ASUS Xonar U7 USB SoundCard Review
For past few years Xonar Soundcards has become quite the diverse player in the audio world and ASUS audio products have been seen everywhere
from the high end external market, HTPC world to the entry level price conscience segment.
The Xonar soundcard line started with the early D series card, with their contributions being the premium STX and PCI 7.1 version, the ST
Deluxe.
Today we are reviewing ASUS Xonar U7, a 7.1 USB Soundcard and Headphone Amplifier. With Xonar U7 Asus is providing an audiophile-grade 7.1
sound system with premium connectivity for your Notebook or PC.
Packing
Xonar U7 comes in a very attractive looking black cardboard box that does well to show off the product. The front of the box has all the stuff
you want to know, headphone amp, microphone input, 114dB SNR, 24-bit/192KHz sampling rate, and Dolby software. The back has all the features in
detail with some graphs showing audio performance details. Technical and System requirements are on the sides.
Inside the box
The accessories which accompany the Xonar U7 are rather simple. You get a Quick Start Guide, driver CD, USB 2.0 cable, and an S/PDIF - TOSLINK
adapter.
Specifications
Features
The wonder of incredible compact audio!
Xonar U7 serves the needs of PC gamers who want to enjoy strong and clear sound via headphones, and is also ideal for listening to music as it
produces much more impactful and detailed audio than onboard hardware. Connecting via USB, it requires no standalone power supply and works
instantly setup-free with no need to open the PC case.
Powerful sound from a compact and portable device
Xonar U7 engineering overcomes the space and power supply limitations of other external USB-based audio devices to offer precise and immersive
7.1-channel surround. Sound processing uses 192kHz/24-bit fidelity for true high definition output, maintaining consistent quality across all surround channels.
114dB signal-to-noise ratio (SNR)
Up to 16X audio clarity compared with the SNR ratios of average onboard audio.
Hyper Grounding technology
Exclusive ASUS Hyper Grounding PCB fabrication ensures strong signal insulation to reduce distortion and interference,
also known as component crosstalk.
Digital to analog converter (DAC)
High quality Cirrus Logic® CS4398 DAC (digital-to-analog converter) brings clarity with excellent signal-to-noise ratio (SNR)
and low total harmonic distortion (THD)
Integrated headphone amplifier pumps up every sound detail
Since many gamers choose to refrain from disturbing others while insisting on the best sound quality, Xonar teams integrate a
headphone amplifier on Xonar U7 to boost immersive headphone output.
Gamer-friendly design
Microphone and master volume controls
Dedicated controls allow gamers to quickly adjust master and microphone volume without pausing games or going into software menus,
ensuring uninterrupted action. The master volume control doubles as a clickable switch for selecting between headphone input (front panel) and
speakers (back panel).
USB convenience
Xonar U7 connects to PCs via USB. It also gets power directly from the USB port, removing the need for a power adapter. Combined
with its compact dimensions, these turn Xonar U7 into a highly portable audio-enhancer that can be easily carried and placed anywhere.
Realistic surround and enhanced sound with new Dolby® Home Theater v4
Realistic surround
Surround decoder ― converts 2-channel audio sources to multi-channel, so you experience more immersive sound
Surround virtualizer ― delivers stunning virtual surround playback for standard stereo headphones or speakers.
Sound enhancement
Intelligent equalizer ― allows users to adjust tone dynamically with easy visual controls
Dialogue enhancer ― improves premium clarity for in-game communications
Volume leveler ― helps maintain chosen volume consistently across audio sources
Closer Look
The Xonar U7 hardware device is a smart, compact black box with gold accent.
On top the device there is a stylish volume controller and three LEDs which indicate whether the audio output is set to the
speakers, dedicated headphone output, or the SPDIF port. You can select these modes by pressing down the volume controller.
Connecting up the Xonar U7 requires a free USB port. Xonar U7 supports USB 1.0 and 2.0 with a selection switch at the bottom
of the device. Full functionality is enabled in USB 2.0 mode and USB 1.0 mode limits the sound card to playback only.
On the front panel you will find a dedicated headphone 3.5mm jack, the microphone / line-in jack, and the microphone volume up
& down buttons.
On the rear panel starting from the five jacks on the left are for speakers. The red and white RCA jacks are for front channel
in 5.1/7.1 setups and L/R in 2 channel setups. The three 3.5 mm jacks are in order from left to right, Side, Center, and Rear. To the right of
the Speaker outputs and left of the USB connector is an SPDIF output.
Inside Look
Component Layout
Detail of all components used inside the device
1. C-Media 6632A - At the heart of the Xonar U7 is a C-Media 6632A
Sound Processor. The 6632A supports USB Audio Class 2, which means it does need a driver, but it also means it has a good bit more horsepower
for DSP effects, 7.1 channel surround, 24bit/192KHz sampling rates, and tight audio specifications. Specifically designed for use in high-end
USB sound cards
2. Cirrus Logic CS4362ACQZ (DAC) – The Cirrus Logic CS4362 are complete,
114dB, 24-Bit, 192kHz, 6-/8-channel, digital-to-analog (D/A) systems that include digital interpolation, fifth-order Delta-Sigma D/A conversion,
digital de-emphasis, volume control, and analog filtering.
3. Cirrus Logic CS4398-CZZ (DAC) – The Cirrus Logic CS4398 is a complete
stereo audio120dB, 24-bit/192 kHz digital/analog converter (DAC) system. This D/A system includes digital de-emphasis, half dB step-size volume
control, ATAPI channel mixing, selectable fast and slow roll off digital interpolation filters followed by an oversampled multi-bit Delta-Sigma
modulator, which includes mismatch shaping technology that eliminates distortion due to capacitor mismatch.
4. cFeon EN39LV010 - 1-Megabit (128K x 8-bit ) 4 Kbyte Uniform Sector,
CMOS 3.0 Volt-only Flash Memory.
5. Cirrus Logic CS5361-KZZ (ADC) - The Cirrus Logic CS5361 is a complete
audio 114 dB, 192 kHz Stereo A/D Converter (ADC) for digital audio systems. It performs sampling, A/D conversion and anti-alias filtering,
generating 24-bit values for both left and right inputs in serial form at sample rates up to 192 kHz per channel.
6. 7. ZA3 are Texas Instruments LME49726 Series Audio Op Amplifiers.
The LME49726 is a low distortion, low noise rail-to-rail output audio operational amplifier optimized and fully specified for high performance,
high fidelity applications delivering superior audio signal amplification for outstanding audio performance. The LME49726 has a very low THD+N to
easily satisfy demanding audio applications ensuring that the most challenging loads are driven without compromise, with output current greater
than 300mA at 5V and dynamic range is maximized by an output that drives 2kΩ loads to within 4mV of either power supply voltage.
8. TPS77601 – At power supply stage Texas Instruments TPS77601 is used.
TPS77601 have a fast transient response, 500mA LDO voltage regulator with RESET output and Power Good(PG) output and is stable with a 10µF low
ESR capacitor. This combination provides high performance and great reliability.
Software
The Xonar U7 sound card software package consists of two different control panels – Xonar U7 Audio Center and Dolby Home Theater
V4.
Drivers were very stable and responsive. Control panel is easy-to-use, accessible and allows you to customize almost anything
you want i.e. sample-rate control, environmental emulation effects, headphone gain, virtual headphone surround and more.
Testing
Test System is Sony VIA S-Series Laptop. Model No. VPCSA26GG.
Benchmarks
Xonar U7
Xonar DX
Pros
Cons
Conclusion
Asus Xonar U7 Sound Card provided us with excellent sound quality during movie, music, and game playback with 7.1 speakers,
but even at its maximum headphone volume, the product always left us wanting just a little bit more.
RightMark Audio Analyzer (RMAA) suite showed that Xonar U7 is way better than onboard audio , specially the headphone amp part
provided super good audio experience. But in 7.1 audio setup Xonar DX is still better than Xonar U7.
If you are an audio enthusiast, and are looking for a superior alternative to the quality of your laptop or desktop's 2.1 analog
onboard sound, then you should without a doubt consider this sound card.
For the performance, portability, and features that the Asus Xonar U7 Sound Card offers, it is a good audio enthusiast
solution.
Source / More Info
Manufacturers Info
Asus.com
Thanks.
Rakesh Sharma
An externally hosted image should be here but it was not working when we last tested it.
For past few years Xonar Soundcards has become quite the diverse player in the audio world and ASUS audio products have been seen everywhere
from the high end external market, HTPC world to the entry level price conscience segment.
The Xonar soundcard line started with the early D series card, with their contributions being the premium STX and PCI 7.1 version, the ST
Deluxe.
Today we are reviewing ASUS Xonar U7, a 7.1 USB Soundcard and Headphone Amplifier. With Xonar U7 Asus is providing an audiophile-grade 7.1
sound system with premium connectivity for your Notebook or PC.
Packing
Xonar U7 comes in a very attractive looking black cardboard box that does well to show off the product. The front of the box has all the stuff
you want to know, headphone amp, microphone input, 114dB SNR, 24-bit/192KHz sampling rate, and Dolby software. The back has all the features in
detail with some graphs showing audio performance details. Technical and System requirements are on the sides.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Inside the box
The accessories which accompany the Xonar U7 are rather simple. You get a Quick Start Guide, driver CD, USB 2.0 cable, and an S/PDIF - TOSLINK
adapter.
An externally hosted image should be here but it was not working when we last tested it.
Specifications
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Features
The wonder of incredible compact audio!
Xonar U7 serves the needs of PC gamers who want to enjoy strong and clear sound via headphones, and is also ideal for listening to music as it
produces much more impactful and detailed audio than onboard hardware. Connecting via USB, it requires no standalone power supply and works
instantly setup-free with no need to open the PC case.
Powerful sound from a compact and portable device
Xonar U7 engineering overcomes the space and power supply limitations of other external USB-based audio devices to offer precise and immersive
7.1-channel surround. Sound processing uses 192kHz/24-bit fidelity for true high definition output, maintaining consistent quality across all surround channels.
114dB signal-to-noise ratio (SNR)
Up to 16X audio clarity compared with the SNR ratios of average onboard audio.
An externally hosted image should be here but it was not working when we last tested it.
Hyper Grounding technology
Exclusive ASUS Hyper Grounding PCB fabrication ensures strong signal insulation to reduce distortion and interference,
also known as component crosstalk.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Digital to analog converter (DAC)
High quality Cirrus Logic® CS4398 DAC (digital-to-analog converter) brings clarity with excellent signal-to-noise ratio (SNR)
and low total harmonic distortion (THD)
Integrated headphone amplifier pumps up every sound detail
Since many gamers choose to refrain from disturbing others while insisting on the best sound quality, Xonar teams integrate a
headphone amplifier on Xonar U7 to boost immersive headphone output.
An externally hosted image should be here but it was not working when we last tested it.
Gamer-friendly design
Microphone and master volume controls
Dedicated controls allow gamers to quickly adjust master and microphone volume without pausing games or going into software menus,
ensuring uninterrupted action. The master volume control doubles as a clickable switch for selecting between headphone input (front panel) and
speakers (back panel).
USB convenience
Xonar U7 connects to PCs via USB. It also gets power directly from the USB port, removing the need for a power adapter. Combined
with its compact dimensions, these turn Xonar U7 into a highly portable audio-enhancer that can be easily carried and placed anywhere.
An externally hosted image should be here but it was not working when we last tested it.
Realistic surround and enhanced sound with new Dolby® Home Theater v4
An externally hosted image should be here but it was not working when we last tested it.
Realistic surround
Surround decoder ― converts 2-channel audio sources to multi-channel, so you experience more immersive sound
Surround virtualizer ― delivers stunning virtual surround playback for standard stereo headphones or speakers.
Sound enhancement
Intelligent equalizer ― allows users to adjust tone dynamically with easy visual controls
Dialogue enhancer ― improves premium clarity for in-game communications
Volume leveler ― helps maintain chosen volume consistently across audio sources
Closer Look
The Xonar U7 hardware device is a smart, compact black box with gold accent.
On top the device there is a stylish volume controller and three LEDs which indicate whether the audio output is set to the
speakers, dedicated headphone output, or the SPDIF port. You can select these modes by pressing down the volume controller.
An externally hosted image should be here but it was not working when we last tested it.
Connecting up the Xonar U7 requires a free USB port. Xonar U7 supports USB 1.0 and 2.0 with a selection switch at the bottom
of the device. Full functionality is enabled in USB 2.0 mode and USB 1.0 mode limits the sound card to playback only.
An externally hosted image should be here but it was not working when we last tested it.
On the front panel you will find a dedicated headphone 3.5mm jack, the microphone / line-in jack, and the microphone volume up
& down buttons.
An externally hosted image should be here but it was not working when we last tested it.
On the rear panel starting from the five jacks on the left are for speakers. The red and white RCA jacks are for front channel
in 5.1/7.1 setups and L/R in 2 channel setups. The three 3.5 mm jacks are in order from left to right, Side, Center, and Rear. To the right of
the Speaker outputs and left of the USB connector is an SPDIF output.
An externally hosted image should be here but it was not working when we last tested it.
Inside Look
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Component Layout
Detail of all components used inside the device
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
1. C-Media 6632A - At the heart of the Xonar U7 is a C-Media 6632A
Sound Processor. The 6632A supports USB Audio Class 2, which means it does need a driver, but it also means it has a good bit more horsepower
for DSP effects, 7.1 channel surround, 24bit/192KHz sampling rates, and tight audio specifications. Specifically designed for use in high-end
USB sound cards
An externally hosted image should be here but it was not working when we last tested it.
2. Cirrus Logic CS4362ACQZ (DAC) – The Cirrus Logic CS4362 are complete,
114dB, 24-Bit, 192kHz, 6-/8-channel, digital-to-analog (D/A) systems that include digital interpolation, fifth-order Delta-Sigma D/A conversion,
digital de-emphasis, volume control, and analog filtering.
An externally hosted image should be here but it was not working when we last tested it.
3. Cirrus Logic CS4398-CZZ (DAC) – The Cirrus Logic CS4398 is a complete
stereo audio120dB, 24-bit/192 kHz digital/analog converter (DAC) system. This D/A system includes digital de-emphasis, half dB step-size volume
control, ATAPI channel mixing, selectable fast and slow roll off digital interpolation filters followed by an oversampled multi-bit Delta-Sigma
modulator, which includes mismatch shaping technology that eliminates distortion due to capacitor mismatch.
An externally hosted image should be here but it was not working when we last tested it.
4. cFeon EN39LV010 - 1-Megabit (128K x 8-bit ) 4 Kbyte Uniform Sector,
CMOS 3.0 Volt-only Flash Memory.
An externally hosted image should be here but it was not working when we last tested it.
5. Cirrus Logic CS5361-KZZ (ADC) - The Cirrus Logic CS5361 is a complete
audio 114 dB, 192 kHz Stereo A/D Converter (ADC) for digital audio systems. It performs sampling, A/D conversion and anti-alias filtering,
generating 24-bit values for both left and right inputs in serial form at sample rates up to 192 kHz per channel.
An externally hosted image should be here but it was not working when we last tested it.
6. 7. ZA3 are Texas Instruments LME49726 Series Audio Op Amplifiers.
The LME49726 is a low distortion, low noise rail-to-rail output audio operational amplifier optimized and fully specified for high performance,
high fidelity applications delivering superior audio signal amplification for outstanding audio performance. The LME49726 has a very low THD+N to
easily satisfy demanding audio applications ensuring that the most challenging loads are driven without compromise, with output current greater
than 300mA at 5V and dynamic range is maximized by an output that drives 2kΩ loads to within 4mV of either power supply voltage.
An externally hosted image should be here but it was not working when we last tested it.
8. TPS77601 – At power supply stage Texas Instruments TPS77601 is used.
TPS77601 have a fast transient response, 500mA LDO voltage regulator with RESET output and Power Good(PG) output and is stable with a 10µF low
ESR capacitor. This combination provides high performance and great reliability.
An externally hosted image should be here but it was not working when we last tested it.
Software
The Xonar U7 sound card software package consists of two different control panels – Xonar U7 Audio Center and Dolby Home Theater
V4.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Drivers were very stable and responsive. Control panel is easy-to-use, accessible and allows you to customize almost anything
you want i.e. sample-rate control, environmental emulation effects, headphone gain, virtual headphone surround and more.
Testing
Test System is Sony VIA S-Series Laptop. Model No. VPCSA26GG.
An externally hosted image should be here but it was not working when we last tested it.
- Intel® Core™ i7-2620M Processor 2.70 GHz with Turbo Boost up to 3.40 GHz
- 8 GB DDR3 RAM
- SSD in RAID 0: 256 GB (64 GB x 4; Serial ATA)
- Windows 8.1 Professional 64-bit
Benchmarks
Xonar U7
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Xonar DX
An externally hosted image should be here but it was not working when we last tested it.
Pros
- Audiophile grade sound quality
- Good headset amp
- Portable
- Easy-to-use software with a lot features
- Stable Drivers
Cons
- No 6.3mm (1/4″) Headphone jack
- No RCA to 3.5mm Audio connector for Speaker Output.
Conclusion
Asus Xonar U7 Sound Card provided us with excellent sound quality during movie, music, and game playback with 7.1 speakers,
but even at its maximum headphone volume, the product always left us wanting just a little bit more.
RightMark Audio Analyzer (RMAA) suite showed that Xonar U7 is way better than onboard audio , specially the headphone amp part
provided super good audio experience. But in 7.1 audio setup Xonar DX is still better than Xonar U7.
If you are an audio enthusiast, and are looking for a superior alternative to the quality of your laptop or desktop's 2.1 analog
onboard sound, then you should without a doubt consider this sound card.
For the performance, portability, and features that the Asus Xonar U7 Sound Card offers, it is a good audio enthusiast
solution.
Source / More Info
Manufacturers Info
Asus.com
Thanks.
Rakesh Sharma
Nice comprehensive review, but those RMAA results look off. Seems like there may be some sort of processing still going on (I mean, xtalk at -6 dB?) - apparently it is advisable to enable "HiFi mode" in Xonar cards. Also check input settings, as I've seen the U7 post 108 dB(A) in loopback elsewhere.
Sure I will try and re-run RMAA, but the image in the link you provided shows device as [DIRECT SOUND] not Xonar U7 as it is shown in my images. Second I am sampling at 96kHz so may be that can create some difference.
Thanks
You can name and rename a test run as desired. He was using DirectSound and happened to leave that part in.
Now that I'm looking at them, your DX results do not seem quite right either - frequency response +3.82, -65.22 dB? Presumably RMAA and the card disagreed on sample rates. If it's any consolation, a significant percentage of RMAA results to be found out there displays issues of one kind or another.
You should know that once you're using the venerable MME/waveOut API in Windows Vista and newer, the soundcard operates in shared mode. In this case the audio format specified in input and output device properties is relevant, and resampling is employed when needed. IOW, whenever you want to change sample rate, there are 3 spots to do that in - RMAA, input device, output device. DirectSound is no better off in this regard.
I suggest you familiarize yourself with ASIO and ASIO4All - it's a bit more tricky to set up, but afterwards you can change sample rates and formats to your heart's content, in just one spot. As a bonus you can select any channel pair, so you can look at surround output performance as well. Extra processing is bypassed as well.
While even the free version of RMAA can use WASAPI lately (called "Win Sound"), it would seem to be using shared mode, not exclusive mode (which would be the one that would permit having the devices set up automagically and bypassing additional processing, like the ASIO based solution does). At least it'll only let you select a sample rate matching current device settings. Possibly they're using the PortAudio library for audio I/O, like Audacity does.
Now that I'm looking at them, your DX results do not seem quite right either - frequency response +3.82, -65.22 dB? Presumably RMAA and the card disagreed on sample rates. If it's any consolation, a significant percentage of RMAA results to be found out there displays issues of one kind or another.
You should know that once you're using the venerable MME/waveOut API in Windows Vista and newer, the soundcard operates in shared mode. In this case the audio format specified in input and output device properties is relevant, and resampling is employed when needed. IOW, whenever you want to change sample rate, there are 3 spots to do that in - RMAA, input device, output device. DirectSound is no better off in this regard.
I suggest you familiarize yourself with ASIO and ASIO4All - it's a bit more tricky to set up, but afterwards you can change sample rates and formats to your heart's content, in just one spot. As a bonus you can select any channel pair, so you can look at surround output performance as well. Extra processing is bypassed as well.
While even the free version of RMAA can use WASAPI lately (called "Win Sound"), it would seem to be using shared mode, not exclusive mode (which would be the one that would permit having the devices set up automagically and bypassing additional processing, like the ASIO based solution does). At least it'll only let you select a sample rate matching current device settings. Possibly they're using the PortAudio library for audio I/O, like Audacity does.
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Good review Rakesh. Do you have the figure for the power consumption? I'm concerned about it overloading the marginal power supply of my mini ITX media PC.
Very professional ad copy and artwork.
Why would a 192k card only have a frequency response to 46kHz?
Why would a 192k card only have a frequency response to 46kHz?
I would have thought all PC sound cards use multi layer designs with ground planes, which I presume is what the copy calls Hyperground, I wouldn't want one that wasn't multilayer and laid out properly.
I am building a PC at the moment and will have a serious look at this one though...
One thought, separating things vertically through the PCB stack is not the best method, proper defined areas for analogue and digital is the way to do it, with any on board SMPSs isolated from the rest both vertically and horizontally with switching loops minimised and switching ground on the same side as the SMPS components only with connection to main ground at one point.
I am building a PC at the moment and will have a serious look at this one though...
One thought, separating things vertically through the PCB stack is not the best method, proper defined areas for analogue and digital is the way to do it, with any on board SMPSs isolated from the rest both vertically and horizontally with switching loops minimised and switching ground on the same side as the SMPS components only with connection to main ground at one point.
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I've bought one and so can now answer my own earlier question. The power consumption is 120ma.
The card is a big improvement over the on-board DAC on my Asus P8H67-I Deluxe mini ITX motherboard. The only issue is that it momentarily drops out once every three or four tracks. Any ideas as to why that may happen?
The card is a big improvement over the on-board DAC on my Asus P8H67-I Deluxe mini ITX motherboard. The only issue is that it momentarily drops out once every three or four tracks. Any ideas as to why that may happen?
Sorry, this does not sound like a review, but like marketing blah blah.
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Then one of the two is probably set up wrong (with the Asus cards it is advised to use "HiFi mode") or subject to a driver bug (any Realtek HDA driver from 2011/12-ish should be updated, as there were some issues with 24/44 operation in particular), or you didn't match levels.
Possibly issues with high DPC latency. Your board has integrated WiFi, I'd update its driver first. If problems persist, run DPC Latency Checker (note: gives inaccurate readings on Windows 8) or LatencyMon.
Hi sgrossklass,
Do I properly understand your above post that properly set up the on board Realtek should be equal in sound quality to the Asus card?
regards,
Eelco
Do I properly understand your above post that properly set up the on board Realtek should be equal in sound quality to the Asus card?
regards,
Eelco
Pretty much so, audibly and in line operation at least. The U7 would obviously measure better, particularly on the recording side, and it would play a hint louder.
Headphone operation is a different kettle of fish. I would expect the onboard output to have higher output impedance (75 ohms typical), with the associated effects. The U7 features about 15 ohms IIRC, but the fancy low-power opamp it uses would be expected to be somewhat unhappy when asked to drive low-impedance loads at non-negligible levels (I mean, 1 kHz distortion is only 90 dB down - with a non-pretty spectrum - when driving 300 ohm HD600s, which should be a walk in the park for any half-decent headphone driver, and extrapolation to 32 ohms and 10 kHz gives some reason for concern). I wouldn't necessarily ask it to drive a HE-6. Nor would I expect a measly ALC892 to do that same, actually, but that one would be more likely to run into hard clipping first.
Headphone operation is a different kettle of fish. I would expect the onboard output to have higher output impedance (75 ohms typical), with the associated effects. The U7 features about 15 ohms IIRC, but the fancy low-power opamp it uses would be expected to be somewhat unhappy when asked to drive low-impedance loads at non-negligible levels (I mean, 1 kHz distortion is only 90 dB down - with a non-pretty spectrum - when driving 300 ohm HD600s, which should be a walk in the park for any half-decent headphone driver, and extrapolation to 32 ohms and 10 kHz gives some reason for concern). I wouldn't necessarily ask it to drive a HE-6. Nor would I expect a measly ALC892 to do that same, actually, but that one would be more likely to run into hard clipping first.
My experience with the Xonar U7 is to use a 30cm USB cable instead of the length supplied in the box.
Are you using this on Windows 10? If so, which drivers? I am getting some crackling noise on my system - all files, all sample rates. The disturbance is worst at the very end of the file (buffers emptying?).
Update: Turning on the Dolby enhancement removes most of the crackle. The audio is now listenable. Just a few tiny crackles at the end of the file.
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