Saturday 30 May 2015

MEASUREMENTS: Computer USB port noise, USB hubs and the 8kHz PHY Microframe Packet Noise

Block diagram of the USB 'PHYsical' chipset. UTMI stands for USB 2.0 Transceiver Macrocell Interface, a standard bus interface for the USB chipsets out there...

One could see the "glass half full" when we run into imperfections in the audio system. For example, it was serendipitous that the Emotiva XSP-1 pre-amp had such a noise-sensitive unbalanced "HT Bypass" input that I was able to detect noise and measure it previously. If it were not for this issue, I probably would never have thought about using the Corning USB 3 optical cable or consider how to isolate the noise originating from the computer USB connection to the DAC simply because in my system using balanced interconnects throughout, I had never had a problem (subjectively audible or objectively verifiable).

As a quick recap, I was able to hear and measure the 8kHz "microframe packet noise" (125us) originating from the computer's USB port. The HTPC is based on the ASUS B85M-E/CSM motherboard running the Intel Pentium G3220 CPU. I use one of the USB 3 connectors although I have tried the USB 2 ones as well and they're just as noisy. So, step-by-step, I can show you how I brought that noise down:

0. Let's start just by showing what "silence" looks like in the system through the pre-amp's analogue out.

If I don't have the USB cable connected to the DAC. Here's what the balanced output looks like from the XSP-1 preamp from 1kHz to 30kHz:

This gives us the "baseline" to start from. There is some inaudible noise in the system - realize that I have my Transporter and Onkyo receiver connected to this pre-amp at the same time so some of this low level stuff could be from these sources as well. Theoretically, if there's no noise at 8kHz (around where that cursor is, hard to "lock" on to 8000Hz as there is no tone there to snap the cursor on to), the noise floor is around -146dB. Ideally then, we'd like to see this when we connect the USB TEAC UD501 DAC to the computer.

I. Nasty 8kHz noise directly from computer to USB DAC:

HTPC --> 10' generic USB cable --> TEAC UD501 DAC --> 6' balanced analogue cables --> Emotiva XSP-1 preamp --> 6' balanced analogue cables --> E-MU USB0404 ADC --> measurement computer

To the left we can see the cursor is locked on 8kHz and the signal is -102.5dB. Note that this might seem low but in fact as it goes through the gain in my monoblocks, I can hear this annoying pitch (along with other electrical noises) as I demonstrated in the video previously.

The 8kHz primary frequency and resonant frequencies up at 16kHz, 24kHz can easily be seen. There's also some noise at 9kHz but below -120dB. Notice that I'm running this FFT out to 30kHz (above audible range just to make sure there's no obvious anomalies up there).

II. Let's put a USB hub between the USB cable and the DAC:

Suppose now we put a USB hub in the path... Remember that in the previous post, I had to do this eventually anyhow because the Corning USB 3 optical cable does not supply power which was needed for my TEAC DAC to recognize the connection to the computer. So let's put in the same Certified Data USB 3 4-port unit that I used in my previous post.

HTPC --> 10' generic USB cable --> Certified Data USB 3 hub --> generic 3' USB cable --> TEAC UD501 DAC --> 6' balanced analogue cables --> Emotiva XSP-1 preamp --> 6' balanced analogue cables --> E-MU USB0404 ADC --> measurement computer

This hub is one of the early 4-port USB3 devices. I think I bought it more than 3 years ago (around 2012). As you can see, placing the hub in the data chain doesn't do much to the noise... It's down to -103.5dB which is just a 1dB improvement. Certainly the distortion does not sound any less annoying!

This shows us that a USB hub itself does not attenuate the conducted USB noise from the computer.

III. Let us now use the 33' Corning Optical USB 3 cable to connect to the computer, same hub as above:

HTPC --> Corning Optical USB 3 cable --> Certified Data USB3 hub --> generic 3' USB cable --> TEAC UD501 DAC --> 6' balanced analogue cables --> Emotiva XSP-1 preamp --> 6' balanced analogue cables --> E-MU USB0404 ADC --> measurement computer

There you go... Clearly the optical cable has done a very noticeable job in attenuating the USB noise from the computer! The 9kHz tone is below noise floor now. As I discussed previously, the optical connection provides significant electrical noise isolation from the computer's USB port. We see the 8kHz tone drop from -102.5dB to -118dB. A very significant difference of 15.5dB. This drop has now rendered the annoying high-pitched tone inaudible to me in the soundroom. Job pretty much done :-).

However, since curiosity is perhaps the most important trait to have in order to ascertain truth and the nature of things (and one which I try to instill in my children), the obvious question is: do other powered USB hubs, likely using other PHY chipsets result in lower 8kHz packet noise?

Well, since I have a few USB hubs at my disposal (and I was wanting to buy a new USB 3 hub anyway for work), let's measure a few in place of the Certified Data model I have used up to now and see if this 8kHz tone changes...

So for the following tests, the connections will go like this:
HTPC --> Corning Optical USB 3 cable --> [USB HUB BEING TESTED] --> generic 3' USB cable --> TEAC UD501 DAC --> 6' balanced analogue cables --> Emotiva XSP-1 preamp --> 6' balanced analogue cables --> E-MU USB0404 ADC --> measurement computer
As best I can, I will use the same USB cables between the hub and DAC (not that I believe USB cables make any difference unless faulty). Unless specified, the included wallwart power supply was used with each hub.

A. Kensington PocketHUB SE 2.0 Model K33141:

This 4-port USB 2 hub has been with me since 2006 at the office! It has worked flawlessly, is fast, never any issues. Good metal construction as well. Probably paid about $25 when it first came out.

Not bad, the 8kHz tone down to -119dB.

B. Cheap generic portable USB 2 hub:

Alright, I was scraping the bottom of the bargain bins to find this baby. I bought it in Beijing in 2009 when I was on vacation there and was desperate to get a cheap hub to do some USB stick data transfers of photos. Often, goods sold domestically are of lower quality "B grade" than export goods although I'm sure things have improved. I paid less than US$5 from some vendor in a tiny stall in one of the busy residential districts. Lightweight, all plastic. The logo says "A1Pro" on the top. At this price it didn't come with its own power supply, I used the same one as the Kensington above (plug and polarity compatible).

Ewwww. Look what happened here! The 8kHz noise is up at -107.5dB which is horrible, almost as bad as straight off the computer USB port and negating most of the benefit from the optical USB cable. Furthermore, notice the noise floor elevation below 14kHz. This is audible as white noise. This is a bad, noisy hub!

C. Logiix 7-port USB 2 [Data Port VII Slim]:

This one costs about US$40. Bought recently so probably newer USB 2 chipset compared to the older ones above. Reasonably hefty power supply providing up to 3.7A.

8kHz noise down at -118.5 dB. Not bad.

D. Kensington UH7000C USB 3 7-port hub:

The most expensive hub of the bunch here - US$50. Released in late 2014 so I'm assuming it's based on one of the newest USB3 chipsets. 4A wallwart, the most substantial power supply of the group. Great for charging devices quickly off this hub (advertised on the box).

8kHz noise at -119.3 dB. Excellent.

Summary and Conclusion:

Okay, let's just cut to the chase.

1. I've shown again that the Corning USB 3 optical cable paired with a powered hub is capable of removing much of the nasty USB noise originating from the computer's motherboard USB connection. The vast majority of the improvement I saw came from the use of this fibreoptic cable.

2. Powered hubs are not created equal. Here's the summary of the improvement over a direct connection to the computer USB port (sorted from worst to best in terms of lowering of the 8kHz packet noise):
Optical USB + Cheap USB 2 hub bought in China = -5.2 dB
Optical USB + Certified Data 4-port USB 3 = -15.5 dB
Optical USB + Logiix 7-port USB 2 = -16.1 dB
Optical USB + Kensington PocketHub SE USB 2 = -16.7 dB
Optical USB + Kensington UH7000C 7-port USB 3 = -16.8 dB
As you can see, the cheap USB 2 hub I bought in China was atrocious and noisy! Not only did the 8kHz tone attenuate by only a small amount, but it even caused audible broadband elevation of the noise floor below 14kHz (see FFT above). Otherwise, the others were all relatively close; essential +/- 1dB difference. Note that I did measure each hub a couple of times using 2 different ports and the relative attenuation remained essentially the same. There are obviously too many hubs out there to measure each one but this sampling did not show any pattern as far as I can tell other than the best performing being the two Kensington products. One was an old USB 2, 4 port model and the other being one of their latest 7-port USB 3 hubs.

Other than the very inexpensive hub bought inside China, the others were essentially playing in the same ballpark.

3. The only lingering question left for me... How does this level of noise attenuation - especially that 8kHz packet noise reduction compare to commercial audiophile products. I posted a few weeks ago after the Vancouver Audio Show about the forthcoming inexpensive (~US$50) AudioQuest JitterBug. As I said, the quote I heard was "reduces noise by 6dB, and jitter by 8%". Hmmm, in the context of what I'm showing here, 6dB doesn't seem like much. Furthermore, I have yet to see anyone demonstrate the effect of jitter with modern asynchronous USB DACs in the analogue output so would question the benefit of this "8%" jitter reduction on the sound quality even if objectively demonstrable in the USB digital signal.

Then for US$175 we have the UpTone Audio USB Regen. Basically, we have a single port hub... But custom built to reduce the packet noise and "ground plane" noise as well as a better, lower noise power supply. I'd be very curious whether this device works in the same fashion as what I've been describing and measuring here. The Corning Optical USB 3 cable + Kensington USB 2 hub costs about US$130 before taxes and you can likely get free shipping off Amazon these days. Nonetheless the price differential is small. However the optical cable and USB hub can be repurposed for other uses in the future. Lots of words and descriptions on the UpTone website along with claims that $0.40 worth of resistors "increased its musical performance by 40-50% - most especially the bass!" presumably using some kind of Squeezebox Touch setup and significant differences in sound between a 'nasty' 6" cable and 'smooth' Supra USB cable. Right, so that's how audio design is done... I see they also looked at the Corning optical cable in that post and makes various claims about audible sonic changes. Strangely no measurements to confirm changes since this is primarily a noise reduction device, isn't it? Needless to say, I would love to get my hands on this USB Regen device (one night would be enough to make some observations :-).

There are of course other products out there like the iFi iPurifier. Notice in that web page they have a little graph showing EMI noise as 34dB from the iPurifier vs. 39dB from the computer USB port. 5dB noise reduction? Similar to the JitterBug; I can see this as realistic.

Okay, to end off... Let me remind everyone one more time that what I'm measuring with the 8kHz noise here is because of the Emotiva XSP-1's sensitivity to noise through the "Home Theater Bypass" input. The 8kHz tone is NOT something I have ever heard / measured coming out of a reputable DAC's analogue outputs! Nor is it something that's found in Stereophile USB DAC measurements. If it were not for this noise sensitivity, I would not even have bothered with looking at ways to attenuate the computer USB noise. Nonetheless, I think this is an interesting real-life demonstration of the noise pollution that can come out of the computer's USB port and a solution that works reasonably well. I did not bother with jitter tests this time as I have never seen the Dunn J-Test change in any substantial fashion with the use of a hub with an asynchronous DAC (see the measurements last time). Think you have a jitter issue? Save up the cash and buy a better asynchronous USB DAC - forget cables and tweak products IMO.


Well, they're at it again... I mean LH Labs and this time it's the Geek Out V2 (US$250+shipping) on crowd-sourcing.

About a year back I actually pre-ordered the first Geek Out but later cancelled as it was taking too long; this time word is that they might be ahead of schedule. Well, I'm a sucker for little DACs and since I don't have an ESS Sabre DAC to play with, I figure this might be fun to have at not much cost. This new model also has a few new features like balanced output and 2 headphone amp power settings (100mW & 1000mW), the 100mW setting should be good to keep the little DAC cool. Looks like it can handle up to DSD128 as well.

Now, if any of you have an interest in getting one, feel free to CLICK HERE and order using my referral code... Apparently it's one of those deals where they'll give me an upgrade or something if a few folks buy. Seriously, no pressure, whatever comes gets measured :-) !

Remember: Don't forget the Digital Filters Test is ongoing! We're at 35 responses at the 1/2 way mark. Thanks to everyone who has experienced for themselves and entered their results. Would love to get to 50 by the end of this month!

Well, it's getting sunny and hot on the West Coast; time to go play with the kids. Hope you're all enjoying the music as we get into June...


  1. To me it appears like a common mode induced problem.
    Most likely caused by a poor ground/wiring layout in the Emotiva.
    Some of the digital signal 'currents' make it into the audio section (via wrong grounding layout ?) and is simply mixed with the audio out from the DAC by the analog section.
    Together with extra noise from a PC, hub, DCDC converters and or the used power supply higher, or lower, amounts of 'unwanted currents' will make it into the audiopart of the Emotiva's circuitery.

    Curious .... could you check if the Corning cable has the same ground on both ends of the cable ?
    Just use a multimeter and see if the ground (and or +5V) are connected on both sides of the extension cord.
    The fact that the cable itself is hybrid (F.O.+copper) makes me suspect USB ground may still be connected, perhaps via a common mode filter so may not be isolated at all.
    In that case there may be some some internal common mode filtering perhaps.
    IMO the reason Corning used fiber optics is because of the length of the (extension) cable which can be longer than a copper cable (for USB3.0 data speeds), not perse for isolation.

    Fortunately in most cases there is none of the packet noises audible and suspect even the uptone regen will also show these peaks, perhaps somewhat lower in level as USB remains USB and the packets will still be there.
    The 'packet noise' will, most likely, still mess with the Emotiva.

    1. Hi Solderdude. I will check tonight how the ground is connected with the multimeter.

      Currently my suspicion is that you are right, the ground is probably still not isolated but that the majority of the noise is actually from the data lines which in this case is attenuated because the data is reconstituted through the optic cables.

    2. IDsonix® USB 3.0 10 Port Hub from Bizarkdeal.

      I have had so much trouble with the powered hubs that I have purchased over the years that I decided to wait before writing a review on this one. We use this hub on a repair bench in an Apple authorized service shop and I have to say that I have been VERY pleased with it. There are 3 banks of USB ports that are all powered separately. I use one bank for my own system and the other two for troubleshooting client equipment ranging from computers and hard drives to iPads & iPhones. There is sufficient power to all three banks to run multiple devices without problems. It is also sufficiently fast to handle multiple external hard drive which can book a computer through the hub. Very pleased with this unit and I highly recommend it for someone in need of lots of extension ports that are separately powered. It also sits nicely on the desk / bench and doesn't all tangled in other equipment.

    3. Before you set out to build a new PC, you should find the best power supply for your build, as it really is critically important. They might not be as exciting as the best graphics card, but every PC component will rely on the power supply for, well, power. hpxjg

  2. I am thinking of improving my audio-pc with low noise regulated linear power supplies and different filters ( for the HDD, fan, USB ) in order to reduce electronical noise going from my PC to my DAC connected via USB.
    Because the above tweaks are rather expensive I am looking into alternatives like the JitterBugger, Uptone Audio Regen, Shiit Wyrd etc.
    My DAC uses asynchronous USB and does not use the 5v in the USB cable coming from the PC/USB-host.
    I just wonder whether for an asynchronous USB DAC the tweaking of my Audio-PC by adding low noise PSU would be beneficial with regard to sound quality?
    Do you think going for a solution like the JitterBugger, Uptone Audio Regen, Shiit Wyrd, could make the expensive tweaking of my audio-PC redundant/superfluous?

  3. the Schiit weird and Uptone do NOT offer any electrical isolation.They are just a hub + clean power supply but all the common mode garbage on the USB out will be passed on via the ground wire to the DAC.
    The jitterbug is just a common mode filter + external power supply and may reduce the common mode currents ABOVE the audible range but won't do nothing for garbage within the audible range.
    It has no reclocking.
    Whether one of those will improve your audio experience depends on factors like 'belief in the product' and the insensitivity of the used DAC for common mode garbage and USB induced jitter.

    1. So do you think that using low noise regulated linear power supplies (LPS) in the audio-PC could improve the sound quality of my DAC with asynchronous USB without 5v input from the USB-host? Would the LPS reduce electronical noise coming from my audio-PC?
      If you had to choose between the Wyrd/Regen/Jitterbug or LPS, which one would you take?

    2. "do you think that using low noise regulated linear power supplies (LPS) in the audio-PC could improve the sound quality of my DAC with asynchronous USB without 5v input from the USB-host? "

      I am afraid I can't answer that question properly as I have no idea if your DAC actually has degraded sound due to problems in the PC's power supply.
      With a proper implementation of asynchronous USB-jitter (or from the PC) should not matter at all as the DAC's clock is the master and tells the PC when to send it's packages and at which 'speed'.
      With synchronous USB the DAC has to 'synchronise' to the PC's clock speed and thus the DAC's clock has a form of dependency on the PC clock.

      "Would the LPS reduce electronical noise coming from my audio-PC?"
      Most likely there will be less common mode noise that is generated in the SMPS so there may be less problems with specific DAC's under specific circumstances.
      You should realise though that every motherboard also has lots of small SMPS's on board that still generate HF garbage.
      A processor always needs a lot of different (and low) voltages IF it is to run fast and these are generated on board using DCDC converters.
      So a linear power supply could potentially reduce (but not eliminate) electrical noises from the PC.
      No guarantees that it does (reduce electrical noises) nor that the performance of the DAC/analog sections and equipment will (audibly) improve.

      "If you had to choose between the Wyrd/Regen/Jitterbug or LPS, which one would you take?"

      That would depend on problems I would be facing.

      IF I were only plagued by RF problems induced by the PC then the Jitterbug (or other) common mode filter + perhaps some ferrites would be better than the other options.

      IF the DAC is quite old and or susceptive to PC induced problems, has known high amounts of jitter or doesn't work correctly when driven from USB directly I would buy another DAC instead.

      IF I were plagued by leakage currents from the PC and would need isolation I would use fiber optics (TOSlink or other) or buy a DAC with galvanic separation inside (ifi micro iDSD for instance).

      If I wanted to use my ears only and see if it does 'something' (purely subjective) I would go for the Wyrd as it is cheaper, looks nice, has a trafo for power supply.
      If I thought the Regen would do better or wanted a smaller (physical) size device I would pay almost double the price of the Wyrd but get an extra SMPS in the audio chain OR would need to invest in an extra linear power supply to replace the stock PS.

      In both cases your PC will still be hardwire connected to the audio chain though.

      Sorry, I can't give a straight forward recommendation and just pick one without having the feeling I would be 'pinning one on you'.
      Audio life would be easy if such answers could be given.

    3. Nice summary Solderdude!

      Complex :-).

    4. Indeed a very nice summary! But confusion remains.

      I bought recently an asynchronous USB DAC (Audio-gd NFB-11) taking its 5v for the USB interface from within the DAC and disposing of some common ground isolation described by the designer/manufacturer as follows: "The Digital Interface applies silver wires isolated transformer for insulate the ground noise between PC and the DAC, which can offer the black backstage and more analog sound flavors.” How this works and to what extent this improves SQ is like "Chinese" to me. As a matter of fact the statement is probably a bad translation from Chinese.

      If it works, the better. But in that case investing in expensive low noise LPS or the likes of Wyrd/Regen/Jitterbug might be an overkill.

      I say "might" because before passing a judgement I will await publications of some in depth research based on actual measurements (hat off for your Archimago!!).

      I hope Archimago that you will find some time in the near future to make a comparative study of the Wyrd/Regen/Jitterbug!

    5. The A-GD NFB-11 ONLY has an isolation transformer for the S/PDIF BNC input, not for the USB input.

      Should you be troubled by a ground loop (not many people are) and need galvanic isolation, which not automatically means a good EMI/RFI attenuation, you could connect the USB via a USB-S/PDIF converter or buy a soundcard for the PC that has S/PDIF out (RCA/BNC) and use that instead.
      No async for S/PDIF though.

      Most people will not have any problems connecting DAC's via non isolated USB connections.

      When one connects lots of equipment together (home cinema, TV, NAS, PC, amplifiers and what not) the risk of getting in 'ground loop' or EMI/RFI problems is bigger.
      Also connecting equipment with 3 prong mains connectors to 2 prong outlets/extension cords is asking for problems.

      Some forms of galvanic separation (transformers) won't attenuate, or just slightly, HF garbage.

      When one is bothered by ground loops galvanic separation may help.
      When one is bothered by HF garbage/RFI/EMI common mode filters and or ferrites may help.
      In rare cases a combination may be needed.
      Get the grounding right and most likely everything works fine, even with USB.

    6. Thanks a lot Frans to take interest in my "noob questions".

      Please enlighten me further?

      First let me refer to an earlier reply:

      "Most likely there will be less common mode noise that is generated in the SMPS so there may be less problems with specific DAC's under specific circumstances.
      You should realise though that every motherboard also has lots of small SMPS's on board that still generate HF garbage."

      I thougth that most of the garbage comes from the main supply (AC to DC converter). If one would replace this witth a low noise regulated linear power supply, would this not give rise to far less garbage? The garbage arising because of DC-DC SMPS conversions on the motherboard would they not be not less "damaging" if the main supply is a low noise regulated LPS?

    7. Most of the 'garbage' does indeed come from the SMPS.... in general.
      Good power supplies with low emission exist and cheap crap that won't pass any tests exists as well.
      A linear power supply would/could reduce some or even most of the 'garbage' substantially.
      Proper grounding / wire routing also may help.

      HF 'garbage' will still exist due to the SMPS on the mobo + all the digital stuff 'switching' and loads drawn so there will STILL be a lot of common mode RFI/EMI stuff but lower in current and ground loops (in the audible range) will be smaller.
      Ferrites should also be used, when using the linear power supply, and have at least 3 windings through the ferrite (not a single pass through a clamp ferrite) IF your goal is to lower HF garbage as well.

      The real question is IF it is really needed at all and IF the DAC indeed doesn't operate properly under these conditions and in essence is faulty or poorly designed.

      What I am getting at is that even when all counter measures are in place and all garbage is lower this doesn't mean the actual signal from the DAC will be 'better' or cleaner in a real (measurable way) as the DAC may not be bothered by all this in the first place.
      MOST DAC's work perfectly fine under normal conditions, even when common mode noise is present.

      In general some subjective found differences are really hard (impossible ?) to correlate to real life electrical changes.

      It also depends on more than just the source (PC), also the rest of the chain can have an influence in a technical sense on ground loops (hum, pithed noises, whines, sounds that clearly don't belong)

      A loss, or degradation, of things like 'imaging, air, separation, realism, being there' etc is often linked to these kinds of things... there is no scientific evidence pointing in that direction though.
      Not counting many 'reports by others' as real conclusive evidence here.

      I would only start heroic actions IF one is actually plagued by noises that clearly should not be there (as in the case of Archimago) otherwise I would just enjoy the music and stop listening out for 'problems'.

    8. Hi Frans

      What an enlightening answer! Things start to become clear for me.

      Do you have some ideas how to test whether my DAC's USB interface might be "poorly designed".

      If I would connect it to a "noisy laptop" running internet, playing video's, moving the mouse, ... all at the same time as streaming audio via USB (push it really to its limits), and than listening/searching for audio disturbances?

      Could this be a way to test the quality of my DAC's USB interface? Have you some other suggestions?

    9. That would not test your DAC, but would only say something about the speed of your laptop.

      If you do not hear ticks, weird sounds, noise(s) or other 'things' in silent passages then there is no reason to worry about the DAC's performance.
      You can for instance play test a CD (or file) with absolute silence or dither only as in this case the DAC is active.

      If you want to test objectively how it performs in your specific setup you will need a PC or laptop with an analog input, the DAC, test programs to measure jitter, distortion etc. and a Y splitter.

      Connect a laptop (running off batteries) to the DAC and the (external ?) soundcard and loop the output to the DAC via decent RCA cables to the input of the soundcard.
      Some programs may have difficulties using 2 sound devices.
      Make a 'base' measurement of the DAC (distortion, jitter etc.).
      The ADC should be decent enough to show the noise floor of your DAC.

      Then connect the DAC to your audio system and to everything else that is normally connected.
      Use a Y connector and connect that to the DAC output.
      Run one output of the Y to the soundcards analog input and the other output of the Y is connected to your stereo system (everything must be on and functioning) as it normally would be connected.

      Make another set of measurements and compare to the baseline.

      Also you can connect the Y splitter further on in the chain and loopback the audio from there.

      As mentioned... if your DAC sounds fine and don't hear things that shouldn't be there you do not have to worry.

    10. Hi Frank
      Just want to let you know that within my technical means I have done a lot of tests during the weekend.
      First I recorded a "silent" audiostream with Audacity and listened if I could discern anything abnormal while playing this file through USB from my laptop. Dead silent, as well while connected to the power supply as while only using its battery.
      Then I compared identical lossless files (16bit/44Khz) played via iTunes set at max volume for bitperfect streaming to my Airport Express connected to my DAC via optical, and also via JRiver with bitperfect Asio settings and my laptop connected via USB to my DAC.
      No abnormalities in the sound reproduction. Although I found that the USB connection for one reason or the other gave a more pleasant sound. This could be due to the DAC's internal electrical design, the AirPort Express’s reproduction of the optical signal, or the difference between iTunes or JRiver. Honestly I have no clue.
      I think I have just joined the "bits are bits"/"objective" audiophile camp.
      Anyway I will not put my money into a tweaked (expensive linear power supplies, SSD, filters, etc.) HTPC, but save it for a better amplifier, and use my laptop as a "music server".
      Lastly I would like to thank you for the good technical information. This really was a great help.

  4. USB Isolation

    For USB Audio Class 1 USB DACs (up to 24 Bit 96 kHz), I was using an OptiCis Fiber-Optic isolation / extension cable (10 Meter). This “cable” does 100 % isolate the input from the output (for sure), but does need a power supply for the output side in order to work (no matter if you have a self powered USB input at the DAC side or a VBUS powered USB input).

    This cable does come with the SMPS but I replaced this with the batterie power supply from SOtM, the mBPS-d2s, and this runs without charging power supply for over a complete show day.

    This was a good combination, for example at HiFi shows, where I could have my computer at my side, and could extend to the USB DAC about 10 meters away. But this does no longer work with USB Audio Class 2 inputs, even if you plan to play a 44k1 song, the input could not be recognized from the computer, because the data rate (even for the handshake) will be too high.

    And so I was thinking to buy the Corning USB cable but reading this blog and one other blog before, makes me thinking, if this is the right cable. I have some questions left:

    1. Does this Corning USB really isolate the input from the output? So when I would use a batterie driven usb hub on the output side, would there be really an isolation (as it is with the above mentioned OptiCis cable) or would be some electrical connection left (as some readers have commented). It looks like, that only the two data lines are isolated for the reason of extension the length of an USB cable, but that the VBUS and Gnd go all the way through with cooper wires. Yes, you can isolate (remove) the VBUS with some DACs but all the common mode suppression would still be destroyed, when the Gnd would not be isolated. And if this would be the case, also a batterie powered output side, would not isolate the computer from the DACs input. The ground would be still running all the way through. This would be an important point for me to know.

    2. Archimago: If you would have used a batterie power supply for your Kensington usb hub, would then the 8 kHz noise be reduced any further (or does this 8 kHz still creep through the suspected ground wire connection, mentioned in my question 1).

    If the Corning USB cable does not really isolate, then it is basically “only” an extension cable, but not an isolation cable.


    1. Hi Juergen, I'd love if you could try the cable and see what you find!

      Given the very narrow nature of the 8kHz tone, I doubt that a battery supply to the Kensington would change the nature of this as it's too coincident with the USB2 microframe packet rate noise one would expect.

      I'll check tonight to see if there is a ground connection as Solderdude suggested above.

      Given that it is capable of USB3 speeds, I think it's certainly worth a try compared to the other more expensive optical cables that can only operate at USB1 speeds! Even if not complete isolation (which is what I suspect), at least in my system, it helps reduce noise to an inaudible level... At least empirically demonstrable - more data to consider than I've seen with the other products!

  5. Hey there guys. Okay. I took the multimeter to the Corning optical cable and indeed the suspicions were correct - GROUND IS STILL CONNECTED.

    However, given my findings, I am of the opinion that the noise reduction I'm seeing is primarily due to isolation of the data lines using optics which I think is *much more important*. That 8kHz tone IMO is much more bothersome than comparatively lower level noise and is originating mainly from D+ and D-, not the 5V and GND connections. Suppression of 8kHz at least in my system provided a nice proxy for quieting down the noise in general.

    Again, love to see what the other commercial products would do in this "test bed".

    1. It's not entirely impossible the ground /power supply lines may have small common mode filters (inductors) and or an additional capacitor in line which may help with reducing common mode garbage.

      Is the total resistance close to 0 Ohm (not counting the measurement leads) or a few Ohm ?

      The ground (and power supply lines) have the lowest resistance and is the path the 'garbage' takes.
      The data lines are usually higher impedance (Around 100 Ohm input R) so noises travel across the lowest impedance they 'see', the ground and power lines i.e.

      On top of that the Data+ and Data- lines are balanced so common mode garbage should not (highly depends on the receiving end topology) be bothered by it but, as you mentioned, also travels along the data wires as well.

      The test bed (ill designed Emotiva) is indeed a good testbed for this kind of tests.
      Not representative for most other equipment though but it is to be expected that if the Emotiva works optimal the 'conducted' noise may also be lower in other applications.

      Thanks for measuring this.

  6. Isolation and Common Mode “Noise”

    Archimago: Thank you for clarifying, that this Corning Glass Fiber USB cable does not isolate the Ground wire, between Computer and DAC.

    As for the 8 kHz micro frame packet noise, of the PHY chip, I have ordered the USB Regen device and will see what this device does.

    Over the last 5 years, I have developed three different USB Inputs with different characteristics. First with the “old” TI TAS chip, for USB Audio Class 1, then with the “first” XMOS chips, for USB Audio Class 2. Both chips have had the 90 Ohm differential USB Data lines included and the latest with newer XMOS chips, that needed the above mentioned PHY for the USB data.

    As Archimago mentioned, when he (or when I) measure just the USB DAC device, I do not have any serious noise at the analog output of a (good) DAC, no matter which of the above mentioned USB Input circuits I am using. But what I have noticed; When I setup a complete hifi system, also with some external devices, then I do measure on some combinations, some “noises”.

    These noises are normally introduced because of different common mode voltages of the different devices and the different behavior of those devices concerning common mode voltages.

    My first USB inputs have had digital signal transformers for isolating the computer from the DAC side. This works good for galvanic isolation, but they are still some stray coupling of the windings of the digital transformer. Even when I use some expensive digital transformers from Scientific Conversion, that have some shielding between the windings, I still do have some couplings.

    So with my newer USB inputs, I do have optical isolation (that are a bit more complex, because I then need also an isolated power supply on the isolated signal side (which is not needed for transformer coupled isolation)), but these optical isolation does have a further reduced coupling capacitance and is more immune against common mode voltages.

    And this is the reason, why I look for a real isolated optical extension for USB Audio Class 2.


  7. Hi Archimago
    Is it possible to re-edit a former post I published?

  8. Hi, Archimago

    Please contact I have some surprises for you.
    Really admire your passion and dedication.


  9. You should get your hands on an Uptone Regen. CA is raving about it, and I'm really curious what if any objective difference could be measured.

  10. "Okay, to end off... Let me remind everyone one more time that what I'm measuring with the 8kHz noise here is because of the Emotiva XSP-1's sensitivity to noise through the "Home Theater Bypass" input. The 8kHz tone is NOT something I have ever heard / measured coming out of a reputable DAC's analogue outputs!"

    Confused here: If you are saying that you haven't seen this on the analog out of a reputable DAC and you have a reputable DAC, how is it that you are seeing this on the Emotiva since you are feeding the Emotiva Analog?

    1. There is a difference between the actual analog output voltage between the output pin and 'common/screen/ground' and the same signal + common mode leakage currents + poor PCB layout in the Emotiva making this audible/measurable.

      So a DAC output signal can be sqeeky clean on its outputs (measurably) but still cause problems in the analog path behind it because of common mode currents and poor cables/PCB design/power supply issues.

  11. Thank you for the information rendering by you and helping me too.So keep sharing such useful information to us .To know about this information i really want to see about it more.I want to share some information on loans. Loans are extremely important when one wants to invest in good projects.To know more about loan related activities and to understand the procedure as well you can visit Low EMI Personal Loan in India .

  12. Hi Archimago, when you tested for ground across the optical cable, did you unplug the equipment at both ends? I am just as surprised by your finding....

  13. That's a really nice article indeed, thank you! Now digging a paragraph taken from "The most common and simple-to-fix EMI error is mistakenly
    tying the shield in the USB cable directly to the ground plane of your system. This allows any noise injected into the ground plane to escape any shielding around your device. " Also "Tying the shield directly to ground would create a direct path from the ground plane to the shield, turning the USB cable into an antenna". More details could be found here:
    I believe my DAC has the GND plane directly connected to USB GND, so now should I really cut DAC's USB plug (or PCB) GND to insert a 1Mohm resistor paralleled with a 4.7nF capacitor between USB GND and DAC GND plane?
    Thank you!

  14. This comment has been removed by the author.

  15. Extremely interesting article.

    What about using the SMAKN® USB3.0 Type A Male to Type B Male Adapter Connector instead of the USB hub ?

    like this:
    HTPC --> Corning Optical USB 3 cable --> SMAKN® USB3.0 Type A Male to Type B Male Adapter Connector --> TEAC UD501 DAC --> 6' balanced analogue cables --> Emotiva XSP-1 preamp --> 6' balanced analogue cables --> E-MU USB0404 ADC --> measurement computer

  16. Wow! Simply awesome! Thank you so much for the details discussed! At BLA ETECH we provide complete solution in Standard & Customized Power Line FILTERS. Spanning in all directions of Electronics Filter applications, ranging from 1 Amps to higher currents of 2500 Amps.

  17. Best USB HUB available on Amazon with best price.

  18. I personally use them exclusively high-quality elements : you will notice these folks during: usb-c adapters

  19. Thank you so much for this valuable information. Visit Kalakutir Pvt Ltd for Food Truck Branding, Vinyl Signage Printing Services and Epoxy Paint Flooring & Coating.
    Epoxy Paint Flooring & Coating