PC to DAC connection?

[zhai]

No, it wasn't grounded. I believe I did pull the power cable when I connected the computer and the amp, but it died anyway.

[peufeu]

You see, the PC itself is usually the only thing that is grounded (and sometimes the monitor also).

But all the other junk that connects to the PC is usually not grounded :

- DSL modems
- USB harddisks
- Powered speakers
- etc

I once had a DSL modem with a diying power supply which was sending high voltage through the ethernet cable. Of course, neither the modem nor the motherboard had ethernet isolation transformers (guess they were too cheap). So I got an electric shock when I unplugged the PC from the wall socket because it was no longer earthed.

Apparently the current was not high enough to trigger the main breaker which is set at 30 mA but it was definitely enough to feel it or burn a few opamps...

Said modem also sent high voltage through the phone line and burnt stuff in the DSLam !

[FastEddy]

Finding problems with USB connections:

Disconnect and remove everything in the PC that is non-essential to boot, load, run the OS and display on screen, first. If OK, then put back the rest of the plug in cards, only, one at a time, but leave any power hungry external peripheral devices disconnected.

Then reconnect the USB / Audio device's * Cable Only * and test it with a volt meter, looking for deviations of both DC voltage out of normal range (+5.00 VDC +/- 2% - on red wire (pin 1) v. 0.00 VDC ground (black wire & shield, pin 4 & 5) AND any riding AC voltage which should be undetectable or less than 0.05 VAC .... an oscilloscope can pick it up, but many AC voltmeters can not. ( I use a "developer's cable" for this: http://industrialcomponent.com/newnex/uf12402.html )

If there is any significant AC riding on the USB power lines, red and black wires (white and green are USB data wires), this is a problem, either with your computer power supply, your motherbaord PCI bus & power filters, your USB port card, etc.

If this AC or any other PC generated noise is being passed through your USB DAC to the NAD, it will probably be detectable on the USB DAC (analog RCA?) outputs ... and your USB DAC is either defective or was a poor design to begin with = unable to properly filter the noise and you only options would be USB to D2D/ SPDIF optical for the isolation, then SPDIF DAC at the NAD ... or complete replacement of the offending PC. (There is another option used by high end audio types and recording studios: Total USB optical isolation: http://industrialcomponent.com/opticis/optusbext.html ... but this can cost more than the computer to impliment.)

....

I have dealt with this problem too many times. Invariably, with 96% surity, the PC power supply is at fault OR some power line related problem like an ungrounded system.

The above also assumes that you are not going to consider a better transport scenario like FireWire 1394 DAC / ADC, where the data can very easily isolated from PC power with proper cabling choices.