USB Powered Dac problem.

[Welder]

Until recently I’ve been using an HRT Pro connected to my laptop. Between the HRT Pro and the laptop I’ve been using an adapted mains powered USB which has a galvanic isolator between the Dac and the Hub.
I sold the HRT Pro along with the powered hub and returned to the HRT 11+ and I’ve been running it off USB power until I get around to building another isolated mains powered USB hub.
Through my amp and speakers all seems fine, but when I connect it up through my headphone amp I get a noticeable hum in one channel akin to a grounding loop problem. If I earth the Dac by touching it the noise goes. If I stand directly in front of the laptop the hum decreases.
It may also be relevant that I use semi balanced interconnects (shield not connected at amp end, only at source)

So, I set about trying to see what could be done to ground the path from the laptop to Dac.
Like most laptops I imagine (?) it doesn’t have a ground lead from power mains plug to transformer to laptop power input.
If I leave an Ethernet cable plugged in (which carries a ground) even if the router is switched off the hum goes.
This would seem to suggest that the laptop could do with grounding.
I find it hard to believe it is just the combination of my headphone amp, Dac and cables causing this problem and wonder if all laptops using a USB only powered Dac have this problem
Has anyone else experienced this problem and does anyone know of a way of grounding a laptop under mains power?

[Vincent Kars]

If you do a search on ground on e.g. http://www.head-fi.org/ you will find a lot of postings.
Plug all the gear in the same power block sometimes help.
Cheater plugs (lifting the ground of the DAC) are often used.
A simple USB isolator like http://electronics-shop.dk/?

Succes!

[Welder]

Thanks for the reply Vincent
I had seen the USB isolator elsewhere.
I adapted this for the HRT Pro; combined it with a USB mains powered hub and a decent 5V power supply.
http://www.circuitsathome.com/measurements/usb-isolator
with a bit of help from a mate.
I use a single point ground conditioned distribution box for all my Hi Fi, conditioning now as
http://theartofsound.net/forum/showthread.php?t=7450

However, an isolator on its own doesn’t solve the grounding problem
I’ve had a look at the head fi forum and apart from the fact that some idiot decides to post something really stupid mid thread I cant find yet a reasonable suggestion for grounding a laptop………..unless you take one of the suggestions and shove a copper rod into the guts and run an earth wire of it

However, it seems I’m not the only one with laptop grounding problems.
Looks like a bit more research is needed.

[Vincent Kars]

However, an isolator on its own doesn’t solve the grounding problem

I expect a isolator to galvanic isolate the 2 components on power and dataline and ground. Wonder what is going on

If you lift the ground of the DAC, does that help?