[pinnahertz]

  Remember that I switched out the power supply on the PC and had the same problem.
Anyway, I now have the plugs, so I'll experiment later tonight.
Thanks!

Did you swap the PS for another identical unit? Same make/model?  We're dealing with design issues here, not failures or random defects.

 

[denti]

  Did you swap the PS for another identical unit? Same make/model?  We're dealing with design issues here, not failures or random defects.

No, different make entirely.

 

[denti]

OK, I attached the ground from the PC to the chassis of the amp. Same noise. Will try connecting the DAC and amp grounds next.

 

[NA Blur]

Can you run a wire from your PC case or at least some of the metal housing on it to the case of your audio device? That should help.

 

[denti]

  Can you run a wire from your PC case or at least some of the metal housing on it to the case of your audio device? That should help.

Thanks. Just tried it. No difference.

 

[denti]

OK, I've got the PC, DAC, and amp all plugged into 3-to-2 prong adapters, with the ground wires running as follows: DAC to PC case; PC to amp chassis; amp to PC case.
 
Same noise.
 
Could this still be a ground loop issue? I'm starting to doubt it.

 

[pinnahertz]

  OK, I've got the PC, DAC, and amp all plugged into 3-to-2 prong adapters, with the ground wires running as follows: DAC to PC case; PC to amp chassis; amp to PC case.
 
Same noise.
 
Could this still be a ground loop issue? I'm starting to doubt it.

Try grounding one unit, possibly the DAC, using the power cord ground, but keep all other wires in place.
 
Is there anything else connected to the PC?  I assume a monitor, but what else?  You'll want to disconnect each device one at a time and listen.

 

[denti]

  Try grounding one unit, possibly the DAC, using the power cord ground, but keep all other wires in place.
 
Is there anything else connected to the PC?  I assume a monitor, but what else?  You'll want to disconnect each device one at a time and listen.

What do you mean by "leave the other wires in place"? I have tried grounding the PC to the amp chassis. Then I added the amp and DAC grounding wires, running to the PC. If I ground only the DAC that would mean disconnecting the ground wires of the others.

 

[denti]

SOLVED IT!!
I just went back to the system just now and it was dead quiet. How'd that happen??
I don't know. So I started unplugging the grounds. DAC ground out. Still quiet. PC ground out. Still quiet.
Amp ground.... aaaand, that's it. I didn't even need to attach the ground to anything. Just having the ground removed does the trick!
 
So, why didn't I hear that last night? No clue.
Glad it's fixed, though. Thanks for your help, pinnahertz!

 

[pinnahertz]

Glad you found the culprit. Ground loops are very often elusive, but the best weapon is persistence.

Hope it all sounds sweet now!

 

[denti]

So I'm told it is absolutely not safe to unground the amp. What to do now?!

 

[pinnahertz]

  So I'm told it is absolutely not safe to unground the amp. What to do now?!

"Absolutely not safe" is pretty strong.
 
The only issue would be a failure where the power hot wire connected to, or partially coupled to the chassis energizing it, and you then touch the chassis while some other portion of your body is grounded.  The chances of that are extremely remote, and would likely blow a breaker.  The chassis is already grounded through the external wire (if you still have it in place) and the signal wires.  
 
The hazards are not zero of course, there's always a risk.  But there's always a risk when you use electrical equipment, even when it's "grounded" with the power cord.  
 
I wouldn't worry, but it's your call.

 

[denti]

This scares me: https://en.wikipedia.org/wiki/Cheater_plug#Safety
 
What are my other options? I'm assuming grounding the USB isn't possible without getting one of those devices that limits the frequency of audio files.

 

[pinnahertz]

Did you read the statistics? You are more than 5X more likely to be struck and killed by lightning in any given year than be electrocuted by an ungrounded device.

Alternative? Get an isolation transformer and power your amp from that, then it no longer needs to be grounded to be safe.

 

[denti]

Did you read the statistics? You are more than 5X more likely to be struck and killed by lightning in any given year than be electrocuted by an ungrounded device.

Alternative? Get an isolation transformer and power your amp from that, then it no longer needs to be grounded to be safe.

So what's the safest way to set this up? Running a wire from the tab on the 3-to-2 plug to the PC case and from the PC case to the amp ground/chassis?