ASUS’ ST/STX range of audio cards were very well received by both reviewers and the public when launched a few years ago, even garnering an Editor’s Choice here at Techgage. It showed, for the first time, that there was a real demand for premium PC audio for dedicated music listening. I say “music listening” for a reason, as surround sound wasn’t made available, but what it lacked in channels, it made up with a built-in headphone amplifier for driving high impedance loads.
However, for those that wanted to eliminate as much system noise as possible by moving the soundcard outside of the PC, they had to wait a little bit longer. In 2011, ASUS launched its first premium USB DAC onto the market, the Xonar Essence One. It had it’s own internal power supply, balanced XLR output, asynchronous USB, 11 replaceable OpAmps, and for some odd reason, S/PDIF. Two years later, we now have a sibling, the Essence STU.
Forgoing the dedicated power supply and XLR outputs, the STU is a back-to-basics DAC (Digital-Analog Converter) and Headphone Amplifier. It’s USB powered to keep things simple, and includes RCA and 6.3mm jack outputs. The USB line is Asynchronous, meaning that the STU doesn’t use the system clock, but instead, it’s own clock; this results in significantly less jitter due to system noise and long cable lengths. Replaceable OpAmps are still available, although there are only three this time.
Internally, there are jumper selectors for disabling the amp so that the STU can run as a pre-amp instead, allowing you to use an external amp. There are also jumpers for a DC servo; this is used so that the headphones are only live after the unit powers up, and shuts down before it loses power, eliminating the ‘pop’ and ‘thump’ sound in power cycles. When it comes to caps, the usual (for ASUS) Nichicon Finegold electrolytics are included, but also WIMA FKP2 series Polypropylene Metal Film caps. There’s a PCM1792A DAC from TI, handling the conversion, and the venerable, but aging TPA6120A2 acting as the amplifier (the 32Ohm minimum load might be an issue for low impedance IEMs).
The reported specifications are definitely audiophile grade stuff: 120dB SNR, 0.00032% THD, 10Hz-48kHz frequency response, up to 600ohm Amp impedance, and sample/bit-rate of 192/24. How all this lives up to real-world use is another matter entirely. The ASUS Xonar Essence STU will be available from late October worldwide with an MSRP of US$399. The price is important, since the Essence One is slightly older, it could be found cheaper, despite its original $599 price-tag. ASUS might have set up a price war with itself.