Musical Fidelity V-DAC II D/A processor Measurements
Sidebar 2: Measurements
To measure the V-DAC II (serial no. SDY0367), I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (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 analyzer.
Unlike the original V-DAC's S/PDIF input, which was limited to 96kHz, the V-DAC II's input operates at sample rates from 32kHz up to a maximum of 192kHz. The Macintosh USB Prober utility identified the product as the "Musical Fidelity V-DAC 24/96," but no serial number. USB Prober confirmed that the V-DAC II did operate in isochronous asynchronous mode, as claimed, and that the USB input would handle 16- and 24-bit data with sample rates of 32, 44.1, 48, 88.2, and 96kHz.
The maximum output level from both of the V-DAC II's single-ended outputs was 2.24V, 1dB higher than the CD standard's 2V and the original V-DAC's 2.08V. If not compensated for, this difference will work in favor of the new products in side-by-side comparisons with the Mk.1 V-DAC. The Musical Fidelity preserved absolute polarity (ie, was non-inverting). Its output impedance was higher than the original version's, but was still low at 99 ohms at high and mid frequencies, and at 124 ohms in the low bass.
Fig.1 shows the V-DAC II's frequency response with sample rates ranging from 32kHz (yellow and gray traces) to 192kHz (blue and red). The response at each sample rate follows the same basic pattern: a gentle rolloff followed by a sharp drop-off just below the Nyquist frequency (ie, half the sample rate). Unlike the original version of Musical Fidelity's more expensive M1DAC, the response with 192kHz sampling does extend an octave higher in frequency than at 96kHz. Channel separation (fig.2) was superb, at >110dB in the treble and midrange, but did decrease in the bass, reaching 100dB at 20Hz. This is presumably due to the increasing source impedance in this region of the MF's tiny wall-wart power supply.
Whether measured using a swept 1/3-octave bandpass filter (fig.3) or with a modern FFT technique (fig.4), the increase in bit depth from 16 to 24 resulted in a 20dB reduction in the level of the V-DAC II's noise floor, suggesting that this inexpensive device has a resolution of around 19 bits. However, the lowering of the noise with 24-bit data (blue and red traces) does unmask some static tones just under 8kHz. Admittedly, these are at a very low level, but their presence does suggest some mathematical limitations in the Musical Fidelity's digital signal processing. Fig.4 was taken with S/PDIF data; repeating the test with USB data gave an identical result, confirming that the V-DAC II correctly handled 24-bit data via its USB input.
Unlike some processors that use a sample-rateconverter chip as a jitter filter, the V-DAC II suffered very little from noise modulation. Fig.5 shows the spectra of its output while it reproduced a 1kHz tone at 0, 60, and 90dBFS. While there is a slight rise in the noise floor as the signal level increases, this is mild, though AC-supply components at or below a still-very-low 130dBFS can be seen in the left channel. With its low noise and very low linearity error (fig.6), the V-DAC II's reproduction of an undithered 16-bit tone at exactly 90dBFS was essentially perfect (fig.7). Not only are the three DC voltages representing this signal clearly resolved, so is the Gibbs Phenomenon "ringing" on the tops and bottoms of the waveform. This waveform also indicates that the V-DAC uses a conventional time-symmetrical FIR reconstruction filter rather than, for example, a minimum-phase or apodizing filter that, all things being equal, tend to give a sound that listeners prefer. Extending the bit depth to 24 gave a well-defined sinewave (fig.8), despite the very low signal level.
Although the V-DAC II did produce some high-order harmonics with a full-scale signal, these were all very low in level, even with the extreme 600 ohm load (fig.9). The subjectively innocuous second harmonic was the highest in levelmore so in the left channel (blue) than the right (red)but was still inconsequential in absolute terms. The picture was similarly excellent on the demanding high-frequency intermodulation test (fig.10), with the second-order difference component lying at 96dBFS (0.0015%).
The V-DAC II effectively rejected word-clock jitter on all its inputs. With 16-bit data (not shown) there was no accentuation of the odd-order harmonics of the J-Test's low-frequency squarewave. No jitter-related sidebands can be seen with 24-bit data via the S/PDIF or USB input (fig.11), though the idle tones just below 8kHz in fig.2 that I mentioned above are present.
The Musical Fidelity V-DAC II may be affordably priced, but its measured performance is almost beyond reproach. Extraordinary!John Atkinson