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USB isolation hub (Gordon, audioengr, John - input wanted)

Posted by pryamomimo (A) on February 22, 2008 at 07:59:59

Wanted to share this with you and ask for any comments or recommendations. I have a standalone DAC with PCM 2902 inside. The DAC is used via SPDIF from my CD player and TOSLINK from my DVD player. I intended to use it via USB in from my laptop (fed from a separate power line), but was concerned about 120 mV of AC between the grounds – clearly not good. In addition, my stereo system is more than 5 meters away from my laptop. So, I came up with this solution.

A USB hub at the end of a 5m USB cable, then 0,5 meter USB cable to DAC. The hub uses a modern NEC 720114 chip. I made it run in self-powered mode off a battery. Still, the 720114 chip needs Vbus from the PC for device recognition (on a separate Vbusm pin), but the USB bus is not used to power the hub (there are diodes installed for this purpose). Then, PCM 2902 in the DAC again needs Vbus for recognition (this time it is the power from the battery via the USB hub, which has some sort of DC line filtering via ferrites and caps), although all of its critical areas are powered internally within the DAC.

This is what I did: first of all, I separated the shield ground lines between the upstream and downstream USB port (had to saw a little across one side of the USB hub PCB). Second, I tried disconnecting the USB ground line running from the PC (by unsoldering the respective pin on the IN socket in the hub). This resulted in a system message stating that USB hub not being recognized properly. To solve this AND provide some sort of noise isolation between PC – hub – DAC, I soldered an inductor (Bourns RLB 100mH) between the USB bus ground line and the hub system ground. Now, my multimeter does not show any AC between the devices. So far, things are working, but it is too early to make any observations re the sound. Also, I do not have a scope at my disposal, so can’t make any measurements as to groundline noise reduction.

This might prove an effective way to provide some sort of isolation between PC and hi-fi system, definitely more accessible than $500 cables. However, maybe Gordon Rankin, audioengr, John Swenson or other interested parties can provide some input on the following questions:

1. In your opinion, is a 100mH inductor an adequate solution? What could be better for filtering the most typical PC noise on the USB ground line? (again, don’t have a scope, so can’t tell what are the offending frequencies).

2. It is my impression that the NEC 720114 chip actually buffers and “reclocks” USB packets, thus potentially helping reduce the resulting jitter, especially on a link over 5 m long. What do you think?

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