also be seen in fig.8—something is not working correctly in the left channel's circuitry, perhaps an issue with this sample of the Burr-Brown PCM1704 chip.
Fig.1 Naim CD5 XS, impulse response (one sample at 0dBFS, 4ms time window).
Fig.2 Naim CD5 XS, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with CD data (20dB/vertical div.).
Fig.3 Naim CD5 XS, frequency response at –12dBFS into 100k ohms, without deemphasis (left channel blue, right red) and with deemphasis (left cyan, right magenta) (0.6dB/vertical div.).
Channel separation (not shown) was superb, at >110dB in both directions from 20Hz to 4kHz, and still 90dB at 20kHz. The analog noise floor (fig.4) was low in level, but with some low-level power-supply–related artifacts at 120Hz and its harmonics present. With dithered data representing a 1kHz tone at –90dBFS (fig.5), the graph actually shows the spectrum of the dither noise used to encode the 16-bit test signal. With undithered data representing a 1kHz tone at exactly –90.31dBFS, the three DC voltage levels described by the data are well defined, but some high-frequency noise can be seen (fig.6).
Fig.4 Naim CD5 XS, spectrum of 1kHz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.5 Naim CD5 XS, spectrum with noise and spuriae of dithered 16-bit, 1kHz tone at –90dBFS (left channel blue, right red) (20dB/vertical div.).
Fig.6 Naim CD5 XS, waveform of undithered 16-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red).
Fig.7 Naim CD5 XS, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).
Fig.8 Naim CD5 XS, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).
Fig.9 Naim CD5 XS, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: CD data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
This Naim CD player's measured performance is overall acceptable, but its error correction is less tolerant of CD problems than other modern players that I have tested. This surprised me. In a conversation I had 40 years ago with Naim's founder, the late Julian Vereker, who had raced saloon cars in the 1970s, he emphasized that a racer can't win if he doesn't finish. For a CD player, the equivalent of finishing a race is making sure all disc errors, no matter how severe, are corrected.—John Atkinson
