Tip #82 (June 1, 2013)
April Music’s Stello U3 Digital Converter
by Dick Olsher


Early this year I fell though the rabbit hole into the wonderland that is high-end computer audio. More precisely, it was a calculated move designed to take advantage of a maturing technology that in the span of just a few years sprouted from infancy to a relatively mature state. The incentive was far more than just a matter of convenience, of being able to manipulate hundreds of music files without rummaging through a pile of physical media. It was about access to high resolution 192khz/24bit and 92kHz/24bit music files, available for download for example from HDtracks.com. But also the realization that standard CD quality files, that is 44.1kHz/16bit data played back off a computer, can potentially be reproduced with less jitter and greater fidelity relative to what even an expensive CD player or transport is capable of. Now that was more than incentive enough for me to embrace computer audio.

The core of my computer audio system consists of a Mac BookPro laptop with 8GB of memory and a solid-state hard drive running Sonic Studio’s Amarra Version 2.5 music player software. From my perspective, Amarra’s support for up to 384 kHz sample rates, memory cache playback, and playlist mode make it a clear winner. Of course it integrates perfectly with Apple’s iTunes, but it can also operate free of iTunes in play list mode. The next order of business was to integrate computer file streaming from the Mac’s USB 2.0 port with an external DAC or CD player digital input. It turns out that not all CD players or DACs possess a USB input and those that do may well not offer an asynchronous USB input. What is an asynchronous input you ask? In a nutshell, it’s an input that incorporates a clock for controlling the incoming data stream. Rather than allowing the Mac’s internal clock to control the data stream, the Amarra software makes it possible for an external low-jitter clock to take control. After all, a computer’s primary mission in life is not to generate a stable clock. And proximity to switching power supplies, CPU, and other digital noise sources could compromise the internal clock’s performance. The idea then is to avoid using your PC’s soundcard or Mac’s onboard DAC. And that’s where April Music’s Stello U3 comes in.


Technical Details

Think of the Stello U3 as a bridge between a computer and an external DAC. There is one USB 2.0, Type B input connector, and both balanced and unbalanced digital outputs. The U3 is a standalone device and is powered directly by the computer’s USB port. The USB standard specifies a maximum current draw of 500 mA at 5 volts for USB 2.0 devices. Since the U3 draws 200 mA, it is well within that spec. The U3 main board incorporates a custom xCORE 32-bit/500 MIPS microprocessor from chip maker XMOS, a leader in USB audio solutions supporting resolutions of up to 24 bits at 192kHz. XMOS, a fables semiconductor company, has been at it since 2005 and has made it possible for small manufacturers to offer low-cost asynchronous USB inputs. Two low-jitter clocks are used to clock the incoming data stream. I should note that this is same board that is also used in April Music’s $3,200 Eximus DP1 DAC/Pre.

The U3 is natively supported by the Mac OS X, so no driver installation was necessary. However, the U3 is bundled with a driver installation CD for Windows OS. An AudioQuest Carbon, 1.5-meter length USB cable, was used to connect the Mac BookPro to the U3. A 1-meter coaxial cable was used to connect the U3 to an external DAC’s SPDIF input. From the Amarra file pull-down menu, one can open the Audio Device Preferences window and re-scan audio devices to recognize the U3. The final step in managing the connection is to select the U3 (identified as XMOS) as the output port.

Sonic Impressions

A major player in all of the listening tests was the EAR-Yoshino model 192 DAC. Well, this is no ordinary DAC. The audio output stage consists of a transformer-coupled tube line stage based on the 6922 dual triode, a design topology that designer Tim de Paravicini has previously deployed in his pro audio gear, and which he likens to that of a small amplifier. A volume pot is included allowing for direct connection to a power amplifier. The EAR DAC incorporates USB, coaxial SPDIF, and Toslink inputs. That allowed me to run the Mac directly into the EAR DAC’s USB input as well as have the U3 bridge the connection into the SPDIF input. A key point in this comparison is that the EAR DAC’s USB input, while being 2.0 high-speed, is not an asynchronous type. That meant that the Mac was allowed to clock the bit stream to the external DAC’s USB input.

Differences between these two modes of file playback were dramatic, the sort of night and day differences worth writing home about. Let me first of all make clear that sonically the EAR DAC is one of the best money can buy. In my experience, a tube output stage is an important criterion for the success of failure of a given DAC. Getting tubes into the digital front seems to civilize harmonic textures and intensify tonal colors. No matter how good the DAC chipset is and how low-jitter the digital to analog conversion might be, I can still plainly identify the differences between solid-state and tube buffer stages. I’ve been a proponent of tubes in DACs for many years, as this in itself seems to be responsible for much of the sonic differences between DACs. One of the advantages of a vacuum tube output stage, as is the case with the EAR DAC, is its inherent spatial integrity. The ability to flesh image outlines with genuine palpability and to portray a soundstage with convincing depth perspective and lateral extension have been and will likely remain the domain of the vacuum tube.

It was in this context, by using an external DAC fully capable of revealing spatial information, that bypassing the U3 resulted in a major loss of spatial integrity. Allowing the Mac to directly feed the digital bit stream to the EAR DAC’s USB input squashed the spatial impression by flattening the depth perspective and defocusing image outlines. Inserting the U3 into the chain brought about a dramatic improvement - a three-dimensional panoramic experience complete with a huge depth perspective and a soundstage populated by precisely defined image outlines. Massed strings were free of residual hardness and digital brightness. The upper midrange has proven to be a frustrating experience for critical listeners searching for a satisfying digital experience and has driven many audiophiles away from digital and back to analog. These listeners are typically put off by a residual tonal hardness and unnatural transient crispness that plague so many DACs. Digital gremlins were absent with the U3 in the chain. Textures were reproduced sweetly and with a level of purity that most digital front ends struggle to match. And it occurred to me that this particular setup might just be cleaner and purer sounding than a CD digital feed into the EAR DAC’s SPDIF input.

For the past several years I’ve been using a ModWright modified Sony XA-5400ES SACD player as a reference digital front end. Using the Sony as a transport allowed me to compare computer files ripped onto the Mac to the that same CD’s digital feed from the Sony SACD player’s digital output. The findings were quite startling, especially since I didn’t expect much of a difference. But for the record, the computer file playback resulted in enhanced microdynamic shadings, purer textures, and an increased sense of transient clarity. However, comparing the ModWright Sony’s analog output to that of the Mac+U3+EAR DAC gave a much closer sonic score card. So in the final analysis, file playback though the U3 easily bested the performance of the Sony as a transport. And not only that. I’ve auditioned a $20K+ transport in the past couple of years and I would venture to say that I could live happily ever after with the Mac and Stello U3 combo even in the face of such pricey ultra high-end competition.


The inescapable conclusion is that the Mac BookPro together with the Stello U3 make for an exceptional virtual transport, and one that can compete successfully at the highest levels. This little black box delivers exactly what it promises, and in my experience makes for an essential bridge between a computer and an external DAC, especially one that lacks an asynchronous USB input. According to April Music, their primary mission is to create components that they themselves would love to own and enjoy. And they maintain that musical nuance, timbre, and staging are the elements that set their products apart from the competition. I’m certainly a believer, and given the Stello U3’s affordability, makes it one of the greatest bargains in computer audio and earns it a most enthusiastic recommendation. It’s a keeper and I suspect that it will remain an essential part of my computer audio system for a long time to come.

April Music
3F Bangbaehill Bldg., 882-3 Bangbae-Dong
Seocho-Gu, Seoul 137-840
South Korea
Web: www.aprilmusic.com

: standard sample rates 44.1, 88.2, 176.4, 48, 96, 192kHz/24bits Audio
Input: one USB 2.0, Type B connector
Outputs: one coax 75 Ohm, gold-plated RCA Connector
      one AES/EBU 110 Ohm, Neutrik XLR connector
Lock indicator: LED on the front panel
Platform Support: natively supported by Apple Mac OS X
             Windows Driver: Windows XP, Windows Vista, Windows 7, Windows 8
Dimensions: 100 X 36 X 100mm (WxHxD)
Price: $495