[pinnahertz]

  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?

How did you get it working and quiet? Please describe in detail.  We'll go from there.

 

[denti]

  How did you get it working and quiet? Please describe in detail.  We'll go from there.

Ungrounding the amp. That's it.
 
I would prefer to leave it grounded, but don't know what USB ground isolators will work with DSD. I'm eyeing the ifi iDefender.

 

[pinnahertz]

  Ungrounding the amp. That's it.
 
I would prefer to leave it grounded, but don't know what USB ground isolators will work with DSD. I'm eyeing the ifi iDefender.

Might be worth a try. It's not getting great reviews, so just make sure you can return it.  
 
IF you'd prefer to have the amp grounded, I assume it's because of the safety scare.  It's overblown, but if it worries you that much, there are two alternatives.
 
1. A pair of audio transformers between the DAC and amp. 
This is the good stuff:
https://www.amazon.com/Jensen-CI-2RR-IsoMax-Isolator-Jensen/dp/B00ASVWYCS/ref=sr_1_37?ie=UTF8&qid=1492738876&sr=8-37&keywords=stereo+ground+isolator
 
I've kept a few Radio Shack ground isolators (transformers) in my kit for decades, but they are no longer available.  They worked well, and were surprisingly good.
 
This is something that will work.  I haven't tested it, but it's similar to the RS one.
https://www.amazon.com/Raptor-RGLI-MID-Channel-Isolator/dp/B00VO89UUS/ref=sr_1_50?ie=UTF8&qid=1492739094&sr=8-50&keywords=stereo+ground+isolator
 
If you use an power isolation transformer you don't have to have the amp grounded for safety, and don't have to have anything in the audio path.   
https://www.amazon.com/300-Watts-AC-Isolation-Transformer/dp/B006NG977W/ref=sr_1_10?s=electronics&ie=UTF8&qid=1492737755&sr=1-10&keywords=isolation+transformer
 
You leave the adapter in on the amp power cord (or it will be grounded, and in this case, that's not what you want).   The isolation transformer creates a safe situation without grounding because you could touch either AC wire and not be electrocuted, or even feel anything, even if you yourself are grounded.  Without the isolation transformer, you will receive a shock doing that.  

 

[denti]

Thanks. The first option is a bit expensive. The second I have used in other situations, and it kills the sound (depletes the bass), so that's out of the question. The third option looks good, though the USB isolator is a tad cheaper, so I'll try that first.
 
If I were to leave the amp ungrounded and without a power isolation transformer, can I do anything to minimize the risk of shock by using the ground tab on the adapter?

 

[pinnahertz]

The RS isolator does not kill the bass. There are several variants, and at some point I'll have to test a few because I use these to solve exactly this problem in studios. Are you certain you used that exact one?

No there's nothing you can do to reduce the already infinitesimal shock hazard with any other connections without reintroducing noise. You'll be using one of the suggestions I've given.

Dumb question, but do you have everything plugged into the same power strip? Might not help but worth a try if you don't.

On the "shock hazard", you should understand the situation fully before drawing conclusions. The AC power at an outlet has two wires, the "hot" and "cold". Hot is energized at 120v relative to cold, which is at or near ground potential. Depending on the style of wiring in the home the third "ground" pin may connect to a third wire that runs all the way back to building ground, but it also may be connected only to the metal outlet box and be grounded via conduit. That's not always a good ground, but it's legal. Building ground is supposed to be a ground rod driven into the earth by 6' or so, but that can degrade over time and not actually be a very good ground. The shock hazard occurs when you come in contact with the hot wire and you yourself are grounded by standing barefoot on a concrete floor, have a hand on a water pipe or fixture, or are touching something that is grounded. Just touching the hot wire without being ground yourself may result in a slight tickle at most, nothing at least. For there to be a shock you have to be part of the circuit by being connected to ground. Since few people stand barefoot on concrete floors while handling live hot wires, or hold a water pipe, the chance of becoming a circuit is rare. And that's if you grab a hot wire.

In your setup your amp is grounded from the standpoint of shock hazard and audio ground, or it wouldn't work. For the chassis of the amp to become energized it must contact the hot wire, or provide capacitive coupling to the hot wire. If the amp was at all designed well, and good materials were used (like wire with good insulation that won't crumble to dust), there's almost no chance of the chassis becoming energized. If it did the energy would flow to ground via the path you've provided likely blowing a circuit breaker. But even if that didn't happen and the chassis became connected to the hot wire, to get a shock you would need to be grounded somehow, and unusual situation for most humans. For the shock to be even close to lethal your skin resistance would have to be very low, like with sweaty or wet skin at both the point of ground and the point of contact with the hot wire. Dry skin has a very high resistance, thousands of times that of wet skin. For all these reasons and a few nor too, there's really no significant risk of electrocution with your setup. What you are actually facing is fear of the unknown.

That said, most people solve this problem with an audio transformer.

 

[DDF]

Intona Usb isolator?

Sent from my SM-P600 using Tapatalk

 

[pinnahertz]

The RS isolator is still available after all.
 
https://www.radioshack.com/products/radioshack-ground-loop-isolator
 
I can't locate my test data, it's probably in a backup somewhere, but I don't have the time to look. 
 
In this link someone else tested it, 9th post down.  "The measured performance was truly amazing for a pair of transformers case and cables selling for only $16.65."
 
They're cheaper now.

 

[denti]

Yeah, that's the one I had. I tried between a phono pre and amp and it was dreadful. Might not be bad in this case. I've got a USB isolator on order. If that doesn't work I'll just use the cheater plug and be done with it.
 
Thanks for your help!!

 

[pinnahertz]

Apparently, there was a version when those were bad, but I've never seen it. I've seen tests of it and it was pretty bad. All the ones I've bought in the last 15 years or so we're fine. It's possible you got one of the bad ones. It's so cheap, I'd start with a brand-new one.

 

[denti]

Got the ifi iDefender 3.0. Works perfectly! Problem solved.