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Not all USB ports are born equal

Soniclife

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Shame there is no out of the DAC measurements. Maybe if Amir has a ton of old computers lying around, and nothing better to do we might find out.
 
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Vincent Kars

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If one does, one measures of course the quality of the DAC and in this case , the quality of the USB receiver.
A “good” DAC will of course cope with this phenomenon.
Even one of the early USB DACs like the Ayre QB9 already applied galvanic isolation.

There is a measurement by Exasound showing the difference between isolated and non-isolated USB: http://www.thewelltemperedcomputer.com/Intro/SQ/GalvanicIsolation.htm
 

Fitzcaraldo215

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If one does, one measures of course the quality of the DAC and in this case , the quality of the USB receiver.
A “good” DAC will of course cope with this phenomenon.
Even one of the early USB DACs like the Ayre QB9 already applied galvanic isolation.

There is a measurement by Exasound showing the difference between isolated and non-isolated USB: http://www.thewelltemperedcomputer.com/Intro/SQ/GalvanicIsolation.htm
Vincent, yes, I agree fully.

It is no surprise that PCs, Apples, etc. are inconsistent, noisy sources via any of their outputs, not just USB. The eye opener in the original article is how variable different ports in the same computer might be.

I think this should be common knowlege. I also think DAC designers should all recognize this and design accordingly. Unfortunately, not all do, but some appear to be well isolated by design from these real world issues. Hence, we have add on devices by the ton, only a few of which might actually be helpful to those DACs needing proper isolation. I also think Amir's measurements of DACs and addons are very helpful.
 

restorer-john

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The 'tests' linked above, show nothing useful whatsoever.

For a start, the USB ports are unloaded. Loading a rail with a fixed current draw is the only way to view ripple and draw meaningful conclusions. The various USB 'ports' are likely under complete control of the OS and being cycled/polled on/off at high speed or in power saving mode as there is no attached device pulling current.

The scope is not isolated from the ground of the PC/Laptop and the computers may or may not be effectively grounded. In short, it's just a whole lot of meaningless screen grabs.

I'm happy to test a bunch of USB ports from a pile of PCs/Laptops on my DSO and post the scope shots, but let's determine a current draw applicable to actually load the rail effectively.
 

admin

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I'm happy to test a bunch of USB ports from a pile of PCs/Laptops on my DSO and post the scope shots, but let's determine a current draw applicable to actually load the rail effectively.
Good points. On this, I think the worst case I have seen is about 150 milliamps. I could measure a few and report back.
 

restorer-john

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IIRC, USB up to 2.0 is 500mA max and USB 3 is 900mA max.

150mA is a useful figure and likely typical of D/As perhaps?

There are plenty of inline USB testers for a few dollars that will indicate the current pulled by devices which may be a useful addition to USB D/A converter reviews.

something like this: https://www.ebay.com.au/itm/USB-Vol...hash=item41ca75a6f8:m:miapXBsvKAY-BVq-Uyds89A

The second usb port on that little tester could be used to monitor ripple on a DSO.
 

amirm

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NorthSky

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That's interesting; which one of the four USB ports from the laptop should I use? Normally I use the closest one to me, a high speed 3.0 (blue) version.
 

trl

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The 'tests' linked above, show nothing useful whatsoever.

For a start, the USB ports are unloaded. Loading a rail with a fixed current draw is the only way to view ripple and draw meaningful conclusions. The various USB 'ports' are likely under complete control of the OS and being cycled/polled on/off at high speed or in power saving mode as there is no attached device pulling current.

The scope is not isolated from the ground of the PC/Laptop and the computers may or may not be effectively grounded. In short, it's just a whole lot of meaningless screen grabs.

I'm happy to test a bunch of USB ports from a pile of PCs/Laptops on my DSO and post the scope shots, but let's determine a current draw applicable to actually load the rail effectively.

Did some loaded USB measurements too, last weekend, will forward'em soon to Archimago to post updated results. On the 8-bit FFT (bandwidth <100kHz) the only difference was around 1kHz...a small spike, but the rest of the graph was basically the same. By loaded I mean: USB port connected to PLAY or to EssenceOne.

Oh...scope was completely isolated, I've used a MBP on it's battery, thank you. Also, I've used a spire on the GND's probe, just to increase the accuracy of the measurements.

However, if someone thinks the process can be improved, please let me know how.

Thank you,
Raul.
 

Theo

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Did you see any difference when the MBP is powered externally?
 

trl

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If you're speaking about MBP's USB port noise, then I wasn't able to find much difference powered vs. battery operated. After all, there are the same LDO's powering the USB ports. I can try finding the original screenshots or to get new ones and post'em here if you want.
 

sofrep811

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Thx. I'm going to try a Raspberry Pi server build and the least I can do is put some parts in it that are supposed to be better than average. Hopefully,I didn't type too fast as I haven't read it yet.
 
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