Wavelength Audio Proton USB D/A converter
Without asynchronous streaming, USB audio functions in adaptive mode: the computer establishes the data-transfer clock, forcing any outboard digital-to-analog converter or other such device to adapt to that rate. Worse still, adaptive mode asks the converter to derive a clock signal from a USB operating frequency that's mathematically unrelated to any digital audio frequencies known to man. It's like putting a driver behind the wheel of a car whose speedometer is calibrated in miles per hour and telling him to maintain a steady speed of exactly 70.13 kilometers per hour, lest he go off the road and die, unpleasantly: Accident avoidance requires one to perform a continual series of high-speed calculations on the fly.
Alternatively, Rankin's Streamlength software, which resides in a ROM chip within the USB device, empowers the converter to do something that USB converters don't normally do: provide its own clock, rather than live or die by the clock in the incoming datastream. Rankin says that this reduces jitter by a significant margin; his success in licensing Streamlength to such manufacturers as Ayre Acoustics, Berkeley Audio, Halide Design, and others might be taken as a sign that it works.
Like other Wavelength Audio products, the Proton contains an almost fanciful amalgam of old and new technologies: a glorious mosaic, shrunk and made electric. Tiny surface-mount resistors and capacitors share space with old-style carbon-comp resistors and ice cubesized Mundorf M-Caps, which in turn share space with a ROM chip, a TAS1020B USB receiver chip with embedded microprocessor, voltage regulators, and a 3.6V rechargeable lithium-ion battery. (History buffs, take note: Lithium-ion batteries were invented in nearby Binghamton, New York, which is also home to McIntosh Laboratories.) There's even a sprinkling of Åber-rare Black Gate capacitors in therethe red ones!
At the center of all this, figuratively and nearly literally, is a Wolfson XWM8721 DAC chip that also incorporates an analog volume control and a headphone amplifier. Within an inch of the Wolfson DAC are two low-jitter oscillators, which generate the signals for both sample-rate families: 22.5792MHz for 44.1 and 88.2kHz, and 24.576MHz for 48 and 96kHz. Four LEDs on the Proton's rear panel correspond with those four sampling rates: visual confirmation that you're hearing what you hope you're hearing.
In an effort to eliminate the hated wall wartRankin wanted his least expensive DAC to be usable while traveling, as well as at homethe Proton's basic functions are powered by the 5V that appears on the USB port, while the critical analog side of the Wolfson DAC chip is powered by the above-mentioned lithium-ion battery, which is recharged while the computer rests between tasksor, in the case of some contemporary machines, even when the computer is powered down. Rankin estimates 25 to 30 hours of playback time per charge; when the battery dips below a certain voltage, the Proton goes into charge mode, which is indicated whenever two rear-panel LEDs of the same sample-rate family are illuminated. (When two unrelated LEDs light upsay, 44.1 and 48kHzthat indicates a hardware failure.) Interestingly, during this product's development, Rankin discovered that the precise value of its power-supply voltage regulator is sonically critical: The 3.6V regulator he initially chose let through too much computer noise. By switching to a 3.3V regulator, Rankin was able to reduce the Proton's jitter level considerably.
All of the above is built into an aluminum extrusion more or less the size of the average moving-coil step-up transformer. Internal build quality was superb, and the review sample looked very nice indeed. In particular, the Proton's rear panel, with its engraved lettering and its neat row of LEDs, had a purposeful look that complemented both my iMac rig and my hi-fi.
Setup and installation
The Wavelength Proton was darn near plug-and-play: Unpacking the review sample, plugging a cable from my iMac's USB bus into the Proton's USB type B socket, and connecting the latter's RCA line-output jacks to the inputs of my Shindo preamp took all of four minutes. (Those RCA jacks are sufficiently close together that the use of very thick cables, or cables equipped with Koala-style battery packs, is mildly challenging but not impossible.) After that, I selected System Preferences from my iMac's screen dock, clicked on Sound, then on Output, and, when "Proton USBDAC" automatically appeared, selected that: another 90 seconds of arduous, grinding labor.
I suppose I could have stopped there, but a couple of essential tweaks remained: First, with the Sound window still open on my iMac, I clicked on Sound Effects, and then, from the pull-down menu halfway down the window, I selected Internal Speakers instead of Proton USBDAC. I've found this to make a notable improvement in the quality of computer-music sound, although I don't know the technical explanation why.
Second, the Proton contains its own analog volume control, which is adjusted from the associated computer. But the last 10% of its range is intended only for headphonesto use that additional range on the line output is to risk clipping the signal. (I'm not a headphone kinda guysorryso the vast majority of my listening was done with the Proton's line outputs driving my music system.) If you're running iTunes on a Mac, the solution is easy: When using the Proton as a line-out device, open the Audio MIDI Setup utility and knock the volume for both channels down to 91%. For playback software (such as Decibel) that bypasses the MIDI utility, or for Windows or Linux machines, use the volume control in the menu bar to approximate 90%.
Combined, those tasks required a little less time than it takes to microwave a serving of Tandoor Chef brand Palak Paneer. Which is excellent, by the way.
The Proton's instruction sheet says that a brand-new unit requires both a battery charge and a period of break-in before sounding its best, and suggests that the best way to do the latter is through normal (as opposed to continuous) use. Done.