Objective Reviews & Commentary - An Engineer's Perspective

November 12, 2011

Asus U3 DAC

asus xonar u3 boxINTRO: In my quest to find a companion DAC for the O2 headphone amp I was disappointed by three different C-Media based USB DACs (such as the FiiO D5) I found a bit more money buys much better performance with the Creative X-Fi Go. But even the X-Fi had some issues—especially driving headphones or when using the PC’s volume control. Asus Xonar soundcards have earned a reputation for decent performance so I wondered how their small “thumb drive” USB DAC performed? At $40 it has the highest street price of the six DACs I’ve recently tested but is still inexpensive by DAC standards. Asus claims “Incredible Sound” and the U3 has a “built in headphone amplifier”.

ASUS XONAR U3: Externally the U3 is very similar to the X-Fi Go with the exception of two odd and useless small grills that look like speakers or microphones but appear to be nothing more than decoration to cover two blue LEDs. The other DACs just have one LED so apparently Asus had to raise the bar. I’d rather the money been spent on improving the audio performance, but perhaps that’s just me. The U3 is another USB thumb drive sized DAC with only a microphone input and a single combined headphone/line output. There are no controls.

asus xonar u3 modesWINDOWS INSTALLATION: The U3 installed smoothly in both XP and Windows 7 without needing any drivers. Windows 7 reported it as a “ASUStek Advanced USB Audio Device” with 4 sample rates available, 8K, 16K, 44K and 48K all at 16 bit. The 8K and 16K options are rare these days and are mainly used for things like voice memos.

SUBJECTIVE SOUND QUALITY: Running the U3 into my O2 headphone amp I first noticed it has significantly higher output than the Creative X-Fi. The sound quality was good with no obvious problems. With the volume adjusted for the U3’s higher output, it was notably quieter than the X-Fi. When connecting headphones directly the results depended on which headphones I used. My DT770 Pro 80s sounded fairly good, my HD650s also sounded OK and played quite a bit louder than with the X-Fi, but my balanced armature Ulitmate Ears SuperFi 5s sounded wrong making me suspect a high output impedance.

asus u3 dscopeMEASUREMENT SUMMARY: The overall results are fairly similar to the X-Fi Go and also the UCA202. The U3’s strengths are its higher output voltage into 300+ ohm headphones and line inputs along with a much lower noise floor and higher total dynamic range. It has similar distortion to the X-fi and UCA202 into high impedance loads. The bad news is somewhat ragged high frequency response. And, for driving headphones, the U3 has a high output impedance around 23 ohms. That means the U3 is only suited for headphones around 200 ohms or higher. Yet the U3 lacks enough output power for many high impedance headphones (such as the Beyer DT880-600s). That leaves it suitable for only a small minority of headphones although it may get loud enough for many tastes with the popular HD600/650—especially with pop music. The ratings use a letter grade from A to F where A is excellent and F is Fail (unacceptable):

Measurement Asus U3 X-Fi Go FiiO D5 CM119 UCA202 TB Micro II
Freq. Resp. 10K +/-1.5 dB C +/- 0.4 dB A +/-1.5 dB C +/-1.0 dB B +/- 0.1 dB A +/- 1.0 dB B
Freq. Resp. 33 ohm +/-1.5 dB C +/-5.0 dB D +/-1.5 dB C +/-6.0 dB F N/A +/- 1.8 dB C
HP Output Imp ohm 23.6 D 7.8 C 0.72 A 5.9 C 47 F 0.95 A
Max Output 10K 2.15V 1.0V 1.5V B 0.95V C 1.12V B 1.34V B
Max Output 33 ohm 0.9V 0.75V C 1.4V B 0.68V C N/A 1.26V B
Max Power 32 ohm 25 mW C 18 mW C 61 mW B 14 mW C N/A 50 mW B
THD+N 0 dBFS 10K 0.01% 0.007% A 0.24% C 0.035% B 0.008% A 0.14% C
THD+N 100hz 10K 0.008% A 0.007% A 0.08% C 0.035% B 0.007% A 0.025% B
THD+N 1Khz 10K 0.008% A 0.007% A 0.08% C 0.035% B 0.007% A 0.02% B
THD+N 1K 33 ohm 0.04% B 0.009% A 0.08% C 0.095% C N/A 0.12% D
THD+N 10Khz 10K 0.008% A 0.009% A 0.04% B 0.090% C 0.009% A 0.11% C
IMD CCIF 10K/33 0.004% A 0.004% A 011% D 0.028% D 0.005% A 0.028% D
IMD SMPTE 10K 0.004% A 0.0005% A 0.80% D 0.012% B 0.002% A 0.02% B
Noise A-Wtd dBu 91.6 B 88.9 C -90.0 C -89.0 C -88.8 C -93.8 B
-90 dBFS Linearity 1.2 dB B 1.5 dB B 0.7 dB A 0.9 dB A 3.8 dB C 0.8 dB A
USB Jitter Jtest VG B VG B Poor D Poor D VG B Poor D


  • Relatively low distortion
  • No bass roll off even into 32 ohms
  • Over 2 Vrms output into 600+ ohms
  • Relatively low jitter
  • Close to ideal 16 bit good dynamic range with relatively low fixed noise
  • Very portable


  • Very high headphone output impedance mostly suitable only for 200+ ohm headphones
  • Not enough output for many 200+ ohm headphones
  • Steep roll off above 15 Khz and associated phase shift
  • Marginal DAC filtering (ripple in the audio band)

BOTTOM LINE: Both the Asus U3 and Creative X-Fi Go are well ahead of the other three similar C-Media based DACs I recently tested. Both the U3 and X-Fi also challenge the previous low cost benchmark—the Behringer UCA202. But the U3 and X-Fi have some significant differences. The much higher output level of the U3 into 600+ ohms gives it a big advantage in dynamic range driving a line input and in driving 200+ ohm headphones directly. While the X-Fi has a lower output impedance, superior frequency response, and much better DAC filtering than the U3. If I wanted a DAC to drive a headphone amp where I would control the volume from the amp I would easily choose the X-Fi. The same is true for powered speakers, etc. If I wanted to use the a software volume control, or drive 200+ ohm headphones directly, such as my Sennheiser HD650s, I’d choose the U3 for its higher output and much better dynamic range despite the high frequency problems.



TECH INFO: I’m not sure what chip(s) are used in the U3 but the results are notably different than I’ve observed from the C-Media, Creative and TI PCM27xxx DACs. If someone knows more, please leave a comment? I mainly tested at 16/44 and ran a few spot checks at 16/48.

FREQUENCY RESPONSE: The U3’s frequency response with a 10K load (such as a headphone amp) was good but not as accurate as the X-Fi Go. There’s some ripple above 1 Khz indicating marginal digital and/or analog filtering and the U3 rolls off rapidly above 15 Khz. It’s not quite as bad, however, as the C-Media DACs. The U3 apparently uses a bipolar power supply to avoid output capacitors as the frequency response doesn’t change much with a 33 ohm load (yellow plot). It’s the only DAC of the six with no significant low frequency roll off into 33 ohms. The massive 5 dB drop across the spectrum into 33 ohms, however, is due to the sadly high output impedance (more on that later). For comparison the X-Fi response is shown in the second graph:

U3 Frequency Response 10K (blue) & 33 Ohms (yellow) -3 dBFS -3 dBu 16-44

X-Fi Frequency Response 10K (blue) & 33 Ohms -3 dBFS -3 dBu 16-44


THD+N vs OUTPUT: This test starts at 10 mV (about –40 dBFS) where noise partly dominates the measurement. The blue plot is into a 10K load and the U3 manages to exceed 2 volts with low distortion. This requires a DC-DC converter to increase the supply voltage or generate a negative supply—otherwise USB DACs are limited to only about 1.4 Vrms by the 4.5 – 5 volt USB power supply. The U3 most likely uses a charge pump generating a negative 5 volt supply for a true bipolar power supply much like the FiiO E5 and E7. The distortion stays under 0.01% into 10K which is fairly respectable. Into 33 ohms, however, distortion is around 0.03% and the amp clips at about 900 mV. That’s still enough output for lots of low impedance headphones (but forget something like the AKG K701). But it’s worth noting even the lowly $20 FiiO E5 headphone amp does much better with most headphones. With higher impedance headphones, like the Sennheiser HD600/650, the U3 will have a few dB more output than the FiiO E5/E7 can manage. But into lower impedances the reverse is true. The lower graph is the X-Fi for comparison (note the horizontal voltage scales are different):

U3 1 Khz THD N vs Output 10K (blue) 33 ohms (yellow) 16-44

X-Fi 1 Khz THD N vs Output 10K 16-44 (yellow) 10K 24-44 (red) 33 ohms 16-44 (blue)


100hz OUTPUT IMPEDANCE & MAX POWER: The U3 badly clips into 15 (or 33 ohms) with a 0 dBFS input. So instead, I started with the maximum output at about 1% THD+N into 15 ohms (700 mV) and then switched to a 100K load. The output increased to 1.79 Vrms yielding an output impedance of 23.6 ohms at 100 hz (it was identical at 1 Khz). The U3 likely uses a 22 ohm output resistor and the amp, wiring, etc. has about 1.6 ohms of output impedance. This is the U3’s single biggest flaw as it limits its use to headphones around 200 ohms or higher or you risk frequency response and bass damping problems. This is per the “1/8th rule” from my Output Impedance article. Balanced armature IEMs, such as those from Shure, Etymotic, Ultimate Ears, etc. will perform very poorly with the U3’s high output impedance. The maximum power at clipping and/or 0 dBFS is:

  • 16 ohm = 30 mW (0.7 V)
  • 32 ohms = 25 mW (0.9 V)
  • 80 ohms = 28 mW (1.5 V)
  • 300 ohms = 11 mW (1.8V)
  • 600 ohms = 6 mW (1.9V)

Here’s the U3 at clipping into 15 ohms with the values shown for 100K in the yellow box:

U3 Max Output THD N 100 hz 100K & 15 Ohms (blue) BW=22 Khz


THD+N 100 hz 0 dBFS 10K: Some DACs struggle with a 0 dBFS signal. Here’s the U3 at 0 dBFS at 100 hz at full volume. The distortion is respectable with everything below –80 dB and a very respectable output of 2.15 Vrms (slightly above the Redbook standard of 2.0 Vrms). There’s nothing much to complain about here:

U3 Max Output THD N 100 hz 10K BW=22 Khz


THD+N vs FREQUENCY: Here’s the THD+Noise plotted from 20 hz to 20 Khz. The 10K plot (blue) is comfortably below 0.01% above 30 hz and often around 0.008% which is respectable performance. Into 33 ohms (yellow), however, things are not nearly as impressive. This is not because the U3 was close to clipping (the output was only about 450 mV). It’s simply the “headphone amp” in the U3 doesn’t much like a 33 ohm load. It’s also possible the DC-DC power supply isn’t happy delivering the required current. The U3 is more likely to have audible distortion into low impedance headphones compared to the X-Fi. Into a line level input they’re very similar. The second graph is the X-Fi for comparison (note the blue and yellow plots are reversed):

U3 THD N vs Frequency ~0 dBu 10K (blue) 330 ohms (yellow) 16-44

X-Fi THD N vs Frequency ~0 dBu 33 ohms (blue) 10K (yellow) 16-44


SMPTE IMD 10K LOAD: The U3 does very well and very similar to the X-Fi into a 10K load:U3 SMPTE IMD -3 dBFS ~0 dBu 10K ohms 16-44


SMPTE IMD 33 OHMS: The U3’s SMPTE IMD into 33 ohms is acceptable but the “mountain” reaching up to –70 dB at the base of the 7 Khz signal is some cause for concern and notably worse than the X-Fi (shown in the lower graph for comparison). Conversely, the X-Fi with its capacitor coupled output has a harder time with the 60 hz signal:

U3 SMPTE IMD -3 dBFS ref ~435 mV 33 ohms 16-44

X-Fi SMPTE IMD TB Micro II SMPTE IMD -3 dBFS Ref ~560 mV 33 ohms 16-44


CCIF IMD 44 Khz 10K: The U3 also does very well here for an inexpensive DAC into 10K. It’s in the same league as the UCA202 and X-Fi in most regards but with a somewhat different spectrum. You can see the steep roll off above 15 Khz attenuating the 20 Khz signal more than the 19 Khz signal. The second graph shows the X-Fi for comparison:

U3 CCIF IMD -7 dBFS ~0 dBu 10K 16-44

X-Fi CCIF IMD -7 dBFS ~0 dBu 10K 16-44


CCIF IMD 44 Khz 33 Ohms: Not surprisingly given the high 33 ohm THD at high frequencies, the U3 also struggles here into 33 ohms. There are many spikes above –80 dB:U3 CCIF IMD -7 dBFS ~580 mV 33 Ohms 16-44


NOISE & LINEARITY: The U3 is the most quiet of the bunch with the A-Weighted noise several dB better than the competition at 91.6 dBu and relatively low raw noise of under 40 uV. Using the max output into 10K this gives a total dynamic range of 2.15/40uV = 94.6 dB. This is impressively close to the theoretical maximum of 96 dB for 16 bit audio. The A-Weighted noise referenced to full output into 10K is 100.4 dB which is a very respectable number. The linearity is merely OK but slightly better than the X-Fi being off by 1.2 dB at –90 dBFS. The spike at around 13 Khz is nothing to worry about but a bit odd. It might be related to the DC-DC power supply in the U3. The second graph shows the X-Fi for comparison:

U3 -90 dBFS Noise Linearity ref 0 dBu 16-44

X-Fi -90 dB Noise Linearity ref 0 dBu 16-44


JITTER: The U3’s performance on the dScope jitter J-Test was a bit odd and inferior to the excellent performance from the X-Fi. It is similar to the UCA202 with the “spread” at the base of the 11025hz signal indicating some significant low frequency jitter. What’s odd is there’s a lack of any notable symmetrical side bands—normally the “markers” for high frequency jitter. The spikes present are more likely noise or distortion artifacts and not related to jitter as they’re not symmetrical. I wish I could correlate this to sound quality but I cannot (see my Jitter article for more). I plan to research jitter more in the future. The second graph shows the X-Fi result for comparison and the third shows the UCA202:

U3 Jitter 11025 hz J-Test 10K ~400mV 16-44

X-Fi Jitter 11025 hz J-Test 10K ~400mV 16-44

UCA202 Jitter 11025 hz J-Test 10K ref ~0 dBu 16-44


TECH COMMENTS: Used to drive an amp, powered speakers, or other source with a line input, the U3 is a reasonable DAC. It has roughly double (+6 dB) the output of the similar Creative X-Fi Go when used to drive a line input or high impedance headphones and substantially more dynamic range giving it lower effective noise. This also makes it a much better choice if you want to use the PC’s software volume control with an external amp. Conversely, the X-Fi has more noise but much smoother high frequency response and is a bit cleaner above 10 Khz. If you’re trying to directly drive headphones, the high output impedance of the U3 should only be used either with headphones around 200 ohms or higher, or headphones with a very flat impedance that do not require electrical bass damping. The 23 ohm output impedance will cause problems with other headphones—especially balanced armature IEMs—and the relatively high distortion in the upper midrange and high frequencies driving lower impedances may also be audible. For most applications, especially driving an amp or line input with an analog volume control, the X-Fi is a better choice. Asus’s decision to put 22 ohm resistors in series with the output really compromised this DAC for use with most portable headphones.


  1. @Anon, I can't find anything in Google referencing the Asus U3 uses the VT1620A. Do you have a link that shows it does? It also doesn't have as high of output as the U3.

  2. I just thought I'd so some googling for you to see what I can come up with:

    Apparently it is a "ASUS UA100 USB Audio Chip" according to the Asus site:

    So, searching for that term leads me here:

    Which claims the underlying "audio chip" is a C-Media chip and a Texas Instruments amplifier, but I think you are a better judge in this respect than I am.

    1. The amplifier is allegedly a TI DRV601RTJR, which is apparently also used on the Xonar DG/DGX cards with 10 Ω output impedance (this information is based only on pictures of the PCB, I do not actually have the Xonar DG). Considering that this chip is only intended for use as a line output buffer driving 600 Ω or higher impedance loads, it does not perform very badly in the 33 Ω tests, but a more suitable amplifier IC could have been used instead, like on the X-Fi Go Pro.
      The Xonar DG/DGX and U3 have a similar price, for which one can choose either the (ideally) higher performance and more features of the internal cards, or the advantages of using an external USB device.

  3. Interesting. Seems weird that they would jack up the output impedance on an ultraportable DAC+headphone amplifier, since that means it is likely to be used with lower-impedance headphones and IEMs. And anyway, the vast majority of headphones and earbuds that the general consumers use are 16 ohms or 32 ohms. Oh well.

    And about DetlevCM's above post, wow those TechReport RMAA figures are way wacky, heh. Their IMD graph shows all sorts of problems for the Xonar U3 that don't exist in reality, never mind whatever they think that they measured for the Acer laptop.

    Maybe "loopback" means that they just hooked up the headphone (or line out) output to the microphone input (or line in if available), rather than the output hooked up to at least some respectable sound card's line in. That would explain some things.

    I wouldn't be too surprised of RMAA just being RMAA though.

  4. It'd be interesting to see whether the FR and jitter improve when the DAC is running at 48K. It's common for many "cheap" DAC chips with a maximum sampling rate of 48K to fudge 44.1K. Running a libsamplerate-based resampler (SRC) on the music playback application is a good workaround for such cases.

  5. @Ferongr, I agree about some DAC chips not benig properly designed for 44 Khz. I did check the frequency response at 48 Khz and it does indeed improve although there was still ripple and early roll off. I also checked the CCIF IMD (19Khz/20Khz twin tone) at both sample rates and it was slightly better into 10K but similar into 33 ohms (where the amp, not the DAC, is the problem). The Jtest for Jitter, however, is specific to 44.1 Khz (the 11025 test frequency is exactly one quarter the sampling frequency with the least significant bit toggled). A different test would have to be devised for 48 Khz to be remotely a fair comparison. The dScope comes with a 44.1 Khz Jtest but not a 48 Khz Jtest and it's not as simple as just using a 12 Khz signal.

    Regardless of the above, the Creative X-Fi Go and Behringer UCA202 prove you can have flat frequency response and, for the X-Fi, extremely low jitter at 44 Khz in an inexpensive DAC. 99% of all music is 44 Khz. Most people are not going to want to implement real-time high quality sample rate conversion on their PC's to use one of these DACs. So unless someone was mainly going to use the DAC for watching 48 Khz DVD's, it seems nearly everyone is better off just choosing a better DAC--like the X-Fi--for listening to music.

    The irony is Creative has had a bad reputation for doing exactly what you suggest--performing well at 48 Khz but not at 44 Khz. But the X-Fi Go is clearly an exception to the rule.

  6. >The irony is Creative has had a bad reputation for doing exactly what you suggest.

    Tell me about it... I'm using an Audigy 2ZS. I would have ditched it a long time ago for a USB solution if it wasn't for the kX project drivers that enable using the internal DSP in great ways.

  7. Wow. Keep up the great work in sifting the crap from the gems. I am personally very interested in the accompanying DAC to the O2. Are there any preliminary decisions on the USB receiver or the DAC chip?

    Also, is there any chance of a version of the O2 for 'bookshelf' speakers, similar to Doug Self's discrete blameless amplifiers or his new 'Class XD' designs, like that of the Cambridge Audio 840a integrated, except relatively easily assembled on a PCB for the DIYer? The only speaker amp that seems to be in the same vein as the O2 is the Beta22, which I find very hard to believe the specs of, especially after the Mini3.

  8. Wow! Thank you so much for this blog. Your objective approach has helped me to assemble a nice "objective" audio system to start with. I like what you are trying to educate and promote some common sense to this high-end audio "uber sounding system" which will only cost you more than your house mortgage.
    Muchas Gracias!!!

  9. @Anon, if you want an amp for speakers I would go with Bruno Putzeys Hypex UcD amp modules or one of the National LME49830 reference designs. For more modest needs, one of the better chipamp reference designs might do the job. I'll know more about the ODA DAC option in the next week or so. The current plan is a daughter board to be used within the ODA enclosure (not the current O2) although it may also work standalone from USB power alone with perhaps slightly degraded performance.

  10. Thanks for your excellent blog. I really appreciate your objectivity. Looking forward to more reviews. I wonder what your approach to loudspeaker testing might look like?

  11. @MattRK, thanks. I've done a fair bit of loudspeaker testing and, at one time, designed (and measured) speakers professionally. The same Prism Sound dScope I use for testing electronics can also do quite a few speaker measurements--see prismsound.com. I also have the SoundEasy software (Bodzio) which is relatively obtuse, old school, and rather rough around the edges, but can make nearly any acoustic speaker measurement the dScope can't do if you're patient enough to figure out how.

    I have calibrated mics and a test area that allows the reflections from surfaces to be easily removed from the results. I also have a lot of respect for the Liberty Instruments Praxis software. There are of course many other options ranging from free to expensive including CLIO, lspcad, LEAP, LMS (Linearx), and Prism Sound is now selling the Loudsoft tools which I've seen some live demos of and have been impressed.

  12. Forget this cheap stuff. ;) I want to see a test of the XMOS reference build... and a write-up on licensing.

  13. You're welcome to write and submit a guest article for consideration akgk171! I'm in the middle of testing the upcoming ODA DAC and it's looking pretty darn impressive considering the expected cost. Perhaps someday it can go up against someone's far more costly XMOS DAC in a blind challenge ;)

  14. Incredible blog you have here! It's very difficult to find objective measurements of audio equipment.

    I just have a inquiry, I'm looking for a cheap S/PDIF interface for a laptop, the Asus and the Turtle Beach both have this output and I was just wondering if some of this measures would affect the S/PDIF channel, in other words, should I go for the cheaper Turtle Beach or the Asus Xonar could have a better digital output?

    Thank you, and keep up the great work, in a snake oil world this blog is like an oasis.

  15. @Anon, if by "cheap" you mean "not a critical application" any of them will probably work fine. You can also get cheapo USB to S/PDIF interfaces on eBay, etc.

    For a critical application you're generally much better off with a single box solution--i.e. a USB DAC rather than USB -> S/PDIF -> DAC. The two box solution involves more clocks, re-encoding the audio data clock, and other things that serve to generally add jitter and degrade performance.

    A single box solution can optimally use a single clock and everything inside is (presumably) designed to play nice together.

  16. I see what you mean that seperate box solutions are not ideal for critical applications.

    I hope you find an ideal single unit USB DAC to match your remarkable O2 project.

    Upsampling would be nice, imho.

    Godspeed, sonic soldier.

  17. Thanks kiteki. For what it's worth, "upsampling" is usually more marketing hype than reality. Virtually all DAC chips available these days are delta-sigma 1 bit oversampling designs running at very high clock rates far in excess of the audio data rate. These DACs already provide many of the benefits that upsampling once did with old school DAC chips.

    Re-sampling audio can create audible artifacts and is generally something to be avoided unless there are some compelling reasons to do it. One such reason is true Asynchronous Sample Rate Conversion (ASRC) as a method of jitter reduction. But ASRC is relatively expensive to do well for a range of native data rates and does not, by itself, guarantee the complete DAC will have low jitter.

    From what I've seen, reasonably priced DACs that perform upsampling often perform worse than those that leave the bitstream at the native data rate. But manufactures do it anyway to try and differentiate their products and have something to talk about in their marketing hype.

  18. Hmm I see thanks for your insights and info on the topic.

    I use the SoX upsampler in foobar when listening to internet radio and I like the enhancement.

  19. Do you have an opinion on the Asus Xonar internal cards? They are marketed as headphone cards and their price seems to range from 40 to 200USD. They've gotten rave reviews from some places, but admittedly the reviewers haven't tested quite as systematically as you.

  20. @Anon, Stereophile reviewed the Xonar ST and thought it was decent. But the headphone output impedance is too high for many headphones. See my impedance articles. If you use it for speakers or to drive a headphone amp, you don't mind needing proprietary (and sometimes problematic) software drivers, and you're lucky to have relatively "quiet" PC (some create electrical noise in internal sound cards) it's not a bad choice.

    1. I think many people would love to see you thoroughly testing the Xonar ST/STX. It seems to be the "go-to" card for headphone enthusiasts looking for a cost-effective solution for DA conversion and headphone amplification.

      I'm personally using the STX with a pair of Sennheiser HD800's. I've used it with the K701 and the HD650 before. I've subjectively compared it to a V-DAC + Heed Canamp combo with all these headphones. As I refined my testing method it became more and more difficult to be certain of any audible differences existing as the time gap between listenings became shorter. In general, I have a hard time differentiating between DACs and amps. Only with an unamped soundcard output do the shortcomings become evident to me in listening. Even the integrated chip on my motherboard can push the HD800's to adequate volume levels, but it sounds very wrong. The headphone jack on a Harman Kardon stereo amplifier sounded fine to me.

      I understand that the output impedance of 10ohms from the STX's headphone jack is well within spec for the 300ohm impedance of the HD800. It seems to drive the headphones to very loud volumes even at the lowest gain setting. Do you think the STX can provide enough power for the HD800's?

    2. John Atkinson at Stereophile already tested the ST/STX and found some weirdness but was generally impressed. Don't be too quick to dismiss the output impedance problem, however, even with 300 ohm headphones. While the ST/STX does have enough power for the HD800, when you get to that level of headphone you may want to consider a source with a near zero ohm output impedance. See this article which shows even with the very similar 300 ohm HD650, lower impedance still performs better:

      Sonic Advantages of Low Impedance Headphone Amps

      That's not to say you should run out and buy an amp or new headphone DAC if you're happy with your Xonar, but it would be far from my first choice for someone looking to upgrade their PC's audio.

    3. In the Benchmark article, the damping factor with the HD650 is about 3 times worse than it is with the HD800 from the Xonar STX or other TPA6120 based headphone outputs. The HD800 also has lower bass distortion than the HD650 (judging from the InnerFidelity PDF files), and that means less distortion is fed back to a high impedance output if the drivers are otherwise similar. The frequency response variation is about 0.13 dB. I would assume if the difference is audible at all, it is likely very minor.

    4. The Stereophile review is not very extensive, there is rather limited information particularly on the headphone amplifier. If I recall correctly, the SNR of the line input was not measured either. They also did not mention (or maybe even know about) the fact that the DAC on the card performs significantly, even if not necessarily audibly worse at 44.1/88.2/176.4 kHz; this could explain some of the "weirdness". I have also seen (not verified) claims of excessive amounts of RF noise from the DAC, especially at the problematic sample rates.

      Since the headphone amplifier on the ST/STX is not particularly good for the majority of headphones, measurements of cheaper models - for use as a line output only - would be interesting. From loopback based tests at least, these seem to perform comparably at 44.1 kHz at a lower price.

  21. I also just noticed your reply on the comment thread of e7 that an internal card might just get lucky in one pc, but in another the noise can be worse, so they aren't dependable.

    This site has a wealth of information, but I haven't yet managed to go through all of the comments. So apologies for that redundant question. I also found a comment on two headphones connected parallel (y-plug) and attenuator... seems like my idea of driving two sets at the same time (couple listening) is doomed to fail (without springing the money for two amps, I guess)...

  22. Great blog. I have JDS labs O2 that supposed to arrive tomorrow but I want to connect a decent DAC/thumb drive to the 02. The x-fi and u3 don't appear to be mac compatible. Do you have plan to test and/or locate the best solution for macs? I love this blog....

  23. @Anon, The U3 and X-Fi ARE OS X compatible. Any DAC that works in Windows with no drivers should also work on Intel Macs with no drivers. That includes virtually every DAC I've published test info on. While the U3 and X-Fi have the option of proprietary drivers that add functionality, they will work just fine (and perhaps even better in some ways) without any drivers in either operating system.

  24. Thank you for the review.

    I still cannot decide what to buy the X-FI or the U3. I read that the X-FI has some buzzing noise on low-mid volumes when connected to headphones - is that true?
    I want to connect it to my laptop and listen to music (With Klipsch S4i). I Use Dell E6400 with a crappy on-board sound card and want to upgrade. My desktop has X-Fi Titanium Fatal1ty Pro, so I think that my expectations are pretty high.

    Thanks for your help.

  25. With the low impedance Klipsch headphones I would go with the X-Fi because the output impedance of the U3 is way too high. The buzzing might be computer/USB port related but I didn't have a problem.

  26. Odd, NwAvGuy: The Xonar's U3 software can control the impedance with 3 modes:

    VOIP Mode OR <32 Ohm headphones
    Pro Gaming Mode OR 32~64 Ohm headphones
    Exciter Mode OR >64 Ohm headphones

    While I assume the last option is where it sends the full 23 ohms. Anyway, for BA headphones (hf5 + addiem) it sounds great on the 1st option, but for higher impedance (DT880 200ohm) headphones, I select the last option.

    This DAC is a decent option for mobile PC users who suffer with poor audio output and it features dolby headphone. My only gripe is that it needs a good USB port with little noise.

  27. @Riot, all the Asus proprietary driver is doing (if you want to bother using it) is limiting the maximum output when you change the "impedance". The output impedance remains the same. This has been documented with other Xonar products as well. The output impedance remains unacceptably high in any of the 3 modes. Asus is just assuming you may want less maximum output for lower impedance headphones (which is sometimes true, but there are many exceptions).

  28. Ah, I see, thanks for your response.

  29. After letting this DAC sit for a while, I resurrected it for a business trip I am now on, pairing it with a little Fiio E5 amp, for maximum portability. As I read the specs, if one is using an outboard amp it's got plenty of power and no output impedance concerns, and if one is not concerned with (or my older ears can no longer hear...) the last third of an octave between 15 and 20 khz, there are no real trade-offs except the 0.2 dB ripple? At any rate, it sounds remarkably good. I initially liked the surround modes, but now can only stand the normal "HiFi" mode.

  30. @Kevin, you're correct. I never tried the Asus "bloatware" driver and software so I can't comment on the various fake surround modes, etc. But the U2 driving an E5 is a decent (if somewhat awkward two device) solution for USB headphone listening.

  31. Hi there. Great review!
    I would have a couple of questions, and i would really appreciate your oppinion. :)
    I own a fairly new laptop, yet the audio card in it seems quite dissapointing - Realtek ALC269 (24 bit, ???khz). Sounds worse on many songs than my LG Optimus One phone.
    1. Would the Asus U3, or Creative Go! (or Go! Pro) make a difference (better audio quality)?
    2. I saw in your review the problem with the output impedance of the Asus. A reviewer in my country said that there is a "hidden tab" specified in Asus's manual that lets you select the output impedance. Is this true, and could this fix the mnetioned problem? Or is it rather a fake fix, by just upping the dB?
    3. The Go! Pro lets you select 24 bit, but is actually marketed as a 16 bit card. The older Go!, that is not in production anymore is actually a 24 bit card, would this make it a bit better than the Pro? (I could still buy the old Go! in a store that has it on stock).
    4. I will be mainly listening to music, some movies, and some gaming (in this order) with a pair of Philips SHP5401 headphones (really good for the price), 32 ohm impedance. Should I: stick with the integrated audio / buy one of the 2 mentioned usb audio cards, or / choose abother audio card?

    1. The U3 or Go might sound better than your laptop's output, it's hard to say. The FiiO E10 might be a better option if it's available for a reasonable price where you live.

      The Asus impedance selection is just a software level limiter. It doesn't change the output impedance at all or the gain. This surprised John Atkinson at Stereophile when he reviewed the high-end Asus Xonar STX sound card. The terminology Asus is using is very misleading.

      I have no idea about the original Go. It may have other problems or it might be a better DAC.

  32. Hello,

    I just bought the asus xonar u3 and I have huge difficulties using it. I plugged the card in, then install the drivers furnished with, and then every apps which need sound bug. I downloaded the last driver from Asus website and the installation never finish. Finally, I can not find anybody with the same problem on web. Do you think it might be something about my realtek chip on board in conflict with the asus xonar. I really need help, i feel stuck with no sound on my laptop (asus nf55s with 7 64 bit). Thank you

    1. You are living proof why I don't install sound drivers if I can help it--especially on 64 bit systems. It might be difficult at this point, but my suggestion would be to uninstall all the Asus software and drivers and just use the native Windows sound drivers which work fine with the U3. You may need to read up on complete driver removal as it can be tricky.

      If you have any System Restore points prior to installing any of the Asus software, after you have tried to uninstall everything, you may want to consider rolling your system back to one of those points which will help undo the likely damage done to the registry. Asus is a hardware company. In my opinion they often do a poor job with software and drivers.

    2. Thank you for your answer.
      As I became mad yesterday, I decided to format the whole computer. Then retried the xonar and it worked "quite" well. The native windows driver didn t recognize it as an asus product and some little freeze bug occured sometimes during a film/game, but it worked at least... The main problem was that the sound was really too loud on my headphones even at minimum level. So, stupid as I am, I decided to retry to install the asus drivers (last update) and the same god damn bug happened again : No more sound! and doenst work as well after a hard reboot (I can t even restart cleanly) What I do not understand is why all the reviews that I read never had an issue a with the software!
      How can I remove the drivers properly ? the system bug when I try to uninstall asus software... Thanks again for your time.

    3. Thanks again,
      I ve restored my system and the xonar work again as an digital usb audio output with the windows driver.
      Is there a way to install asus driver without using the software ? Because the sound is really not that good right now. Ty

    4. I'm not sure what else to tell you. If you're really hearing a difference in sound it's probably because you had some of the fake DSP processing (i.e "surround", etc.) enabled with the Asus driver/software. The U3 delivers very accurate sound without using any Asus software or drivers (the proof is in the review above). If you want the DSP bells and whistles, you need to either figure out how to make the Asus software work (I would suggest a PC support forum, not this blog) or use other DSP (like some of the add-ons for the Foobar2000 player, etc.).

  33. Hi, I noted you said they don't have enough power for the DT880 (600ohm).

    I previously owned the 250 ohm version and they get ear deafening loud. I ordered a 600 ohm model to replace them.

    Have you personally tested the U3 with the 600 ohm version?

    If they can't drive them properly, I'll just use my desktop amp: O2.


  34. I have a U3 and am currently using it with AKG Q701s (and sometimes with AT AD900s and AD700s).

    I'm planning on getting an ODA when it comes out, and am considering getting an ODAC at that time. How much of a benefit will the ODAC be over the U3 if I am just using the U3 to output to the ODA at line level?

    1. Good question... the ODA (or the O2) can certainly do a better job with the Q701 than the U3's marginal headphone amp can. The ODAC is also a true 24 bit DAC and easily outperforms the U3. But how much of a difference you will hear it harder to say. If you leave the volume all the way up on your PC and control the volume with the ODA, you may not hear much difference. But if you want to control the volume in your operating system or software player, I'd recommend a true 24 bit DAC with 20+ bits of effective resolution like the ODAC.

    2. Hey Lenny, does the Q701 clip when being used with the Xonar U3?

  35. On a few pieces of music with very high dynamic range I've heard it clip, but that was at an uncomfortable volume. At normal listening levels it's fine.


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