Wavelength Audio Proton USB D/A converter Measurements

Sidebar 3: Measurements

I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system to perform the measurements on the Wavelength Proton (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. Source for the test files was my MacBook, fitted with 4GB of RAM and running Mac OS10.6.8 and Pure Music V1.8.

Wavelength warns that the Proton's line-level output will clip with full-scale signals, and advises owners to set the Proton's internal volume control to 90% in AudioMidi Set-Up. (The remaining 10% of the volume control's range of adjustment is to allow a full-scale signal to be output from the Proton's headphone jack.) I made sure the Proton's internal battery was fully charged before I performed any tests.

The Mac's USB Prober utility reported that the product was the "Proton USBDAC" from "Wavelength Audio, ltd.," and that it operated with 24-bit resolution in "Isochronous asynchronous" mode. Sample rates handled were 44.1, 48, 88.2, and 96kHz. With the volume control at 90%, the maximum output at 1kHz from both sets of outputs was a lowish 899mV, and both outputs preserved absolute polarity (ie, were non-inverting). The headphone output still clipped with the Proton's volume control set to 100%, with an output level of 1.4V. Backing off the volume control to 94% eliminated the clipping, at which point the output level was 1.13V.

The line-level jacks featured a low output impedance of 33 ohms at high and middle frequencies, but this rose to 1504 ohms at 20Hz, presumably due to the presence of an output coupling capacitor. The Proton should be used with a preamp having an input impedance of at least 10k ohms if the bass is not to sound a little lean. The impedance from the headphone jack was an appropriately minuscule 1.5 ohms at high and middle frequencies, rising slightly to 4.6 ohms at 20Hz.

The Proton operated correctly with data sampled at rates from 44.1 to 96kHz, the appropriate LED on the rear panel illuminating for each rate. The frequency response into 100k ohms with 44.1kHz data (fig.1, blue and red traces) was flat almost to the top of the audioband but then began to roll off, reaching –0.9dB at 20kHz. With 96kHz data (fig.1, cyan and magenta traces), the output was –0.5dB at 30kHz and –3dB just above 40kHz. Channel separation (not shown) was superb, at 110dB in both directions at 1kHz, and still 89dB (L–R) and 99dB (R–L) at 20kHz. The Proton's impulse response (fig.2) indicated that it uses a conventional linear-phase FIR filter.


Fig.1 Wavelength Proton, frequency response at –12dBFS into 100k ohms from balanced outputs with data sampled at: 44.1kHz (left channel blue, right red), 96kHz (left cyan, right magenta). (0.25dB/vertical div.)


Fig.2 Wavelength Proton, response to single sample at 0dBFS, 44.1kHz-sampled data (4ms time window).

Linearity error with 16-bit data was negligible down to below –105dB (fig.3), the traces in fig.4 representing a dithered 1kHz tone at –90dBFS peaking at that level and being commendably free from either power-supply–related or harmonic spuriae. However, both this graph and fig.5, which was derived using an FFT technique, also show that the increase in bit depth gives only a slight increase in dynamic range. The use of a battery for the Proton's power supply does reduce the maximum output level at the expense of dynamic range. Consequently, the waveform of an undithered waveform at –90.31dBFS (fig.6) is overlaid with analog noise, obscuring the three DC voltage levels described by the data.


Fig.3 Wavelength Proton, linearity error of left channel, 16-bit data (2dB/vertical div.).


Fig.4 Wavelength Proton, 1/3-octave spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (top), 24-bit data (bottom). (Right channel dashed.)


Fig.5 Wavelength Proton, 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).


Fig.6 Wavelength Proton, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

With the Proton's volume control set to 90%, a full-scale 1kHz signal gave a regular series of distortion harmonics from the Proton's line output (fig.7), though these are all at or below –93dB (0.0028%). Commendably, the level of these harmonics didn't increase significantly when the load impedance was dropped to just 600 ohms (not shown). With a 1kHz signal at –10dBFS (fig.8), all the higher-order harmonics disappeared, leaving the second and third harmonics below –114dB (0.0002%). Intermodulation distortion was also low, with all products at or below –90dB (fig.9). But this graph also shows that the Proton uses a fairly slow reconstruction filter, with the aliasing products at 24.1 and 25.1kHz easily visible.


Fig.7 Wavelength Proton, spectrum of 1kHz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).


Fig.8 Wavelength Proton, spectrum of 1kHz sinewave, DC–1kHz, at –10dBFS into 100k ohms (left channel blue, right red; linear frequency scale).


Fig.9 Wavelength Proton, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

The Miller-Dunn J-Test signal is not really diagnostic for a USB datalink, where the clock is not embedded in the data. However, fig.10 shows the spectrum of the Proton's output while it decoded 16- and 24-bit versions of the J-Test signal. The odd-order harmonics of the low-frequency squarewave almost all disappear when the 16-bit data (cyan and magenta traces) are replaced by 24-bit data (blue and red traces). However, two pairs of data-related sidebands are emphasized and don't disappear: those at ±229 and ±689Hz. The measured level of these sidebands, according to the Miller Analyzer, was equivalent to just 130 picoseconds peak–peak of jitter, which is low enough to have no audible consequences. But these sidebands shouldn't have been present at all, given the Proton's asynchronous USB connection. Perhaps they stem from some kind of interaction within the circuit akin to Meitner and Gendron's "Logic-Induced Modulation" . . . ?


Fig.10 Wavelength Proton, 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 USB from MacBook (left channel cyan, right magenta), 24-bit data (left blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Overall, the Wavelength Proton performed well on the test bench. Its only obvious fault was its limited ultimate dynamic range, due to the use of a battery supply with limited voltage capability.—John Atkinson

Wavelength Audio, Ltd.
3703 Petoskey Avenue
Cincinnati, OH 45227
(513) 271-4186

NigelR's picture

This is a very interesting and extremely comprehensive article.

Nice- heard about the converter from a friend and needed more info. Anyway- just a thank you. I think that you have pretty much covered everything here. Greetings



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GeneZ's picture

I read in the Wavelength article:

"Drummer Michael Clarke's ride cymbal throughout the Byrds' Mr. Tambourine Man (ripped from CD, Columbia CK 64845) sounded appreciably less hashy and artificial through the Wavelength Proton than through Furutech's ADL GT40."

That was not Michael Clarke on the record unless it was a live in concert recording.  Michael Clarke was a pretty face they drafted to travel and play live with the band.  In the studio, it was the famous studio drummer Hal Blaine behind the set.






John Atkinson's picture

Thanks for the info.


John Atkinson

Editor, Stereophile