4 Asus Xonar STU

ASUS Xonar Essence STU USB DAC Review

  1. Introduction
  2. Specifications and Product Pictures
  3. Installation and Initial Impressions
  4. Sound quality
  5. Conclusion

Creative and ASUS have been making a lot of PC gaming audio cards for many years now. ASUS created a stir in the discrete desktop sound card market when they launched the STX and then the ST sound cards that were made more for music lovers than hard-core gamers. They featured (at that time) unheard of features in sound cards like featuring a built powerful headphone amp and offering swappable OPAMPs.

However, these cards require either a PCI port (STX only) or a PCI-E port (ST only) to work. So using these cards on say laptops or on some mATX motherboards is a problem.

So, ASUS has launched a USB version of the STX and ST cards in the form of Xonar Essence STU. The ASUS Xonar Essence STU is based on the same PCM1792A chip that is found in both the ST and STX. That said just having the same chip does not mean the STU will sound exactly the same as the ST and STX. The implementation of the circuit around the DAC is very important. There are many instances of two DACs using the same chip sounding very different.

Asus Xonar Essence STU isn’t merely the STX/ST cards in a box with a USB connection. ASUS has added a few more tweaks to the sound to justify the higher price as well as some additional features like multi OPAMP switching (for further tweaking of the sound as the STX/ST had just one OPAMP that could be swapped), a dedicated volume control, ability to use the STU as a pre- amp and more.


Bus CompatibilityUSB 2.0 high-speed
Audio PerformanceOutput Signal-to-Noise Ratio (A-Weighted) (Front-out) :
●120 dBOutput THD+N at 1kHz (Front-out) :
●>0.00036 %(-108 dB)Frequency Response (-3dB, 24bit/192KHz input) :
●10 Hz to 48 KHzOutput/Input Full-Scale Voltage :
●Unbalanced Output : 2 Vrms ( Vp-p)
●Headphone : 7 Vrms ( Vp-p)
ChipsetAudio Processor :C-Media CM6631A High-Definition Sound Processor
Sample Rate and ResolutionAnalog Playback Sample Rate and Resolution :
●44.1K/48K/88.2K/96K/176.4K/192KHz @ 16bit/24bitASIO Driver Support :
●44.1K/48K/88.2K/96K/176.4K/192KHz @ 16bit/24bit
ConnectivityAnalog Output
●1 x 6.3 mm jack (1/4″) Headphone out
●2 x RCA (Un-Balanced)Digital
●2 x S/PDIF in (1 x Optical (Toslink)/1 x Coaxial)
●1 x Aux in 3.5mm jack (1/8”)
Operating System●Windows® 8 , 32bit/64bit
●Windows® 7 , 32bit/64bit
●Windows® XP , 32bit/64bit
●Mac OS X *
Accessories●6.3mm to 3.5mm stereo adapter x 1
●Driver CD x 1
●Quick Start Guide x 1
●USB cable x 1
●Power cord x 1
●Power adapter x 1
NoteSwappable operational amplifier:
●I/V: LME49720 x 2
●LPF: LM4562 x 1 Output impedance:
●Line out: 100 ohm
●Headphone out: 10 ohm Clock:
●Crystal clock: 12MHz
●Oscillator: 45.1584MHz (44.1kHz);49.152MHz (48kHz) Power supply type:
●Power consumption(Max): 12W
●Power input jack: DC in 12V
●Power input range: AC 90~264V*Xonar Essence STU supports OS X 10.6 or above for playback.

The bundle has everything most people would require for full functionality of the DAC including a useful 3.5mm – 6.3mm jack to use the larger and more durable 6.3mm headphone socket.

Those wanting to use the Coaxial or optical inputs will need to purchase a separate wire which is the norm for every budget DAC.

What could have been added at this price point are a few OPAMPs to try out swapping OPAMPs, but most people wouldn’t wish to dabble in such an endeavour.

One positive that should be mentioned is ASUS’ commitment to older OS like XP which is fully supported by the STU. Windows XP still commands a formidable 42% market share (among Microsoft OS) and Creative has disabled support for XP with its competing Recon 3D and ZX based sound cards.


Installation is very straightforward. Just plug the bundled USB cable into a free USB port and attach the bundled power adapter into a power outlet and the DAC is recognized by the OS. You will have to install the drivers on the bundled CD or download the drivers from the ASUS website.

Now here is where the stark differences between ASUS and its nearest competitor Creative arise. The STU doesn’t come with even rudimentary software. Any Creative sound card (aside from the basic budget ones) comes with a huge bevy of software tweaks bundled along with the cards. In comparison the lack of even the Dolby software that comes with ASUS own Xonar U7 which is at a lower price point is a little surprising.

That said most people don’t use these software tweaks for a music oriented card. I would place sound quality much higher than software tweaks anyways.

The build quality and finish of the STU is good with a brushed aluminium finish of the external casing. However I wasn’t very happy with power switch. The switches are recessed into the case to prevent accidental changes but you cannot really tell if the power button is on or off. So if you plug in the DAC when the power button is left on there are sparks at the plug which isn’t very nice.

Other than that there are no complaints. The volume knob has a nice long throw and is sufficiently damped to allow for smooth changes of the volume. ASUS has also provided lots of connectivity options like Optical/SPDIF, Coaxial and USB digital input, a 3.5mm analog input, headphone and unbalanced RCA output. So like the ST/STX sound cards the STU lacks 5.1 channel support which is a downer considering the lower end ASUS Xonar U7 supports 5.1 channel output.

One special mention has to be the OPAMP tuning. ASUS provides a circuit with option to swap 3 OPAMPS. ASUS has to also be commended that their official manual on OPAMP swapping includes a detailed guide from an end user on head-fi which shows what sound one can expect by swapping the OPAMPS. That said its a bit disappointing that ASUS didn’t include any spare OPAMPs in the bundle for testing. However since the sonic characteristics of OPAMP swapping is mentioned in the manual one can purchase only those OPAMPs to get the desired sonic signature.

Other features worth mentioning are a selectable gain switch for using both high impedance and low impedance headphones/earphones, DC Servo jumpers (to minimize the pop up noise when using the built in headphone amp) and jumper settings to bypass the volume control (to use the STU as a pre amp when connected to a dedicated amplifier).


The STU was tested as a DAC/headphone amp and as a standalone DAC connected to a separate headphone amp. I used the following headphones to test the DAC – Sennheiser HD650, Yamaha YH-100 modded, RHA Audio MA450i (review sample from RHA Audio). There was also an EHHA headphone amp used for the pure DAC testing. The STU was compared to my Music Streamer II (pure DAC) and an el cheapo Fiio E10 (DAC/AMP). Due to testing constraints I was unable to compare the STU’s USB input with either the coaxial or optical input.

First testing was when using the STU as a standalone DAC when connected to the EHHA headphone amp.

The STU has a very detailed sound and for once I felt it gave a real fight to the Music Streamer II which is a pure 2 channel DAC with just USB input. However it costs half as much as the STU but lacks a lot of features compared to the STU.

The treble and mids on the STU were very close to the Music Streamer. However the bass was the let down. The sub bass seemed to be cut off almost completely and the mid bass wasn’t that good either.

Soundstage was appreciably wide but the USP of the Music Streamer II is its 3D soundstage and the STU couldn’t beat it here.

Compared to the cheaper E10 it was a no contest. The E10 is a bit muddy and there isn’t a ton of detail so the STU was much better here. That said the E10 had better bass in impact and depth. However the E10 has a noticeably mid bass hump which makes the bass more muddy than defined. The E10 can compete somewhat against Creative solutions like the Recon 3D and ZX or even with the ASUS U7 but against the STU it is out of its league.

Using the inbuilt amp on the STU showed that the amp compensated for the deficient bass by boosting the sub bass which results in a much improved performance.

However my EHHA amp has noticeably better soundstage and slightly better detailing. Mids sounded about the same.

The built in amp was however very competent and could drive any of my headphones with gusto. There was no lack of power whatsoever.


While I was very impressed with the lower end sibling, (Xonar U7), I was less impressed with the ASUS Xonar Essence STU. The mediocre bass was a rather glaring fault considering the U7 faced so such issues.

That said the DAC has a good build, good amount of tweaks available and swappable OPAMPs is always a plus. Also the treble and detailing is a level above the U7 or even Creative ZX and Recon 3D sound cards.

It would be great if ASUS can bundle few OPAMPS to let users get a feel of OPAMP swapping in future.

The STU costs $399 on Newegg and about $406 on Amazon US. For that price I am a little hesitant to wholeheartedly recommend it. Its still a good quality product that’s well made but ASUS’s own U7 DAC has better bass performance and costs far less than the STU. Its the mediocre bass performance that’s holding back my recommendation of a value for money product.


For those who want a USB sound card that’s sounds not too far off from the STU and has 5.1 channel support to boot I would wholeheartedly recommend the Xonar U7 to anyone for the following price:


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