Asus U3 DAC

INTRO: 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.

WINDOWS 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.

MEASUREMENT 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

FIRST CLASS:

• 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

ECONOMY:

• 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 SECTION

---

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:

 

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):

 

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:

 

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:

 

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):

 

SMPTE IMD 10K LOAD: The U3 does very well and very similar to the X-Fi into a 10K load:

 

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:

 

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:

 

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:

 

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:

 

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:

 

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.