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ASUS Xonar Essence STU

  1. ClieOS
    Another Swiss knife of an DAC/amp.
    Written by ClieOS
    Published Jul 20, 2013
    Pros - All-in-one design, good sounding DAC with upgradable features
    Cons - Not the best sounding amp section. 10 ohm output impedance. Slight hum when not grounded.
    ASUS ventures into the sound card business in 2008 and released their first audiophiles targeted Xonar Essence series in mid-2009. The STU is the latest addition to the line-up. According to ASUS’s press release, the STU is intent to be an USB version of the PCI-e based ST/STX models, with extra features no less. Price wise, the US$399 price tag places it right in the middle of the Essence family, doubling from ST/STX, but half of the flagship One MUSES edition. But the real question, how does it stand on itself?
    SNR (A-Weighted):
    Line Out: 120dB
    Headphone: 117dB
    THD+N at 1kHz:
    Line Out: 0.00036 % @ -108dB
    Headphone: -101dB
    Frequency Response (-3dB, 24bit/192KHz input): 10 Hz to 48 KHz
    Output/Input Full-Scale Voltage:
    Unbalanced Output: 2 Vrms
    Headphone: 7 Vrms
    Bus Compatibility: USB 2.0 high-speed
    Audio Processor: C-Media CM6631A
    S/PDIF: TI PCM9211
    DAC: TI PCM1792A
    I/V: LME49720 x 2 (Swappable)
    LPF: LM4562 (Swappable)
    Headphone Opamp: TI TPA6120A2
    DC Servo: TI OPA2132 (Swappable)
    Sample Rate and Resolution
    Analog Playback Sample Rate and Resolution:
    44.1K/48K/88.2K/96K/176.4K/192KHz @ 16bit/24bit
    ASIO Driver Support:
    44.1K/48K/88.2K/96K/176.4K/192KHz @ 16bit/24bit
    Output impedance:
    Line out: 100 ohm
    Headphone out: 10 ohm
    Crystal clock: 12MHz
    Oscillator: 45.1584MHz (44.1kHz); 49.152MHz (48kHz)
    Analog Output
    1 x 6.3 mm jack (1/4") Headphone out
    2 x RCA (Un-Balanced)
    2 x S/PDIF in (1 x Optical / 1 x Coaxial)
    1 x Aux in 3.5mm jack (1/8”)
    1 x USB 2.0
    Accessories and Build Quality
    My review unit of STU is an early sample. Though it should be the same as the retail unit, it didn’t come in a retail packaging so I am not sure whether there are other accessories being included or not (though the question is, does it need any extra accessories?) . Basically, the unit comes with an AC wall adapter as well as a vertical stand, a 6.35mm to 3.5mm stereo adapter and USB cable of course. The AC wall adapter looks like a miniaturized notebook adapter and it pumps out 12V, 3A of power. While STU is designed to seat flat on the desktop with its 4 non-removable feet, it actually doesn’t look half bad standing vertically either. The stand has rubber strips that clip onto the body using friction and hold it up firmly. The unit is heavy enough (with stand, 780g) that it is not going to move around much.
    The build quality is excellent overall – not surprising given ASUS already has a good reputation as a PC maker. On the back there are the gain switch for headphone-out, RCA line-out, AUX-in to use STU purely as an amp, both optical (Toslink) and Coaxial S/PDIF inputs as well as the USB input. On the front, there are the power switch and the input switch that lets you choose between USB, coax, optical and aux inputs. There are also two volume knobs – one for headphone-out while the other is for the RCA-out on the back, just in case if you want to use STU as a preamp. Last but not least, there is the ¼’ headphone-out. The front panel also has a row of white LED to indicate which input you are using, the gain setting as well as a ‘bit perfect’ mode. The ‘bit perfect’ LED will only lights up when you are using the STU’s ASIO driver on the PC.
    Gain, Hiss, Pop and Channel Balance
    Though the RCA-out does output full 2Vrms line signal, it is controllable via the front ‘preamp’ volume knob. It is a useful feature if you are using STU as a preamp to feed a power amp. But for those who rather want a fixed 2Vrms output, you can set the bypass jump inside (which is accessible once you open the top panel) to bypass the volume knob all together. I have done some RMAA measurement over the line-out on both maxed-out and bypassed mode, and the result comes back identical. If you didn’t bother to set the bypass jumper, maxing the preamp knob should be almost just as good. The headphone-out has two gain settings. The low gain is about -2.8dB while the high gain is about 10.5dB. Unlike your typical gain switch which instantly changes the gain with a flip, ASUS implemented the gain switch in a rather considerate manner. To avoid accidental gain change, you must unplug the stereo jack first. Then and only then the unit will allow you to change the gain.
    Hiss is pretty much a non-issue in low gain as it remains dead silence even when maxed out. It does begin to hiss very faintly when the volume knob passes 12 o’clock in high gain, but any headphone that needs so much volume is unlikely to be sensitive enough to pick up the hiss anyway. I did notice the unit ‘hums’ a little when it is not connected to USB (i.e. when using the S/PDIF input from a portable source). It seems to be a symptom of improper grounding as it disappears as soon as when it is connected to USB or when the casing is grounded (*touched by hand). This shouldn’t be a problem as long as the STU is used mainly as an USB DAC with the occasionally S/PDIF input (which means it will be connected to USB all the time). But it might be something to consider if you are planning to use it as a standalone unit.
    With the default setup, the STU doesn’t have much of any click and pop issue during startup. ASUS does however implemented an OPA2132 based DC servo to deal with any possible pop or DC offset issue. RMAA measurement shows that the DC servo circuit doesn’t really affect the performance so my suggestion is to leave it on. You can bypass it if you like. However, I do strongly advise you keep the DC servo on if you have rolled the opamp on I/V or LPF stage (*more on opamp rolling at later discussion). This is especially important if you have used MUSES01 as it does create a rather big pop during startup.
    If you pop open the casing, you can find two variable resistors next to the TPA6120A2 chip (light blue ‘blocks’ with screw on top, one on the right and the other on the left of the chip). In case of channel imbalance, you can turn the screw on of them until the channel is balanced. So how do you know the channels are balanced? The best way is to play a fixed tone (i.e. 1kHz) and use a multimeter to measure the AC voltage of headphone-out. If both channels matched, then the channels are balanced. Supposedly the unit should be well balanced when you received it, but it is also good to know there is something you can do if it is not.
    Size Comparison (left): iFi Audio iDAC, STU and Leckerton Audio UHA-4
    Sound Quality
    On spec, STU really looks good. I have no reason to doubt ASUS’s number. But just in case, I did some RMAA measurement of my own and the result is pretty positive for both line-out and headphone-out: mainly flat FR curve and low noise. Power wise, the TPA6120A2 on the STU’s headphone-out behaves similarly to FiiO E9 / E09K and FireStone Audio Fireye HD (all three use the same TPA6120A2 chip) – good in voltage but only okay on current, especially into low impedance load. This is probably because of the high output impedance of 10 ohm required by TPA6120A2, which can be a downer if you mainly use low impedance multi-drivers IEM and coloration is an issue. It would have been better if ASUS was able to lower the output impedance by bypassing the output resistors with inductors. iFi Audio has used the same trick on iCAN with good result.
    Subjectively, STU’s amp section is very similar to that of FiiO E9/E09K. The only difference is that FiiO has a fraction better bass slam which gives it a better impression on dynamic. Beyond that, I’ll say they are within 95% of each other. Still, STU amp section doesn’t quite compete with iFi Audio iCAN that employs the same chip. iCAN just sounds more authority, tighter, better textured and grander in overall presentation. It is not to say STU’s amp section sounds bad – just not the best of what I have heard of TPA6120A2.
    Now to the DAC section of STU – it will take USB or S/PDIF input with just about every bitdepth and bitrate commonly used. Also, it supports asynchronous USB mode as well as a build-in ASIO driver for those of you who crave for some bit-perfect moment. The DAC chip itself is the current TI’s flagship PCM1792. The sound of PCM1792 reminds me a lot about the (TI’s previous flagship) PCM1704 in HifiMan HM801 – silky smooth, warm, full and well textured. It is a musical and relaxing sound that leans toward an euphonic presentation and very much on the opposite of ESS Sabre’s aggressive and dynamic sound that tends to highlight micro-detail and transparency. Perhaps not the best way to describe them, but I thought they represent a good case of what analog vs. digital sound is about. Technically, I won’t say PCM1792 in STU sounds any lesser compared to any EES Sabre’s chips (including the basic ES9023 in ODAC to the higher end ES9018 in HifiMan HM901), but going back and forth for comparison always leaves me the impression that, while ESS wants you to hear each music note as clean as possible, the TI chip wants to envelop and submerge you inside the music. That’s how I feel every time I switched from one to the other. But do take note that I am not talking about any major day and night difference – it is really the subtleness of sonic character between them that gives the different feelings. Ignoring the comparison, STU still sounds very transparent and neutral on its own.
    Last but not least – the sound of the DAC section can be ‘tuned’ by rolling in different opamps. That’s what next in the discussion:
    Opamp Rolling
    One of the big selling points of the Essence series is that it allows the user to roll in different opamps combo to fine tune the sound. In the STU, there are 4 opamp you can swap out, including the dual opamp on the I/V stage (converting the current output of the DAC chip to voltage), one on the LPF stage (Low Pass Filter, for cutting out unwanted high frequency over 20kHz) as well as one for the DC servo (for eliminating DC offset / pop during startup).  In theory, the DC servo circuit should have minimum effect to the final sound so I didn’t bother to swap its opamp. I did listen to the headphone-out with the DC servo enable / disable and didn’t notice any obvious sound quality difference. So I focused only on the I/V and LPF stage. With I/V opamp rolling, I kept using the stock LM4562 for LPF. With LPF opamp rolling, I used the stock LM49720 for I/V instead. One important thing to note is that the opamp voltage supply is about +/-12V, so you must be sure the opamp you swap in is at least rated for that voltage.
    I/V opamp rolling (LPF: LM4562)
    LM49720 Stock– well balanced, clean, airy and open but slightly to the warm side.
    LM49720 ‘Metal Can’– almost identical to stock, but just a tap fuller in body.
    OPA2604 – denser, not as airy and open as stock.
    OPA2209 – bass note is a little lean, good height.
    OP270 – Also dense, but a little better soundstage than OPA2604.
    OP275 – Leaner and brighter than stock, almost grainy. Good soundstage.
    AD8066 – Even brighter than OP275, but without the graininess. Slightly flat in soundstage but good width.
    OPA2227 – Forward, full and bright, slightly grainy.
    OPA2134 – More mid centric, lack soundstage.
    MUSES01 – Similar to Stock, with better dynamic and impressive soundstage.
    MUSES920 – full and forward, like smoother OPA2227.
    LPF opamp rolling (I/V: LM49720)
    LM4562 – stock, again: well balanced, clean, airy and open but slightly to the warm side.
    LM49720 ‘Metal Can’ – Similar to stock, slightly less dense and more open in the mid.
    OPA2604 – thick and slow, grainy.
    OPA2209 – Full, similar to OPA2604 but without being overly thick and slow.
    OP270 – Lean, slightly lacking in bass, slightly bright.
    OP275 – Well balanced, though soundstage isn’t great.
    AD8066 – Very airy and open, great soundstage but a little bright.
    AD8620 – Like a tuned down version of AD8066, with better body and smoother presentation.
    OPA2227 – warm and slightly dull, but sparkly on the top.
    OPA2134 – similar to OPA2227, more mid centric and not as sparkly.
    MUSES01 – Similar to stock, but slightly better dynamic and soundstage.
    MUSES8920 – somewhere between OPA2227 and OPA2134.
    Short notes:
    • I happen to have TO8 packaged LM49720 with DIP8 adapter, commonly referred as the ‘metal can’ version of the opamp. They are also usually regarded to be better sounding their DIP8/SO8 counterparts - in the case of LM49720, I’ll agree. But it isn’t really a night and day difference and not something I will lose any sleep over.
    • LM4562 and LM49720 have been long rumored to be the same chip with different name. Overall, they are indeed very similar in presentation. But LM49720 tends to sound more open and relaxing without losing any detail. I’ll call that a (small) win.
    • The stock combo is easily one of the best combos beside triple MUSES01. I don’t really see a strong need for upgrade given how capable the stock combo is. If you are on a small budget, I’ll suggest LM49720 for the LM4562 in LPF. AD8066 is also an interesting LPF choice as it gives unrivalled soundstage, but at the cost of texture and body. For a slightly bigger budget, perhaps MUSES01.
    • LPF rolling tends to have more impact than I/V rolling.
    • Though also come from the same MUSES family, MUSES8920 is nowhere as good sounding as MUSES01 in STU.
    • Triple MUSES01 is by far the best sounding combo. It bests the stock combo with better dynamic and speed, plus a more opened and natural soundstage that is very well layered. But given MUSES01 is hard to find and very expensive (US$50 a pop, and you need three), I am not sure it will be the smartest investment to convert the $400 STU into a $550 unit. Still, it is unbeatable in sound.
    • RMAA measurements have been carried out on quite a few of the combo. Not surprisingly they measured more or less identical. It is very likely the resolution of RMAA is just not good enough to tell the difference between these opamp combos.
    FiiO X3 coax-out to STU sounds rather excellent.
    Sum Up
    Overall, STU is a solid unit. Features wise, it is a compact all-in-one unit that can do them all – S/PDIF, USB, and analog, no problem there. As far as sound quality goes, it is a better DAC than it is a headphone amp. Roll in some MUSES01 and pair it with a better desktop amp, and we could very well have a top-end setup there - though that could very well be just as expensive as STU’ big brother, the Essence One MUSES edition.  For a purely PC-as-source situation, I know I will likely reach out to the iFi Audio iDAC + iCAN combo, but they won’t do S/PDIF and will cost more together. The similarly priced FireStone Audio Fubar HD is also a jack-of-all-trade, but USB is limited to 24/96 while not as refined sounding overall. The only upside is that Fubar HD can be found for half price, so it is not a total lost especially for those with tight budget and willing to settle for slightly less. At the end, for being a mid-range model designed with versatility in mind, I think ASUS has done a pretty good job. The best part is, not only STU is an audio multi-tool, but there is enough room left for upgrade that will give future value to the user - that’s very much the X factor for STU and gives it a slight edge over the competition.
    A thanks to ASUS for the review sample.
      NullZero, Stylus and raoultrifan like this.
    1. View previous replies...
    2. HeadAmp
      HeadAmp, Sep 27, 2013
    3. HeadAmp
      HeadAmp, Sep 27, 2013
    4. ClieOS
      I only see it being sold in the East (China, Taiwan, etc) from official retailer. Haven't really seen it online or from the West.
      ClieOS, Sep 27, 2013
  2. project86
    High value mid-priced DAC/amp from Asus
    Written by project86
    Published Jun 17, 2013
    Pros - Lots of features, clean and clear presentation with nice details, good looks, upgrade potential
    Cons - Could be a little bright or bass light in the wrong system, output impedance not ideal for low impedance headphones

    Asus is a well known tech company. They make laptops, desktops, tablets, motherboards, video cards, and a few other odds and ends. Around here, they are most known for their highly regarded Xonar Essence series of sound cards. The Essence ST and STX are likely among the most widely used "entry-level audiophile" products out there - especially when you consider the massive number of PC gamers who eventually become interested in quality sound. John Atkinson covered the ST and STX at Stereophile a few years back and concluded they were definitely worth the investment. Though not perfect in sound or measurement (but what is?), they offered a very high amount of bang for the buck. 
    Several years later, we see Asus continuing their expansion into the audio market. The big news of last year was the Asus Xonar Essense One - a relatively expensive (compared to the ST/STX cards) stand-alone DAC with built in headphone amplifier and preamp capabilities. It's received plenty of industry praise including this review at InnerFidelity. Asus clearly has aspirations of being taken seriously by audiophiles and it seems to be going well thus far.
    Yet there was a problem. Between the ~$200 price tag for the ST/STX, and the $599 starting price for the Essence One (climbing to $899 for the Muses Edition), there's a rather sizable gap. Asus needed something to fill that hole in their lineup. Enter the Essence STU. At $399, the STU fits square in the middle between the sound cards and the One - not just in pricing, but also in design and features. It shares elements of both designs and hopefully reaches a sweet spot between them. We'll see. Asus was kind enough to loan me a review model for what I believe is an exclusive HeadFi first-look. 
    If the Essence STX is the base model sound card that works with PCI-Express, and the ST is the slightly improved version of the same in PCI only form, the U in "STU" logically stands for "USB". It departs from the internal sound card form factor of its siblings, instead living in an external enclosure. It's a good looking box - smartly designed, small and unobtrusive, able to lay flat or prop up on its side via an included stand mount. In size and shape it reminds me of a router - it should fit most anywhere. In general appearance, it has shades of vintage stereo receiver - a bit of retro design, on a much smaller scale of course. It does seem rather light in weight, which is in contrast to some other DACs with heavy-duty enclosures. But do those actually do anything for the sound? 
    The front panel is dominated by two volume knobs: one dedicated to headphones, the other for line out. Asus does provide an option for fixed, full volume line out via internal jumper. Aside from those knobs we get a 1/4" headphone jack, a power button, and a single button for cycling through the various inputs. The STU has dedicated LED indicators for each of its four inputs, as well as an LED showing "bitperfect" connection via USB. 


    Around back we find digital inputs in the form of optical and coaxial SPDIF as well as USB. Asus claims playback capabilities for all resolutions up to 24-bit/192kHz, which includes the asynchronous USB input. This is one of the few instances where the claim of 192kHz over Toslink actually proves true - most DACs have trouble locking on a signal over 96kHz, even from a pristine source. There's also a 1/8" jack labeled "AUX" for use with a smartphone or tablet. Lastly, we find a switch for selecting high or low gain on the headphone output. Asus doesn't specify exactly what the gain is for each setting but it seems like good mix of "low enough" and "high enough". This is actually an improvement over the Essence One, where the Muses Edition has gain set via internal jumper, and the base model has no gain adjustment options at all.


    The STU internal componentry seems smartly executed. It starts with some of the stronger points of the ST/STX and builds off that solid platform. The design breaks down into separate sections for power, digital, and analog, and the analog portion is such that most companies would claim it as true balanced design. For their part, Asus uses the term "Fully Balanced Stereo Representation". We can argue all day about what constitutes a "true" balanced design (dual independent DAC chips?), but the STU only has single ended outputs, so it's not like Asus is trying to trick us. 
    SPDIF signals enter through a TI PCM9211 which is a new DIR I haven't seen used anywhere else yet. Specs are excellent, appearing to match the industry-leading Wolfson WM8805 at 50ps RMS period jitter. The PCM9211 also has an integrated analog to digital converter, which is used to digitize signals coming in from the AUX input. If I'm playing a song from my iPad via analog connection, I'm obviously not too concerned with maximum fidelity, so this extra conversion step doesn't seem like a problem to me. USB signals are handled by the C-Media CM6631A asynchronous chipset - note the "A" in that title. Schiit used the original CM6631 in their Bifrost and Gungnir USB boards, and it didn't support the 176.4kHz data rate. They now include the newer "A" version in their "Gen 2" USB boards. Asus also uses the same CM6631A in their Essence One.
    Also on board is a Cirrus Logic CS2000, used for clock regeneration while attenuating jitter in the process. It's a hybrid analog and digital phase-locked loop, essentially a junior version of the "JET Technology" circuit used in the Weiss INT-202 FireWire interface ($1885). This works in conjunction with a trio of discrete clocks running at 12MHz, 45.1584MHz (for 44.1kHz signals and their multiples), and 49.152MHz (for 48kHz signals and their multiples). 
    Actual D to A conversion is handled by a PCM1792A, which is the current top of the line chip from Texas Instruments. I/V conversion is handled by a dedicated LME49720 opamp for each channel, while a single LM4562 opamp handles low-pass filtering. Fun fact about those particular opamps - they are identical. So why the two different names? National Semiconductor won all sorts of awards for the LM4562 when it first came out. They had a substantial marketing campaign built around it. A few years later, they came up with the idea of having a dedicated naming convention for high performance audio chips, using the special LME designator. But the LM4562 was already very well known and people specifically asked for it by name... so they couldn't get just get rid of it. Thus both parts co-exist, each with their own distinct datasheet containing identical information. Strange right? Yet I've seen designers and opamp rollers alike make the claim that each model sounds different, despite their similarities. I thought this was hogwash until I met someone who actually worked on the LM4562 project. He mentioned some minor variations in construction between each version of the opamp... the type of variations where nothing really should sound different, but nonetheless could. At least it gives a theoretical reason for why some people find them different, though I'm still not completely convinced. 
    Anyway, sorry for the tangent... getting back to the STU. As in their sound cards and the Essence One, the STU features socketed opamps in the important locations. They encourage users to experiment with rolling different opamps to see what the result is. In my listening section I'll discuss a few that I tried and the results I obtained. But I like the fact that the included opamps are already of high quality - if a person doesn't like messing with opamps, it certainly isn't required. There are also 3 more LM4562 opamps on board which are not socketed. These are used as buffer stages and supposedly don't influence the sound as much as the I/V or LPF section.
    The headphone stage of the STU is based on the TI TPA6120A2. In this way it is similar to the ST/STX, and different from the Essence One which instead uses a pair of LME49600 opamps. Output impedance is 10 ohms which is common for devices using the TPA6120A2, as the specs for that chip recommend adding output resistors for stability. But some other devices like the iFI iCAN and Audinst HUD-mx2 use the same chip but achieve much lower output impedance while maintaining stability. So it's definitely possible, and I wish Asus had done the same. In this case the 10 ohm output impedance means the STU won't be ideal for most balanced armature based IEMs. Even some full size lower impedance headphones such as Grados will have audible frequency response interactions and less electrical damping than they otherwise could. On the plus side, Asus implements a DC servo on the headphone output to eliminate any power-on thump noises. It's defeatable via internal jumpers in case someone finds it to be a sonic compromise. It features a socketed OPA2134 opamp which can be swapped out for different flavors of sound. Options are good to have. 
    Other odds and ends: SNR (A-weighted) is listed as 120dB for the line out, 117dB for headphone out. THD+N (at 1kHz, A-weighted) is -108dB for line, -101dB for headphone. Full scale line out is the industry standard 2Vrms, while the headphone out can swing up to 7Vrms. As mentioned, headphone output impedance is 10 ohms, while line out is 100 ohms. Asus ships the STU with an outboard power supply that reminds me of the type used for a small laptop or netbook. It beats the typical "wall wart" style PSU included in some DAC in this price range, and the STU also has some dedicated power supply regulation on board. I love a fat toroid or R-core transformer as much as the next guy, but this solution seems to work well enough and helps keep the STU very compact. Asus does mention a possible upgrade to a linear PSU but it's not something they provide at this time, so you'd have to use a different brand. I didn't have anything to try so unfortunately I can't comment on this potential upgrade.
    Power supply section on the left, digital in the middle, analog on the right:

    Headphone section (towards the top of this pic:

    USB receiver:

    Toshiba TPC8107 MOSFET in power supply section:

    cFeon EEPROM on the right, empty spot on the left where Cirrus CS2000 would be"

    PCM9211 DIR:

    PCM1792A DAC chip:

    Analog output stage (I have Muses 02 opamps in there are time of pic):

    Jumpers at the volume pot for line-out:

    DC servo with socketed OPA2134:


    I used the following equipment to evaluate the Asus Xonar Essence STU.
    Transports: Dell Inspiron 17R running Windows 7, Foobar2000 and Fidelizer, either direct through USB or else through an Audiophilleo AP1 with PurePower battery supply, Toshiba HD-A35
    AMPs: AURALiC Taurus, Violectric V200, Icon Audio HP8 mkII, Yulong A100, Matrix M-Stage, Firestone Audio Bobby, Stax SRA-12S, Leckerton UHA-6S mkII
    Headphones: Stax SR-007mkII, Sennheiser HD800, Denon D7000, V-MODA M-80, HiFiMAN HE-400, beyerdynamic T1, Westone ES5, Sensaphonics 3MAX, Cosmic Ears BA4, Heir Audio 4.A, Heir Audio 8.A, 1964 Ears V3
    Power is handled by an APC S15 and a Yulong P18, all cables are Cabledyne Reference. The STU was burned in for well over 100 hours prior to critical listening. 
    Cabledyne Reference cables:






    With Icon Audio HP8 mkII


    My initial exposure to the STU was to use it in a realistic setup; the type that might be owned by the target demographic. To that end I used what I consider a "decent but nothing-spectacular" laptop, feeding the STU straight from USB. I plugged the V-MODA M-80 directly into the integrated headphone jack of the STU and got to work listening. I played some Mozart, some Metallica, some Miles, and some Michael Merenda, to cover most genre bases. And guess what? This setup sounded very respectable no matter what I threw at it. Good bass extension, clear mids, detailed treble with little harshness. Soundstage wasn't massive but it didn't feel overly constricted either. The STU seemed able to resolve complex passages well enough, and it had good tonal accuracy on the simple-yet-difficult-to-get-right stuff like piano or cymbals. Definitely a good start to my experience with this unit, and if I don't stray from the perspective of the target market, I find nothing to complain about.
    However, since I'm accustomed to using more expensive equipment, something did in fact sound a little off about the STU. It wasn't a glaring fault, but I did feel the bass was a bit more loose and undefined as compared to my usual M-80 experience. To test my theory, I swapped over to the Leckerton UHA-6S mkII, a sub-$300 portable amp/DAC unit which should be in the same league as the STU in general terms. What resulted was a subtle reduction in clarity accompanied by an obvious improvement in bass definition. Then I had a lightbulb moment - I had forgotten about the output impedance issue. The M-80 impedance falls between 30 and 35 ohms. The Leckerton has an output impedance of less than 1, meaning no chance of interactions. The STU, with its 10 ohm output impedance, is not free of that issue, and I believe that's what I was hearing on the low end - a less than adequate damping factor. 
    To keep this in perspective - a "normal listener" (which I mean in the least condescending way possible) would probably not notice any issues whatsoever, unless maybe you set up an A/B comparison and specifically walked them through it. But without a reference point, the STU sounds great. And even with a knowledge of this small issue, I was still able to enjoy it well enough. 
    Next I switched headphones to see what worked and what didn't. The STU was able to drive the Sennheiser HD650 with authority, sounding very good for such a compact and relatively affordable all in one unit. The 300 ohm impedance of the HD650 is high enough to not be susceptible to the higher output impedance. I found I could comfortably use the low gain setting with the volume cranked high, or use the high gain setting with the volume knob turned lower. The HD650 is often called "dark" but from the STU it seemed nice and smooth without being overly veiled. I think I could be quite happy with this setup. 
    The STU also did quite well with the HiFiMAN HE-400. Despite a lower impedance, planar headphones seem far less affected in the way their dynamic counterparts can be. The HE-400 sounded clean, dynamic, full bodied, and was lots of fun, especially with rock and metal. The Asus had plenty of drive and bass extension was impressive. Once again, a combination I can highly recommend. 
    The Denon D7000 was good but a little sharp up top. Bass could be tighter as well, probably due to the output impedance mismatch, but the overall result remained mostly enjoyable. I'd say STU users would be more likely to own a D2000, maybe with some damping mods, and that could make a really nice pairing. I haven't heard the funny looking D7100 and its siblings so I can't comment on those.
    The Audeze LCD-2 (latest version in bamboo) made a great pairing with the STU. There was plenty of headroom and the slightly lean character did nothing to stop the LCD-2 from having ferocious bass when called up to do so. Soundstage, not an extremely high point for the Audeze cans as compared to some of its competition, was reasonably spacious and well defined if not particularly a standout attribute. Clarity was high and overall timbre was very believable. Again, I could happily live with this combo as my main setup, even speaking as a snob who is used to far more expensive gear. 
    I then tried the 600 ohm beyerdynamic T1. It's a headphone I enjoy at times yet find merely decent at others, and it really has to do with system synergy. I tend to prefer it with a smoother amp such as the Violectric V200, and it sounds great with tube amps as well. I was surprised at how decent the T1 sounded driven straight from the STU. It sounded fairly enjoyable. And yet.... if I was used to something like an HD650, and got the T1 as my first example as a flagship headphone, I'd be somewhat disappointed with these results. Not because it sounds bad, but the price difference approaches $1,000, and I don't hear nearly $1,000 worth of improvement. It's a very inoffensive sound, clean and clear, but lacks a degree of body and soul. Apparently the STU just doesn't gel well with the admittedly picky T1 sound signature. It's not a power issue, as the STU has plenty of headroom... it's simply a signature mismatch. I've heard worse, but also much better with the T1. I do realize this is a somewhat unrealistic pairing though - the STU user would be far more likely to use a DT880 or T70 or one of those types. But I don't own any of those, so T1 is all I can speak for. 
    Next I tried some IEMs. I found that on low gain, the STU amp section is very good for IEMs, with no hiss and a clean background. Channel tracking is nearly perfect as well. Too bad the high output impedance causes unpredictable variations in the frequency response - if not for that, the STU would be perfect for IEMs. As it stands I got good results from some of them (1964 Ears V3, Cosmic Ears BA4), and not so good results with others (Lear LCM-5, Heir 4.A). IEMs based around dynamic drivers probably do better or at least perform more consistently. Again, this is one of those things where non-headphone nuts might not notice or care - I suspect the average user would still be very pleased with most of these results, despite my picky complaints.
    I was able to determine that I like the amp section of the Leckerton UHA-6S mkII a bit more than the amp section in the STU. Even despite the impedance issues, the Leckerton seems more weighty on the bottom end, which anchors the performance and gives it a more complete feel. That's nothing to be ashamed of - the Leckerton is a killer little unit! On the flip side, as a DAC, the Asus is definitely superior. Faster, more articulate, bigger soundstage; basically a whole different class above, or maybe several classes. Based on this, I come to the conclusion that the Asus STU, like many integrated DAC/amp units, is a DAC first and an amp second - in contrast to the Leckerton which is primarily a portable amp, with the DAC feature as somewhat of a bonus. Which makes sense to me - home users are more likely to add a stand-alone amp down the road anyway, so it's logical for Asus to throw more resources at the DAC portion. 
    Having figured out that last bit of info, I went on to test the STU as a DAC only. Here it fared pretty well. Overall I'd characterize it as a "fast" sounding DAC, somewhat on the analytical side, with good detail retrieval and precise imaging. It does fall somewhat on the lean side, so it still requires a bit of care in system matching. I liked it with the Matrix M-Stage a lot more than I did with the Yulong A100 - the Matrix being more muscular while the A100 is maybe too much of a good thing, the results being too bright for my taste. Headphone matching was along the same lines - HD800 and T1 were not ideal pairings even when using a darker/smoother amp, and I imagine K701 (and variants) go the same way. But other headphones can be a very good match - HD650 chief among them, but also HE-400, LCD-2, and Thunderpants. I even ran the Asus in my Stax rig with an SR-007mkII, and the result was quite enjoyable. All this to say the Asus Essence STU is a worthwhile DAC when matched with the right associated gear. 
    A few other quick notes before I move on to comparisons:
    *I ran the STU without enabling the DC servo. I never had issues with thumps or pops so it didn't seem necessary. But I'm glad the option is there just in case. I enabled it briefly and didn't notice any glaring SQ deficiencies, but I'd have to spend more time before definitively concluding that it doesn't have it's own sound. Swapping opamps in a headphone amp has been known to increase DC offset to the point where it becomes a problem, though this design doesn't power headphones directly from a socketed opamp so perhaps that doesn't apply. If I did feel the need to enable DC servo, I'd probably swap out the OPA2134 for an LM4562 just to match the rest of the system. If I didn't, it would probably bug me, whether or not it actually made a difference in sound. Being in the signal path, it is certainly conceivable that the OPA2134 could color the sound, and probably in a negative way - but it didn't seem like an obvious problem on quick listen. 
    *I couldn't tell any difference between the digital inputs. Further, I didn't notice differences between high-end and basic transports. I used a Toshiba HD DVD player (yes, early adopter here, still have a collection!) which is just a mediocre transport, and couldn't tell a bit of difference between it, or a $1500 Audiophilleo AP1 with PurePower, or basic USB from a laptop. This is a good thing because the STU target market probably doesn't want to bother with a really high end transport. 
    *I used the SD Card Player in my Resonessence Labs Invicta to test Toslink, and was impressed with the 192kHz playback capabilities. I've got lots of far more expensive DACs which can't do that. 
    *The device has some type of soft delay. Power on takes a few seconds, and switching inputs takes a bit of time too. This helps with the lack of pops and thumps. 
    *The front panel LEDs are not overly bright. They could be even more dim and I wouldn't complain, but this is a step in the right direction, especially compared to a lot of blazing LEDs found on other gear.
    *The included stand, to prop the unit up on its side, actually works better than I thought. It's just plastic with a small bit of rubber lining in the right spots, so when I first opened the box I wasn't too impressed. But it seems to hold the STU tight enough to be sturdy even as I plug and unplug headphones. Downside? The DAC itself has feet on the bottom, so those will just be sticking out into space when running a vertical setup. It looks kind of awkward. I notice the promo pictures always show it from the "good side" to avoid this. Also, the DAC is on the lighter side, so heavy cabling could pull it when side mounting. Still, It's a nice feature for those with limited space. 
    *There's an internal jumper for bypassing the line-out volume control. This removes the potentiometer from the signal path and theoretically improves sound quality. Default setting is for volume control to be enabled. I went back and forth a few times and thought I maybe heard a very small difference. Certainly not night and day, and I wouldn't mind always running in variable mode if that's something I'd use at times. In fact, I did use it when running with a pair of active monitors, and it worked just as expected. Nice to have the option either way though.
    The STU lands in a somewhat unique price bracket. It costs more than a lot of the budget equipment out there, but less than most really "serious" audiophile gear. I don't have another $400 DAC on hand so you'll have to bear with these comparisons.
    Audinst HUD-mx2 ($250): The Asus sounded more open and transparent than the little Audinst. It had a more believable soundstage too, with imaging being more precise. The Asus also sounded more even, no matter what input was used - the Audinst has a different tone over Toslink compared to USB, which I suppose could be considered a benefit if you like the option of a more relaxed signature. The Asus also has the advantage of accepting USB signals higher than 96kHz which is becoming more of a useful feature as more and more albums are released in the higher formats. 
    The DAC advantage is clear, but the amp section is less obvious. While the Asus was slightly cleaner and more airy, the Audinst was extremely similar. Both use the same TPA6120 for amplification so I suppose it makes sense that the sound isn't far off - but I've heard the TPA6120 sound very different in other designs (like my Kao Audio UD2C-HP), so it's not always the case. Audinst does some modification to the circuit resulting in a 2 ohm output impedance, making it far more suitable for low impedance headphones and especially multi-armature IEMs. 
    Ultimately the Asus is the superior product, but the Audinst remains a good value at the lower price. For someone on a tight budget I might recommend adding that cash to the headphone fund rather than buying the better DAC. Then again, someone with a higher quality headphone like HD650 will hear a very worthwhile improvement with the better DAC. 
    Parasound Zdac ($475): This is a very different sounding product compared to the Asus. Warmer, richer, more smooth and creamy, it's all about musicality rather than hyper-detail. It could be a much better choice or a far worse choice depending on your system and preferences. It adds XLR outputs but subtracts preamp functionality and USB connectivity beyond 96kHz. The headphone section uses the same TPA6120 with the same 10 ohm output impedance, so neither one is ideal for IEMs. Personally I'd choose the Zdac every time for headphones like the T1 and HD800. For an LCD-2 or HE-400, the Zdac is still good, but I'd give the edge to the Asus. They split the difference with the former flagship Sennheiser models, with Zdac loving the HD600 and Asus doing better with HD650 (though both are quite good either way). 
    In the end I think each DAC lines up well with the philosophy of their brand. Asus is a high-tech company and the STU excels at detail and accuracy, using a variety of advanced techniques to get there. Parsound, on the other hand, is a far more experienced audiophile firm, and really has the analog design expertise to pull off a musically compelling sound. A lot of it probably ties in with the power supply and analog stages rather than digital wizardry. Either of these units, when used in the right context, should give years of faithful service, and I can confidently recommend both. 
    Resonessence Labs Concero ($599): The Concero is truly a giant killer in my opinion - a tiny USB powered box that puts a lot of fancy audiophile DACs to shame, despite any price differences. The Asus doesn't have the same level of realism, which becomes especially noticeable with well-recorded classical. Concero is more transparent and clear, and reveals more layers in each track - assuming they exist in the first place. The Concero also has more "swing", while the STU comes off as a bit lighter in tone. 
    This comparison highlights the difficulty faced by Asus in trying to make waves in this market. There are just so many other exceptional choices, and there ends up being an ever smaller window in terms of pricing, where their offering can compete. In the case of the STU, I can see the argument - "should I spend $400 on the Asus or save another $200 and go for the Concero?" Yes, the Asus does have more features, so it's got that going for it, but the Concero is just tough competition based on pure SQ. So it really depends on what sort of device one is looking for - can you give up the larger selection of inputs and features for a distilled, high performance USB DAC? If yes, Concero, if no, STU. 
    Asus Xonar Essence One ($599): I was able to borrow the Essence One from a friend, in order to compare to its STU sibling. This was a stock unit rather than the Muses variation and my friend had never rolled opamps. To be honest, I don't think they sound all that different. The One seems more capable as a headphone amp, but as a pure DAC the differences are a lot smaller. Certainly smaller than ST versus Concero. I didn't get as much time to compare them as I would have liked, so I'm not making this a definitive proclamation at this point. But so far I'd say the STU might be the better buy if you intend to run an outboard amp and thus nullify that advantage. With the option of adding a linear power supply and better opamps down the road, a gradually upgraded STU would still cost less than the base model One. And the user wouldn't be paying for things like XLR outputs which they might never use. Makes sense to me. 
    Asus was kind enough to send along a few sets of opamps for my rolling adventures. I also had a few of my own, so I was able to get a good feel for the capabilities. Since it only takes 3 opamps, it's not that difficult or expensive to try. Compare that to the One which has 11 socketed opamps to mess with. I have to reiterate that the stock setup is very nice - some people might find it better than any of the variations I tried. So rolling is merely optional and not in any way required.
    First I tried the Muses 01 (I refuse to type it in all caps like the company does....) which is used in the Essence One Muses Edition. This is a very expensive J-FET opamp - somewhere in the realm of $50 each, if you can even find them. I've never been completely blown away by this chip, despite the high price. But I have heard it sound pretty good, which ends up being the case here. It's got a squeaky clean, hyper detailed character to it, an extreme "Hi-Fi" sound that detail lovers would kill for. Soundstage is correspondingly large as well. In that respect it does bring the STU up to a higher level. The downside, in my humble opinion, is that it results in a somewhat unbalanced presentation. The STU was a bit on the brighter side already but it downright shimmers and gleams with the 01 installed. It therefore becomes too bright for my tastes unless paired with the warmest, smoothest amp and headphone combinations. Using the Violectric V200 with the LCD-2 works well enough but even then, I tire of the sound sooner than later and end up wishing for the original flavor to return. I can see how some people might like this result but I'd call it overpriced for what it is. 
    From there I went with the OPA2604 which is a notoriously picky opamp. It can sound really good in the right circumstances, but generally sounds bad when rolled into a circuit not specifically designed around it. That's exactly what happens here. Compared to stock, the sound is muddy and indistinct, yet still manages a thin, grainy feeling, all with a flat, two dimensional soundstage. This is definitely not an option I recommend.
    After that experience I went to the tried and true OPA2134, as found in the DC servo. This is a workhorse opamp found in many applications, and generally sounds decent if not spectacular. It followed the same pattern in the STU - good resolution, nice bass extension, a little softer up top than the stock configuration, and not as detailed overall. This is a cheap opamp and might be a good alternative option for someone who finds stock a little too bright, though it comes at the expense of some definition and realism. Personally I like the original setup better - and I suspect there are better choices out there for a more musical presentation.
    Finally I tried the Muses 02 bipolar opamps. Now that's more like it! I've never used this one before, but I find it to be a great match in the STU - better in a lot of ways (but not all) than the stock LME49720/LM4562. The best word to describe it is natural - it's warm, organic, yet still very detailed, placing the STU closer to the Resonessence Concero than any other opamp I tried. Bass impact finally makes a grand appearance, though I'd still never call the STU a bass monster. Top end has just the right amount of sparkle without going overboard. The only thing stopping it from being a clear upgrade all around is the soundstage, which is just slightly smaller than the stock configuration. It doesn't feel particularly restricting, but it's just not quite "there" in the same way as the original configuration. Overall though, I'd say the Muses 02 is a clear winner, and a major upgrade to the Asus Essence STU. For my tastes, this is an ideal combination, though of course there are many other opamps I have yet to try. AD797 would be a good idea, as would OPA627 on dual adapters for each spot, AD8610, OPA209, AD8397..... the list goes on, but I didn't have those handy so I have no idea how they would perform. And I can think of a dozen others which may be good candidates as well. The Muses 02 is spectacular, and even at $150 for the three of them it seems worth it to my ears, but you may be able to approach the same level for a lot less using other opamps. 


    A quick note - my review loaner is an engineering sample which does not quite match the final configuration. Specifically, mine doesn't have the Cirrus CS2000 on board for reclocking. It looks like all the supporting hardware is there but the chip itself is absent. Astute readers will recall this being the case with the Essence STX as compared to the ST. People having experience with both cards describe a general increase in sound quality, specifically as it pertains to realism and imaging. The STX is reportedly somewhat hazy and indistinct in comparison to the ST, which I speculate is due to improved clocking performance. In any case, the implication is that I'm not quite getting the full experience from my particular STU, and retail versions should be improved by either a small amount or maybe even a large one.
    Back to the comparisons - unfortunately I don't have access to a computer equipped with an ST or STX. But even if I did, I suspect there's enough variability in there to confuse matters. A gaming PC with a really nice PSU is going to have the advantage over a basic system with some voltage sag. But I did ask Asus for their thoughts on why the STU is superior. Their answers confirmed what I already thought, so here's a list:
    1) Asynchronous USB is a huge advantage over internal PCI or PCI-X interface. You might be familiar with the arguments for asynchronous versus adaptive USB modes? Well, the internal sound cards operate in the same way. Timing will not be as exact and thus jitter will not be as low. 
    2) Better opamps by default. The LM4562 and LME49720 were popular options to be used with the ST and STX. The STU already has them. 
    3) Gain selection for headphone use. This is critical when using a range of different headphones with different requirements.
    4) Mirrored PCB layout for lower cross-talk.
    5) Dedicated volume control, complete with jumper bypass option. This is a big one. With an internal sound card, all volume control must be done in the digital domain. Now, I've had good experience with digital volume control, and I don't find it as objectionable as some other reviewers. 32-bit implementations can be essentially transparent in my experience, and even 24-bit can be quite good - especially at higher volumes where less attenuation is called for. The STU is a 24-bit implementation which doesn't worry me so much. But how high do you normally run your volume levels? Maybe 50 percent? 75? Or far less when using sensitive headphones? I find myself at 30-40 percent quite often on my desktop system. With the ST and STX cards, that would result in a significant loss of resolution. The STU on the other hand uses a reasonably high quality Alpha brand potentiometer (similar to the one found in the Essence One) allowing for no loss of resolution. Of course, analog pots are not perfect, and I get the slightest bit of channel imbalance with IEMs (at extremely low levels - as in nearly silent - I hear one channel drop out before the other). And we can only assume some loss of transparency is taking place by even using a potentiometer at all, though I'd say it would still be less than digital attenuation in this case. 
    6) Better components - the STU uses higher quality parts such as WIMA FKP2 capacitors where the ST/STX counterparts use some yellow (unknown to me) variety. It also has a more advanced power supply section, complete with DC-DC conversion and ultra-low dropout voltage regulation. 
    7) Distance. The ST/STX units have dedicated shielding to keep out unwanted interference. But they still, by their very nature, are forced to live inside the PC case which is full of other components. The STU solves that problem - as an external device, it isn't subject to the same exposure. 
    8) Options. It bears repeating once again that the STU has way more functionality than the internal soundcards. Gain adjustment, independent volume controls, selectable DC servo, etc. 
    9) Lastly, and this is another big one for me - it gets rid of the PC requirement. Maybe this is obvious, but I feel to the need to expand on it. Yes, computer audio is growing very quickly, and I'd wager far more new audiophiles use computer-based systems rather than optical media. And yet, within the "computer audio" genre there is a growing subsection of users who don't necessarily want to use an actual desktop PC. Some people stream from their laptop - in fact based on the numbers there are more and more laptops and less desktops out there as each day passes. Others build dedicated small form factor machines running VortexBox. Some might even use a Blu-ray player or other network enabled media streamer for UPnP playback from a NAS. Others simply use a Squeezebox Touch or other affordable dedicated device. The point is - none of these situations allow for an internal sound card like the Essence ST. So Asus is wise to bring a device to market which can accommodate all of these scenarios. 
    The Asus Xonar Essence STU seems destined to become a popular product. I'm proud to have received an early prototype for review, and even more proud that Asus is reaching out to the HeadFi community for feedback. It's a tough place to start - they could easily have reviews done by various gaming and PC websites with little experience in terms of audiophile gear, likely resulting in rave reviews and awards. Instead they came to HeadFi where we know all about the tough competition in this segment. Asus answered all my technical questions but then stepped back and let me write whatever I want. For all they know, I'm writing a scathing critique and effectively killing some pre-release buzz before the product really launches. 
    Thankfully I didn't have to write such a review. The Essence STU is a very worthwhile device that definitely deserves to be heard. Yes, it competes in a very tough market filled with great equipment, and because of that it doesn't quite stand out in its field the way their ST/STX products did. But I find that it has enough sound quality and features to justify its existence, and I honestly can't think of anything else I'd recommend at the price. Spending a bit more will get you a different sound (Parasound Zdac, Yulong D100 mkII), possibly better for some systems but worse for others. You have to move up to $599 for the Resonessence Labs Concero before getting a significant improvement, and even then it comes at the expense of multiple features.
    For it's large feature set, great sound quality, customization potential (opamps, linear PSU), and general good looks, I give the Asus Essence STU my recommendation. And remember - my review unit didn't have the full configuration yet. Adding the Cirrus reclocking chip may prove to be a minor improvement at best, or it could be significant. If that was the case the STU would be an even stronger value. Either way, it's definitely a product worth looking at. 

    So it turns out there was some confusion regarding the CS2000 clockgen on my STU loaner. My unit actually does have the CS2000 on board:
    It's not clearly labeled so I was unaware of it being there, and Asus thought they had sent me an early engineering sample that didn't have it. Turns out the "empty" spot on my loaner was a place where they considered putting some other reclocking chip, but decided against it. 
    What does this mean to you, the reader? Probably not much. I guess there isn't some hidden performance potential waiting to be unlocked - what I heard was as good as it will get, which is still pretty darn impressive. I still definitely recommend the STU for what it is. 

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    1. View previous replies...
    2. project86
      I'm not sure on the release dates for any region at this point. All I know is "pretty soon". Sorry!
      project86, Jul 14, 2013
    3. shyamelge
      I checked with Asus in my country and they say it may take 3-4 months for this product to arrive.
      I have a question.
      Can one use the same swapable op amp (as listed in this review of STU) for Asus Essence One basic model (released in 2010 or so)?
      Is it possible to swap op amps on the basic Asus Essence One without using any soldering equipment?
      If yes, can anyone suggest an op amp, more suitable for vocal jazz and western classical music?
      shyamelge, Jul 29, 2013
    4. shyamelge
      For Asus Essence One model, where would you prefer to put the combo of op amps LM4562 and LME49720?
      shyamelge, Jul 31, 2013