The first thing I noticed about the Asus Xonar Essence STU was it's diminutive size, it's a lot smaller than I was expecting. It is big on features though, having separate high quality potentiometers for the two volume pots (yes two), headphone (6.3mm) and pre-amp outputs (RCA), inputs for USB, optical and co-axial SPDIF, a 3.5mm analogue input for connecting a DAP or phone and a clever gain toggle switch that does nothing when headphones are connected, you have to remove them to change the gain. There is a bank of LEDs to show which input you are using (next to the selector button) and an extra LED for telling you when you are listening to bit perfect audio which only seems to work when using the included ASIO driver, it doesn't seem to register WASAPI in exclusive mode, which is a shame.
Inside the case the STU shares a great deal with it's sound card sibling, the Asus Xonar Essence STX (they really seem to like long names). Both the STU and STX use the Texas Instruments Burr-Brown PCM1792A, a premium quality DAC. This feeds into a couple of op-amps, two LME49720 in the case of the STU and two NJM2114D in the case of the STX. The 49720 is a popular upgrade for the STX so makes a lot of sense for using it in the STU. Both the STU and STX then use the TPA6120A2 amplifier (so does the iFi iCAN) to amplify the signal for output to headphones. Along with most other users of the amp, the output impedance is around 10ohms, which makes it less suitable for using with IEMs. This relatively high impedance can be reduced with the use of inductors, as evidenced by iFi with the iCAN (<1ohm) and it's a pity that Asus didn't do the same. The asynchronous USB is handled by the C-Media CM6631A, which is also used in the USB cards for the Schiit Bifrost and Gungnir DACs, so the STU is in some very good company. Aside from the ICs, the widespread use of Wima and Nichicon capacitors help keep standards high.
For listening tests I used Sennheiser HD 650 and AKG K702 headphones and for comparison I used the Asus Xonar Essence STX sound card and Aune T1 external tube DAC and amp fitted with a Mullard 6DJ8.
Music used:
Dire Straits - Love Over Gold (24/96 FLAC)
Rush - A Farewell to Kings (24/96 FLAC)
Rush - Moving Pictures (24/96 FLAC)
Sade - Diamond Life (24/96 FLAC)
Gillian Welch - The Harrow and the Harvest (16/44.1 FLAC)
Kimbra - Vows (16/44.1 FLAC).
First up were the Sennheiser HD 650s, which the STU handled without any trouble at all, on low gain with the volume cranked up to 3 o-clock or high gain with the volume at about 11 o-clock, either way, they sounded great. The sound was very detailed, clear and natural with a nice bass punch and suited every track I threw at it. 24bit 96KHz versions of Telegraph Road (Dire Straits) and Xanadu (Rush) were very impressive, the DAC managed to drag so much detail from the high resolution tracks that it sounded as though I was there in the recording studio. How much of this was down to the high bit rate and sample rate or the ultra high quality re-mastering is anybody's guess but the result sounded very good indeed. The modded AKG K702s faired almost as well, with volumes in the same ball park as the HD 650s which just goes to show how inefficient they are as they have 1/5th the impedance of the Sennheisers. The K702s and the STU didn't have quite the synergy that the HD 650 and STU managed, probably because both the headphones and amp can be a little bright, so a touch of EQing within MusicBee was necessary to keep my ears happy. Noise was conspicuous by it's absence, there just wasn't any. With the music stopped and volume cranked there was just blissful silence.
Comparing the STU with it's sibling, the STX, proved to be anything but easy as they are so much alike, using the same DAC and Amp. I think the best way of describing the STU is that it's a refined and improved STX. The fact that it has an analogue volume control actually makes a big difference because it makes the use of native ASIO possible without having to adjust the volume in each separate application like the STX, which only has digital volume control (which can cause sound degradation if the bit depth is set to 16). The STU uses more high quality components, has asynchronous USB (for lower jitter), better stock op-amps and most importantly, it's not inside the noise factory known as the PC. It would be easy to say all these improvements make a huge difference but the truth is, they don't. The differences in sound quality between the STU and STX are quite subtle, the STU does sound very slightly smoother and clearer but there really isn't a lot
in it.
I was more surprised when I added the Aune T1 into the mix because again, the differences were subtle and I was expecting more. The T1 DAC uses the TI PCM1793, which is a little brother of the PCM1792A used in the STU, so the sound was always likely to be similar. Where the T1 differs from the STU though, is that it uses a 6DJ8 family vacuum tube in the buffer stage instead of op-amps and by careful selection of tubes the sound can be tailored to suit the listener. The T1 also has a high quality linear PSU which should mean less noise gets added to the audio signal. A linear PSU has been rumoured as a possible future upgrade option for the STU but is not available at this time. Considering I've changed the default tube to a 40 year old Mullard I was astonished at how close the T1 sounded to the STU. There were differences, the STU was slightly clearer and more detailed while the T1 was smoother and had a slightly larger sound stage but it says something that in order to hear these differences I had to use the AKG K702's because of their analytical nature and several hours of switching back and fore between the three amp/DACs.
The Asus Xonar Essence STU is a good DAC/Amp combination, it sounds good, uses good components and has plenty of features. The question is though, is it worth £300 when the STX II is £175 with some of the STU's improvements plus MUSES op-amps and the original STX and the Aune T1 are both £135? Firstly an internal sound card may not be an option for a lot of builds, specially those with dual graphics cards as there may not be a free and clear PCIe slot in which to fit it. There is also the problem of electromagnetic interference and in some builds it is just there and there is nothing you can do about it. If either of these scenarios are true then an external USB DAC/amp becomes a necessity and the STU begins to look more tempting. While the Aune T1 is a lot cheaper than the STU, it also has less features....... and the only digital input is USB...... and those tubes do have a limited lifespan.
Overall I think the STU is a very good piece of audio equipment but at a slightly too high price point, I think £250 would make it a no-brainer.
Final thoughts....
Once I had finished testing, I had the STU, the STX and the Aune T1 sitting in front of me and the one I chose to listen through for eight hours solid is the STU.
I think that says a lot.
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