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In Reply to: RE: USB isolation hub (Gordon, audioengr, John - input wanted) posted by pryamomimo on February 22, 2008 at 07:59:59
Very interesting post, I've actually been thinking along these lines for awhile.First off, inductor in the GROUND line is very bad, almost certainly going to make things worse.
A few general thoughts on ground loops:
first notice the word LOOP, in order to have a ground loop you must have a loop, a single ground wire does not a loop make, in order to have a ground loop you have to have at least two ground paths. So if the ONLY electrical connection between the laptop and the DAC is the USB cable, you cannot have a ground loop and you are fine. So if your laptop is running off batteries and there is no other connection to anything else, just the USB cable you can't have a ground loop.The problem occurs if you have the laptop plugged into the wall, and the ground of the USB cable is connected to either the neutral or the safety ground, then presto, ground loop. Laptops actually have an advantage here because a lot of the power supplies do not connect to the safety ground and are isolated from the neutral. If yours has a three pin plug then things get a bit more complicated. Use the meter and measure the resistance from the ground on the USB port to the ground pin on the supply. If the resistance is high you are fine, if its low then you are going to have a ground loop through the power line.
For long distances and or ground isolation I like the optics cable, its not cheap but really does work well, BUT as Gordon said you need to get rid of the supply that comes with it and make your own high quality supply. Its not hard, the thing barely uses any current so a big honking supply is NOT needed.
On the reclocking and hubs, this is an interesting subject that I have been looking into recently. It turns out that all hubs that I could find DO reclock, but they use a PLL to generate the clock. They use either a 6 or 12MHz crystal and a PLL based frequency multiplier on chip to generate a 48MHz signal used to read the input and clock the output. Since the power supplies are rerely done to "audiophile" standards, and they usually get driven off VBUS anyway, the feed to these PLLS is usually noisy as all get out so the clock jignal has very bad jitter.
I searched and searched and found a hub chip that allows you to bypass the on chip PLL and feed in an external 48MHz clock. So it looks like its possible to build a decent reclocking hub using a really good PS and a very low jitter 48MHz clock. With good board layout it should be possible to make a hub whoose output jitter is at least an order of magnitude less than any other USB connection. The only problem is I forgot to write down the chip number! (you'd think I'd remember my memory is now the proverbial steel sieve) If I can ever find this again I'll post it.
John S.
I found the chip! Its the TI TUSB2046B, digikey has them.
Edits: 02/22/08Follow Ups:
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