Naim CD555 CD player Measurements
I tested the Naim CD555 from its RCA jacks. The maximum output level was 2.04V RMS and the player preserved absolute polarity; ie, was non-inverting. The error correction was good, though not in the same league as the best players I have measured; the CD555 produced audible glitches when the gaps in the data spiral on the Pierre Verany test CD reached 1mm in length. The Naim ran very slight fast, with a positive frequency error of 286 parts per million (0.03%), though this is not enough to be audible as a pitch error.
The player's output impedance was very low at midrange and high frequencies: 17 ohms at 1kHz and 28 ohms at 20kHz. However, its impedance at 20Hz was 378 ohms, presumably due to the use of an output coupling capacitor. Though this is still a low source impedance in absolute terms, the contrast between it and the lower impedance at higher frequencies will result in premature bass rolloff if the CD555 is used with a preamplifier of very low input impedance. This can be seen in the player's frequency response when tested into the punishingly low 600 ohms load (fig.1, bottom pair of traces below 50Hz), where the output reached –3dB at 24Hz. Into 100k ohms, however, the player's output was flat to 10Hz (fig.1, top traces)—and even into 8k ohms, which is about the lowest impedance the player will ever see in real-world systems, the bass response extended to 20Hz.
The response with pre-emphasized data (fig.1, bottom traces above 50Hz) was flat, a contrast to the earlier Naim CD5 player, which had a mid-treble depression in its de-emphasis curves. (The CD5x was okay in this regard.) Channel separation (not shown) was excellent at better than 90dB in the midrange and bass, but decreased to an okay 62dB at 20kHz, due to capacitive coupling between the channels.
Fig.1 Naim CD555, frequency response at –12dBFS into (from top to bottom at 100Hz): 100k ohms and 600 ohms without de-emphasis, 100k ohms with de-emphasis (right channel dashed, 0.5dB/vertical div.).
While the Naim CD555 uses Burr-Brown's PCM 1704 DAC chips, now rather long in the tooth, its performance in the digital domain was superb—or at least better than the intrinsic resolution of the CD medium. The noise floor shown in the 1/3-octave spectral analysis of the player's output while it decoded data representing a dithered tone at –90dBFS (fig.2) is merely that of the dither noise recorded on the test CD, as is the apparent linearity error below –100dBFS (fig.3).
Fig.2 Naim CD555, 1/3-octave spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS, 16-bit data (right channel dashed).
Fig.3 Naim CD555, left-channel departure from linearity, 16-bit data (2dB/vertical div.).
Repeating the spectral analysis with a wider (200kHz) bandwidth and playing back data representing a –1LSB DC offset is shown in fig.4. This graph unmasks some power-supply components—60Hz in both channels, 120Hz in the right channel—though the fact that these are at 130dB below full-scale means that they are, in effect, nonexistent. Fig.4 also reveals that the Naim does not have the rapidly increasing amount of noise at ultrasonic frequencies that results from the extreme amount of noiseshaping used in contemporary DAC chips. Even so, the CD555 accurately reproduces the waveform of a 1kHz sinewave recorded without dither at exactly –90.31dBFS (fig.5). The three discrete DC voltage levels are clearly resolved and the waveform is symmetrical, although a very slight DC offset is evident at around 20µV.
Fig.4 Naim CD555, 1/3-octave spectrum with noise and spuriae of –1LSB, 16-bit data (right channel dashed).
Fig.5 Naim CD555, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data.
The distortion introduced with a full-scale 1kHz tone was virtually negligible, at 0.0009% left and 0.0007% right (both figures are the true sums of the harmonics). An FFT-derived spectrum of the CD555's output into a fairly low impedance, 8k ohms (fig.6), indicates that the benign second harmonic is the highest in level, at just 104dB, though the fifth and sixth harmonics (circled) are also evident. Dropping the impedance to 600 ohms increased the level of the third harmonic to –86dB (0.005%), which, though still very low, reinforces the impression given by the response measurements that the Naim player should not be used with preamplifiers having an input impedance much below 3k ohms or so. Going back to an 8k ohm load and reducing the signal level to –90dBFS produced a spectrum with the harmonic components at the level of the background noise (fig.7).
Fig.6 Naim CD555, spectrum of 1kHz sinewave at 0dBFS into 8k ohms (linear frequency scale).
Fig.7 Naim CD555, spectrum of 1kHz sinewave at –90dBFS into 8k ohms (linear frequency scale).
Intermodulation distortion was also vanishingly low, the 1kHz difference component resulting from an equal mix of 19kHz and 20kHz tones, each at –6dBFS, lay at 0.0007% left and 0.0011% right (fig.8).
Fig.8 Naim CD555, HF intermodulation spectrum, 19+20kHz at 0dBFS peak into 8k ohms (linear frequency scale).
The Naim CD555 performed well when it came to rejecting word-clock jitter, the Miller Jitter Analyzer indicating just 245 picoseconds peak–peak of jitter. Data-related jitter sidebands (fig.9, red numeric markers) were almost at the residual level of the test signal, most of the jitter coming from sidebands of unknown origin at ±1345Hz (purple "6") and ±1460Hz (purple "8").
Fig.9 Naim CD555, high-resolution jitter spectrum of analog output signal (11.025kHz at –6dBFS sampled at 44.1kHz with LSB toggled at 229Hz), 16-bit CD data. Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Other than indicating that it should not be used with those rare preamplifiers that have input impedances of 1k ohm or less, the Naim CD555's measured performance is beyond reproach.—John Atkinson