Digital Processor Reviews

I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system to measure the Musical Fidelity M1CLiC (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, and first updated the M1CLiC's firmware.

Looking first at the M1CLiC's performance as an analog preamplifier, the input impedance was usefully high at close to 100k ohms at low and middle frequencies, dropping inconsequentially to 48k ohms at 20kHz. The analog input preserved absolute polarity (ie, was non-inverting), and the gain from the fixed-output jacks was just under unity. The maximum gain from the variable outputs was 11.85dB. The source impedance from the fixed-level outputs was a still low 146 ohms at high and middle frequencies, rising slightly to 171 ohms at 20Hz, presumably due to a series coupling capacitor. The impedance from the variable jacks was 47 ohms at high and middle frequencies, rising to 76 ohms at 20Hz.

The analog frequency response was extremely wide, the output at 200kHz dropping by just 2.6dB (fig.1). Note the superb channel matching in this graph. Channel separation via the analog inputs was >110dB below 400Hz, decreasing to a good 77dB at 20kHz (not shown). THD+noise at 4V into 100k ohms from the fixed outputs—at least twice as high as the Musical Fidelity will be asked to deliver in practical use—was less than 0.003% below 4kHz, rising to a still-low 0.015% at 20kHz. The output clipped at a very high 9.7V into 100k ohms, again much higher than will be required in real life. Some electronic volume controls will overload with even moderately high input levels. However, with the M1CLiC's control set to unity gain, the preamp circuit didn't overload until the input level was 9.785V, well above what is necessary.

Turning to the digital performance, the maximum level at 1kHz was 2.016V from the fixed outputs and 8V from the variable outputs, the latter below the level at which the analog stage clips. The TosLink input locked to S/PDIF data with sample rates up to 192kHz, and the coaxial input locked to 176.4 and 192kHz data. Apple's USB Prober utility identified the M1CLiC as "USB Audio DAC" from "Burr-Brown from TI," and confirmed that the rear-panel computer USB input was restricted to 16-bita data with sample rates of 32, 44.1, and 48kHz, operating in the usual isochronous adaptive mode. The rear-panel iPod input takes the data in digital form, but is restricted to sample rates of 48kHz and below, of course.

The M1CLiC played WAV files with bit depths of 16 and 24 with sample rates from 44.1 to 192kHz from a USB RAM drive plugged into the front-panel USB port. It also played FLAC files with sample rates up to 192kHz, including files at 176.4kHz, which Jon Iverson had problems playing via his network with the Twonky uPnP software and the Kinsky controller running on his iPad. However, though it would play AAC and MP3 files, the M1CLiC wouldn't play Apple Lossless files, displaying the message "File Format Error." To my surprise, the M1CLiC did play the audio tracks of MP4 movie files, though it didn't display the video.

Fig.2 shows the Musical Fidelity's digital frequency response with WAV files via its S/PDIF inputs and its front-panel USB port. The response at 44.1kHz (green and gray traces) starts to rise in the top octave before dropping sharply just below the Nyquist Frequency (half the sample rate). With 96kHz data (cyan and magenta traces), the rise peaks by 0.2dB at 20kHz, then rolls off by 3dB at 40kHz. Like Musical Fidelity's M1DAC, which Sam Tellig reviewed in March 2011, the M1CLiC's response with 192kHz data (blue and red traces) doesn't extend any higher in frequency than it does at 96kHz, presumably due to the product's use of a sample-rate converter chip ahead of the D/A chip to minimize the effects of datastream jitter. The left channel is about 0.1dB higher in level than the right. Channel separation below 4kHz with the digital inputs was better than with the analog inputs, at >105dB R–L and >110dB R–L, though at 20Hz this decreased in both directions to 86dB, as with the M1DAC.

The M1CLiC's digital performance was very similar to the M1DAC's, though I found it less sensitive to grounding issues when hooked up to either Audio Precision analyzer. The top two pairs of traces in fig.3, taken by sweeping a 1/3-octave bandpass filter from 20kHz to 20Hz while the M1CLiC decoded dithered data representing a tone at –90dBFS, indicate that the increase in bit depth from 16 to 24 drops the noise floor by almost 20dB in the treble, which is easily enough resolution to allow the D/A to decode a tone at –120dBFS (bottom pair of traces). This is close to 19-bit resolution, which is excellent performance and confirmed by FFT analysis (fig.4). Note the absence of power-supply–related artifacts in these two graphs, though some very low-level harmonic spuriae visible in fig.4 have been unmasked by the drop in the noise floor with 24-bit data. Repeating the analysis in fig.4 with 24-bit data on a USB stick plugged into the front-panel USB port gave the same result, confirming that the M1CLiC correctly decodes 24-bit data via this input.

The Musical Fidelity's linearity error with 16-bit data (fig.5) was dominated by the recorded dither noise down to –120dBFS, suggesting that the error was negligible. As a result, and with the very low level of analog noise, the M1CLiC's reproduction of an undithered 16-bit sinewave at exactly –90.31dBFS was essentially perfect (fig.6), with a symmetrical waveform and the three DC voltage levels well differentiated. The time-symmetrical Gibbs Phenomenon "ringing" on the wave's tops and bottoms is also clearly visible. With undithered 24-bit data, the result was a good sinewave despite the lack of dither (fig.7). Noise modulation was low well away from the signal frequency (fig.8), and was of the order of the changes introduced by the Audio Precision's gain-ranging circuitry. However, there was a shift in the noise floor close to the signal at 0dBFS, and sidebands appear at the power-supply–related frequencies of ±120Hz. As these lie at almost –130dB, they will have no audible consequences. This graph also reveals that the odd-order harmonics of the power-supply frequency that result from magnetic interference from the M1CLiC's AC transformer are negligible.

Like the M1DAC's, the M1CLiC's harmonic distortion was superbly low, and dominated by the subjectively innocuous second and third harmonics (fig.9), though the third harmonic from the 'CLiC's unbalanced output is a little higher than it was from the 'DAC's balanced output. This graph was taken into the benign 100k ohm load; the picture didn't change significantly into 600 ohms (not shown). Intermodulation distortion was also very low, even into 600 ohms (fig.10).

Like the M1DAC's, the M1CLiC's rejection of jitter was superb. Fig.11 shows the spectrum of its output when reproducing 16- and 24-bit versions of the Miller-Dunn J-Test data stored as WAV files on a USB stick inserted into the front-panel port. The behavior was identical for both coaxial and TosLink S/PDIF data: with 16-bit data (blue and magenta traces), the odd-order harmonics of the low-level, low-frequency squarewave all lie at the residual level and are not accentuated in any; with 24-bit data (cyan and red), all that can be seen is a slight broadening of the skirts of the spike that represents the 11.025kHz tone, a pair of very low-level sidebands of unknown origin at ±1230Hz, and a pair of sidebands at the supply-related frequencies of ±120Hz. However, these sidebands are too low in level to be resolved by my Miller Analyzer.

To my surprise, the ±120Hz sidebands disappeared when I repeated the 16-bit J-Test with my iPhone plugged into the rear-panel port (fig.12), though they reappeared with an iPod Classic (not shown). Despite operating in the jitter-prone adaptive USB mode rather than the theoretically better asynchronous mode, the M1CLiC's USB input, fed 16- and 24-bit J-Test data, performed as well as did its S/PDIF inputs.

Overall, Musical Fidelity's M1CLiC offered superb measured performance.—John Atkinson