CD Player/Transport Reviews

I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system to measure the Simaudio Moon Evolution 650D (see www.ap.com and the January 2008 "As We See It"); for some tests, I also used my vintage Audio Precision System One Dual Domain and the Miller Audio Research Jitter Analyzer. As well as playing CDs containing test-tone files, I drove the Moon Evolution 650D with S/PDIF data from the Audio Precision analyzers and USB data from a MacBook running Mac OS10.6.8 and Pure Music 1.8.

The Moon 650D's S/PDIF inputs successfully locked to datastreams with sample rates of up to 192kHz and passed 24-bit data. Tested with the Apple USB Prober utility, the Simaudio's USB input was identified as "USB Audio DAC" from "Burr Brown from TI," and was limited to 16-bit data with sample rates of 32, 44.1, and 48kHz.

The maximum output of the 650D was 2.1V RMS from both its balanced and unbalanced outputs, both of which preserve absolute polarity (ie, are non-inverting). (The XLRs are wired with pin 2 hot.) The output impedance was to specification, at 100 ohms from the unbalanced outputs and 200 ohms from the balanced. With CD playback, the Simaudio's error correction/concealment was superb, there being no audible glitches or error flags in its digital output with the Pierre Verany Test CD until the gaps in the data spiral reached 2mm in length.

The Moon 650D's frequency response conformed to the same basic shape at all sample rates, other than rolling off very sharply just below half the sample rate (fig.1). Note the superb channel matching in this graph. Channel separation (not shown) was superb, at >125dB in both directions below 1kHz, and still 115dB at 20kHz.

Fig.1 Simaudio Moon Evolution 650D, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), 192kHz (left blue, right red). (0.25dB/vertical div.)

For consistency with my measurements of digital products, I assess a product's resolution by sweeping a 1/3-octave bandpass filter from 20kHz to 20Hz while it decodes dithered data representing a 1kHz tone at –90dBFS. The top pair of traces in fig.2 were taken with 16-bit data; the spectrum is dominated by the dither noise. Increasing the word length to 24 bits (middle traces) drops the noise floor in the midrange and treble by 20dB, implying resolution of almost 20 bits. This is close to the state of the art, and easily enough to allow the Simaudio to readily resolve a tone at –120dBFS (bottom traces). FFT analysis (fig.3) confirms this superb resolution, with a very low noise floor below the –150dBFS limit of this graph and a complete absence of harmonic or power-supply–related artifacts.

Fig.2 Simaudio Moon Evolution 650D, 1/3-octave spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS, with: 16-bit data (top), 24-bit data (middle), dithered 1kHz tone at –120dBFS with 24-bit data (bottom). (Right channel dashed.)

Fig.3 Simaudio Moon Evolution 650D, FFT-derived spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS, with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red).

Linearity error with 16-bit data (fig.4) was negligible to below –105dBFS, and the combination of ultra-low noise and superb linearity readily allowed the Moon Evolution 650D to resolve with perfect waveform symmetry the three DC voltage levels described by undithered 16-bit data at precisely –90.31dBFS (fig.5). Extending the word length to 24 bits gave a superb sinewave (fig.6). Some other DACs that I have tested recently have suffered from noise modulation; ie, their noise floors varied with the level of the signal. The degree of this modulation with the Simaudio player was of the same order of the residual changes due to the Audio Precision's automatic input gain-ranging (fig.7).

Fig.4 Simaudio Moon Evolution 650D, linearity error, 16-bit data (2dB/vertical div., dBr vs dBFS).

Fig.5 Simaudio Moon Evolution 650D, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

Fig.6 Simaudio Moon Evolution 650D, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

Fig.7 Simaudio Moon Evolution 650D, FFT-derived spectrum with noise and spuriae of dithered 1kHz tone at –40, –60, and –90dBFS.

Harmonic distortion was very low, with the third harmonic highest in level (fig.8), though at –110dBFS the word highest is relative. The 650D's very low noise floor did allow some higher-order harmonics to be seen. This graph was taken into the benign 100k ohm load; dropping the impedance to 600 ohms didn't change the picture at all! The same was true for the demanding high-frequency intermodulation test (fig.9). Even into 600 ohms, all the intermodulation and aliasing products were at or below –120dBFS (0.0001%)!

Fig.8 Simaudio Moon Evolution 650D, spectrum of 1kHz sinewave, DC–10kHz, at –0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fig.9 Simaudio Moon Evolution 650D, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).

Tested for jitter rejection via its S/PDIF input, the spectrum of the 650D's output decoding a 16-bit version of the Miller-Dunn J-Test data showed only the residual harmonics of the LSB-level low-frequency squarewave (fig.10, cyan and magenta traces). The Miller Analyzer estimated the jitter level at just 69 picoseconds peak–peak, which is really below the limit of its resolution. With 24-bit J-Test data (fig.8, blue and red traces), all that can be seen in the spectrum is the central peak representing the 11.025kHz tone and the random noise floor. With 16-bit USB data (fig.11) and CD data (not shown), again, all that can be seen in the spectrum are the residual harmonics of the low-frequency squarewave. This is state-of-the-art jitter rejection.

Fig.10 Simaudio Moon Evolution 650D, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data via 15' TosLink S/PDIF from AP SYS2722 (left channel cyan, right magenta), 24-bit data (left blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Fig.11 Simaudio Moon Evolution 650D, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit data via USB from MacBook (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Reading through what I have just written, I see that I have used the adjective superb six times and the phrase state of the art twice. That pretty much sums up the Simaudio Moon Evolution 650D's measured performance. It's hard to see how it could be any better!—John Atkinson