Asus Xonar Essence STU USB DAC and Headphone Amplifier
Input: Asynchronous USB, 1 RCA Coax S/PDIF, 1 Toslink S/PDIF, 3.5mm Aux
Output: 1 pair unbalanced RCA, 1 x headphone-out 1/4" jack
Dimensions (H x W x D): 1.5" x 8.25" x 6"
Weight: a couple of pounds
Availability: Through Authorized Dealers
The Little Asus That Could
I was impressed with the Asus Xonar Essence One MUSES Edition that I reviewed back in November of last year (see review) finding it fun and engaging to listen to. The new Essence STU due to be released later this month is the Essence One's little brother offering much of the same functionality of its bigger brother minus the latter's XLR outputs. Inside the STU we've got the Texas Instruments (TI) PCM1792A DAC, the 32-bit/192kHz-capable Cmedia CM 6631A asynchronous USB receiver, the TI PCM9211 S/PDIF receiver, and the TI TPA6120A headphone amp. The STU wraps all of this tech into a slender iPad-sized dark gray chassis with an analog volume control for the RCA outs and a separate volume control for the heapdhone out. Add in a two-position Headphone Gain switch and you've got yourself one fulsome package for a hair under four hundred bucks. Nice!
The STU sports a "Dual clock design and dedicated oscillators that support multiples of 44.1kHz (45.1584MHz) and 48kHz (49.152MHz) independently." as well as TI NS-LME49720 and NS-LM4562NA Op Amps. You can swap out these Op Amps to tailor the STU's sound to your ears preference. I was not sent any additional Op Amps to try out so I will not be reporting on this aspect of the STU's performance but suitable Op Amps are available from vendors including Digikey or Farnell from a few dollars a piece up to $50.
The async USB and S/PDIF inputs are around back along with the 3.5mm input, power inlet for the external switching power supply, headphone gain switch (Low/High), and RCA outputs. The front panel hosts the aforementioned volume controls, 1/4" headphone jack, on/off switch, and an input selector push button switch with associated white LEDs. There's also a "Bit Perfect" LED that lights up when using the USB input with the Asus ASIO drivers. The controls feel nice and firm if a tad light weight but considering the STU's cost, the whole package looks pretty sleek dressed as it is in silver and nearly black dark gray. There's also an included plastic stand that allows for vertical placement taking up less desktop space than the more traditional horizontally prone position.
According to Asus, the STU boasts an impressive 120dB signal-to-noise ratio (SNR) and, "Analog signals are separated from digital signals on different sections (No Cross Over rule) and on different grounds (Hyper Grounding technology) of the PCB to prevent cross-talk. Furthermore, the PCB and traces are symmetrically balanced in order to preserve signals and reject spurious noise." With headphone output impedance rated at 10ohms, the STU is more suitable for high impedance headphones. You can also select between capacitor coupling or an onboard DC servo with internal jumpers "...to minimize pop noise which may occur when powering on". Asus also includes a 1/4" to 3.5mm adapter and a USB cable in the package.
While you have the STU's hood up, you can also bypass the STU's internal volume control with jumpers and adjust L/R balance with dedicated variable resistors. All in all, this is one flexible unit. I mainly used the Asus Xonar Essence STU via its USB input in my desktop setup with my ADAM A3X active speakers and in my main system with the Pass INT-30A.
Fit for a Desktop Near You
I preferred listening to the Asus Xonar Essence STU USB DAC and Headphone Amplifier in my desktop system mainly because I believe that's where Asus sees it fitting and it sounded better there than in my main hi-fi. Let's get that critical exercise over with up front. My main gripes with the STU when mated with my MacBook Pro, Pass INT-30A, and DeVore Fidelity The Nines is it presented a rather heavy-handed sound. Music was a bit amorphous-sounding where the musical image was rather dense and the differentiation between the various players was less distinct than I've heard from other DACs. In this regard, the 24/96-capable AudioQuest Dragonfly (see review) paints a more detailed and delineated picture but it does not offer up as full-bodied a sound as the STU. The Dragonfly also paints a flatter picture where the Asus offers up a more rounded, solid, yet compact presentation. The Dragonfly is also USB-only with a single 3.5mm analog output.
I've also heard more air and space in my music as offered by the Meridian Explorer (see review), but again the Asus outweighs the Explorer with a more fulsome sound. There's more apparent meat on them bones although the Explorer also offers up a richer tonal palette. Another reason I say the Asus is better destined for the desktop is it does not offer a remote and I prefer having my headphone amp on my desktop but that's a site-specific preference to be sure. But to finish off the Asus for far-field hi-fi listening, while it offers a pleasant sound overall with no etch or glare, it also muted tone colors and tended to bunch the music up into a tighter than natural-sounding package. Everything gets a tad homogenized.
On my desktop with my ADAM A3X's the Asus was much more at home. The ADAMs do a wonderful job with upper frequencies adding a real sense of sparkle and life which mated well with the STU's otherwise darkish-sounding presentation. The STU's weighty sound was also welcome in the nearfield giving music a nice physicality and it acquitted itself very nicely even with less than ideal streaming sources. I do most of my music exploration listening on my desktop which consists of lots of music streamed from SoundCloud, Bandcamp, NPR's First Listen, Pitchfork's Advance, and of course Boomkat and Bleep to name just a few, and I completely enjoyed my time with the Asus in this regard.
I tend to equate a darker, richer presentation with fun and a lighter-weight, more detail-oriented sound with an intellectual approach to listening. That puts the STU USB DAC square in the fun camp according to my ears. Ergonomically, I also preferred having the STU's volume controls within arm's reach and the vertical orientation also makes it easier to adjust the volume controls giving you more space around the knobs for your fingers. Because the STU only sits an inch or so off the desktop when positioned horizontally, that makes the tiny-ish volume knobs a bit awkward to adjust.
As a headphone amp driving my Audio Technica ATH-W1000s, the STU was also well suited to the task. There was plenty of drive, detail, and delicacy. There was also a touch of dryness in the upper registers but I'd attribute that as much to the Audio Technica's as to the Asus. Bass response was good but not exceptional falling on the loose side of tight so if you are a bass lovin' freak you may want to look elsewhere for that big booty and here the NAD D 1050 comes to mind (see review). I also tried the STU's 3.5mm input with my iPhone as source (and DAC) and the sound was as expected from the iPhone more or less with that tell-tale sameness to tone colors and overall dark slightly shut in presentation but it was certainly pleasant enough to enjoy some tunes in a pinch.
A Good Desktop DAC and Headphone Amp
I'd say the Asus Xonar Essence STU USB DAC and Headphone Amplifier makes a very good desktop music playing companion. The digital inputs can handle up to 24/192 playback, the overall presentation is fit and fun, its analog volume control functions well without any obvious sonic degradation, and the headphone amp offers some flexibility with its two gain settings. Add in the ability to alter the Asus' sonic character by swapping Op Amps, and you could build yourself a custom-tailored component to taste for not much dough.
Also on hand and in use during the Asus Xonar Essence STU review: AudioQuest Dragonfly USB DAC, Meridian Explorer