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May 2013
April Music Stello U3 Digital Converter
One of the greatest bargains in computer audio!
Review 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/24-bit and 92kHz/24-bit music files, available for download
for example from HDtracks.com. But also the realization that standard CD quality
files, that is 44.1kHz/16-bit 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 $3200 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 S/PDIF, 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 S/PDIF 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.
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 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.
Conclusions
The
inescapable conclusion is that the Mac BookPro together with the Stello U3 makes
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.
Specifications
Type: Digital to analog converter
Resolution: 44.1, 88.2, 176.4, 48, 96, 192 kHz @ 24-bits
Input: one USB 2.0 type B connector
Outputs: Coax 75 Ohm gold-plated RCA and AES/EBU 110 Ohm Neutrik XLR
Lock indicator: LED on the front panel
Platform Support: Apple Mac OS X
Windows Driver: Windows XP, Windows Vista, Windows 7, Windows 8
Dimensions: 100 X 36 X 100 (WxHxD in mm)
Price: $495
Company Information
April Music
3F Bangbaehill Bldg., 882-3 Bangbae-Dong
Seocho-Gu, Seoul 137-840
South Korea
Website: www.AprilMusic.com
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