samedi 28 avril 2012

Test - Stello U3 (English version)

April Music, founded in 1998, is a South Korean company editing 3 brands: Aura, Eximus and Stello. A highly skilled staff, often musician themselves, is surrounding Simon Lee, the founder. He is taking a particular care at manufacturing his pieces of electronic to produce highly musical elements. Passionate, well known for having a particularly sharp ear, he is offering his customer a wide range of electronics. Today, we are going to put our attention on a very useful accessory to compensate the lack of an optimized USB input on a converter.

Computer audiophile music has this unique interest in the fact that it allows combining of a whole variety of elements, either at transport level, either at computer or streamer level; It is bringing a quality of output which could not be hoped for people with limited budgets a few years ago. At another level, streamers are providing us with an integrated solution (transport + dac) and at the opposite, computers often shared for domestic usage are not so well optimized for audiophile music.

With the massive arrival of DACs on the consumer market, in parallel, one has developed accessories to transform computers into high performing “state of the art” transports. As of today, the USB connection from manufacturers is all in all equivalent using proprietary drivers or not, it is resulting of the price drop and popularization of USB chips; On the other hand, the margin of progression at operating system level, playback software and S/PDIF conversion section remains important. This is naturally why USB accessories manufacurers including Stello turn towards this direction.


Stello U3 is manufactured as a rounded squared metal box, rigid, rather heavy which straightly inspires confidence and will show up as very resistant to:

- Interferences as of metal made;
- and to vibrations as it is very rigid.

The bottom is totally covered with a thick anti-skating rubber, an excellent choice to insure device stability. Until set, it does not move anymore, even connected to thick and heavy connectors/cables.

Connectors have a particular good treatment that deliver a prosperous yet warm image through their gold plated RCA and XLR Neutrik plugs. Providing with 110 Ohms symmetric link on top of regular S/PDIF 75 Ohms is an excellent idea because of device market positioning. Numerous high level converters are often equipped with an optimized symmetric section, offering them computer connection at least cost and therefore sustaining audiophile investment.

On this board where components are not on top of each other, a lot can be found. A 13MHz clock is dedicated to XMOS processor management, where two other external clocks are used to generate 44,1 KHz and 48 KHz frequency multipliers. As on some other products of the same type, the reduced size of the XMOS component, its powerful 500 MIPS (millions of instructions per second) and its power consumption optimization (an average of 112mA) allows to plan a good reserve of current to draw for the rest of the components. In addition, a red LED on the face shows up when the device is powered and locked by the computer. Taking the product positioning in consideration, we would have appreciated to have an optional battery-powered supply.

So, Stello offers a product which is perfectly in line with the new manufacturing standard of consumer market into providing USB components out of XMOS. On Mac an Linux system, the box does not require any specific pilot to work, relying on the embedded operating system one. On a Microsoft Windows platform, things remain unchanged as Audio Class 2.0 is not implemented and the Redmond firm does not plan, neither communicates any roadmap concerning this support on Windows 8. So, it stills require a driver, which by the way integrates DS, KS, WASAPI support but also an ASIO implementation.

Our tests have been performed using the Kernel Streaming mode. In the next section, we will explain how to make a device setup for this mode

Let’s take note that XMOS does not develop its own asynchronous driver, but rely on a partner, a German company called Thesycon. This well know driver which does not really change from one product to another, offers a very poor integration at operating system level. Recalling me about 90’s drivers, settings have to be adjusted in a startup group application that remains in the tray and thus consume resources.

Thesycon control panel allows adjusting of fundamental parameters such as streaming buffer size for asynchronous transfer. You can adjust it from 1 to 32 milliseconds. I recommend to directly setting it to 1 ms; if it works fine this way, no need to make any step to adjust this value anymore and generate unnecessary jitter:

Let’s take note that ASIO buffer size is also available from the same screen, it can be set to a minimum of 2 ms only (from 176 to 768 samples)

All the other parameters are grayed, except volume adjustment that I advise you not to change. Changing volume would introduce additional jitter and might waste bit perfect flow by doing on the flow data changes.

In a nutshell, Stello U3 device does not derogate to April Music’s rules, “built to last”. Immediately, the box impresses, we now need to check whether this very positive feeling on its built transforms into facts on its musical performance.

System used for the review: 

Especially built for digital music playback:
  • Apple Extreme switc with Full Duplex GB ports
  • Ethernet Cat 6 shielded and braided cables, plugs are also shielded
  • Laptop with an external switched power supply based on Core 2 Duo with 6 MB RAM, SSD disk of 80 GB. PC is liked to network at 1GB/s Full Duplex.
  • Operating System is Windows 7 Ultimate x64 SP1, tuned for audiophileoutput conforms to the PC Audiophile.
  • A Phonic FireFly DAC powered on battery
  • A wireworld platinum Digital S/PDIF cable.
  • A NAS of 2 TB linked to the network at 100MB/s Full Duplex
  • An iPhone 4 to be used as remote control paired to wi-fi network on the Apple Extreme access point
  • Software to play tracks is foobar linked with JPlay V4 (Beach + Hibernation/Fullscale) or JPlay mi
  • ni in mode Hibernation/Fullscale
The rest of the output chain is at iso perimeter to compare the elements:
YBA Diamond, Pre/Amplifier based on DIY Vecteur I6.2, ProAc D28.


First contact is very positive, we are definitely in presence of a well performing transport system. Transparency is perfect evidence, like the feeling of being totally free of color on the medium range. The soundstage is very well defined, one of the best I have ever tested with such a device. Voices are splendid, rich in details and clarity, but never too dry. Eric Clapton on Next time you see her (slow hand 24/96KHz) offers us a world of nuance in the course of his scratched voice, full of veracity.

Let’s take note that, as usual, it is only a question of taste to like this kind of musical signature or not. You easily forget about the Stello box, music takes over technical aspects, dynamic is really present, forte pressure and jump from one range to another seemed to me very realistic.

On HD tracks, Stello transport is without any concession, let’s say brilliant! Listening to Monty Alexander on Calypso Blues (24/96KHz) delivers a very realistic percussions and a rich and surprisingly varied piano, it was easy to guess piano brand at first notes! We are now very close, as long as the S/PDIF cable is also of high grade, to the best transports I could ever hear and especially close to famous Linn Akurate DS2 streamer.

Less fleshly and ethereal than the hiFace EVO style, one will willingly raise volume up without any “digital hardness” feeling in the higher range. On well-engineered recordings, like those from Diana Krall, music is especially fluid and breathes. On From this moment on (24/96KHz) out of the same name record, swing from brasses charms us as well as the deep voice from the singer. Romeo and Juliet from the Russian national orchestra (Alexander Vedernikov - 24/88.2KHz) perfectly simulate the big orchestral mass without any excess or tension.

In order not to derogate to the rule, we have also used our usual test tracks. We have especially appreciated to listen to the classical Take Five where it was really easy to hear with very accurate precision the chords move of the bass. 

Another good point, applause and crowd atmosphere were both very well transcribed; On Sade live, one will also appreciate the accurate precision of the scratched voice accents from the singer. Plane separation is naturally also excellent; Space between instruments and voices is very precise. Apparent bandwidth seems to be perfectly coherent, without any feeling of holes or lack at the start or end of the audio range. Extinctions of notes generate from time to time a frustrating feeling, not because of the system, but because of engineering recording lacking of precision. Lower part of the bass range is clean, never giving a feeling of being slow. 

In front of Internet radio flow, the Stello U3 box does not reacts a very exceptional way. I mean that you can feel compression too much. Listening to high definition channels (256 or 320Kbits/s) brings a better feeling, but seems to be a bit tiring to listen for hours. It might be interesting to swich EQ on at this time!

In terms of listening enjoyment, this box offers an equal pleasure on all kind of music styles, transparency and respect of timbres appeared to me in perfect line with requirements to listen to classical or jazz concerts as well as rock. In a nutshell, I would say that voice performances are brilliants. Cecilia Bartoli delivers here a strikingly true Sposa relying on a truthfully chord background. Lastly, Katie Melua is close to us, at a few inches on Piece by piece bathed in a firm and coating bass line.


As I like to say, using devices such as U3 remains very touchy at a pure technical point of view. It requires pairing the right pieces of electronics to get the expected result; today, we are in front of a very stringent product, but so stringent that a mistake of taste is not to exclude. A few more fleshly products or more colored could outstrip it in the heart of a few audiophiles attached to a specific sound signature!

As a conclusion I think that U3 deserves its own place on a stringent and robust pantheon of audiophile computer world. Even if at the technical level, there are no special very high level components compared to competition, assembly and topology seem to reach target, this is what, at the end of the day, counts.

Savor without any restriction!

Manufacturer link :

Link on French Audiophile forum :

6moons (anglais) :

Spécifications : 

compatible with asynchronous USB Audio Class 2.0 mode
bit perfect audiop stream up to 192kHz
2 external clocks at low jitter

22.5792MHz/24.576mHz to generate multipliers of 44.1kHz/48kHz
44.1, 88.2, 176.4, 48, 96, 192kHz/24bits Audio
1 USB 2.0, Type B connector
galvanic isolation and up to 192kHz
1 RCA  75 Ohms, gold plated
1 Neutrik XLR AES/EBU 110 Ohms, gold plated

LED on front panel

Os Support: Windows XP, Windows Vista, Windows 7Apple Mac OS X natif

Dimension :
100 X 36 X 100mm (WHD)

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