Performance

The ASUS Xonar STU comes full of some of the best ICs for audio amplification, and its headphone amplifier is definitely one of the best for medium- to high-impedance headphones. The STU also features a gain switch, which allows you to toggle between a low and high gain. The low gain is sufficient for most headphones and in-ears.

The performance of the STU was compared to these different DAC/amp combos: JDSLabs O2+ODAC, ASUS U7, and Sunrise Ray DA-P1.

The STU performs very well when hooked up to the HiFiMAN HE-500s; however, it did not seem as competent in the bass department as the O2+ODAC. It had better clarity than the Sunrise Ray DA-P1 and is in a completely different class than the ASUS U7's headphone out, which is no surprise. The sound quality of the STU's headphone out is also a small notch above that of the STX with upgraded op-amps. The STU is not good with in-ears, and it shares this problem with the STX despite having a better volume control. Whether its output impedance or another design decision makes it sound a bit sloppy with in-ears is not clear, but it does not sound as detailed as the O2 with in-ears, which the analytical UE IERMs highlight well, but also the JH-AUDIO JH16Pro and UE PRMs suffer deficits in this area.

Both the STU's and STX's gain favors full-sized headphones. The STU and STX go well with a set of high-impendance, hard-to-drive headphones, like the Sennheiser HD 580/600/650/800. The STU delivers a very balanced sound, with KRK System's KNS-8400s, which does not change once you move up to a set of HiFiMan HE-500s, but not even the STU can power the extremely power-hungry HiFiMan HE-6s. Using the very easy-to-drive QPAD QH-90 headset has the STU sound great—there is no noticeable difference by going from the STU to the O2+ODAC except for a small change in tonality.

STU's headphone-amplifier performance is alright for its price and size, but its qualities as a DAC are remarkable. We took the line out and used it to feed our O2 amplifier, and the results were very pleasing. The STU's DAC has the same detail-level as the ODAC, which is incredible, but it is also spacious-sounding, which is great for most recordings. The sound stage seems wider and sometimes deeper. Both the ODAC and the STU offer a completely black background without any noise at all—you cannot hear any noise with even the most sensitive in-ears. Tonality is completely flat, so this is not the DAC for your if you want a warm, comfy sound. For the rest of us detail-oriented audiophiles, the STU is a very good purchase.

ASUS ships the amp with LME4562 and LME49720 op-amps, which are among the best around. You can, however, like with the STX, change the two I/V LME49720s and LME4562 to something of your choice. You would normally upgrade the op-amp configuration to what the STU ships with, so swapping makes little sense unless spending a lot of time and money on a marginal change in sound signature is alright with you. The stock LME op-amps are near perfect technically.