USB Ground Loop

With a 170 motherboard you most likely only have PCIe slots.  
I have a lot of USB devices and do not have any "ground loop" problems.  
 
What have you done to determine its a ground loop problem?  I've seen and worked on a lot of ground loop problems but never one blamed on USB.

USB interfaces can be influenced by improper grounding of devices that are plugged into it but also by the PC power supply itself, even with self powered interfaces. What you hear is not the typical 50 or 60 Hz hum but higher frequency interference that is very annoying in the monitoring chain and also printed on recordings.

Years ago I suffered from these issues. That was before building dedicated treated rooms with power supply totally isolated from the rest of the building. I threw money at power conditioners, DI boxes, ferrit core USB cables but could not solve it. Interestingly once everything was hooked to a separate power line plus a power conditioner for safety the same PC and interface combination went totally quiet.

So it is not an easy thing to fix. I'd try with plugging everything into the same power socket, try different ones as you will have different circuits in your house and it does make a difference if your AC or washer/dryer are in the same circuit. Also try with nothing plugged into the interface and then add slowly to see if a particular synth, FX or likewise makes it worse.

The PCI slots on newer motherboard (e.g. Z170 boards) are not true PCI but bridged to the PCIe bus.  I have two PCIe five port USB 2.0 cards installed in my Z170 system.  Work perfectly.   

Just a thought, you may want to consider PCIe, since that is direct and not bridged.  Just in case.  That's because I'm superstitious when it comes to converters/adapters/bridges, etc. Also, your next PC might not even have PCI slots, so with PCIe, you could redeploy the card.

Not saying it won't work, but if you have issues, you may wonder.  I don't even like USB hubs.

I just add these cheap header cable brackets to my rear panel and connect them to an extra USB header supplied on my mobo.
 
Like this one:  https://www.newegg.com/Product/Product.aspx?Item=9SIAE4P5VW2097&cm_re=usb_bracket-_-9SIAE4P5VW2097-_-Product

Thanks guys, I ended up getting this:
 
https://www.newegg.com/Pr...x?Item=N82E16815158172

husker
As soon as I unplug the TRS cable from the Moog, the noise simply disappears.  I have tried different cables, but it has no effect.   Would balanced vs. unbalanced cables have an effect?  The output of the Moog is unbalanced, but the cable is a balanced cable.  (I don't have an unbalanced cable to test).
 

if that's the case, a simple DI box plugged between the Moog and the interface should fix it

Abacab, I would tend to agree, but the noise happens whether or not the Moog is actually plugged into the wall or not.  I can have the power totally unplugged, and the noise still occurs.
 
This is driving me insane, so I spent a few hours in the middle of the night on this.  I also seem to have the problem on the "left" output of my Nord.   The "right" output does not experience the issue.  Both outs of the FA are OK as well.
 
Basically, if I turn the the input up on the Focusrite on either of those two "inputs" I get that wicked buzz - it matters not if the power is actually plugged in or not.  As soon as I unplug the cable, the noise disappears.

Back when I mainly used external synth modules, I ran them all through a small Mackie audio mixer first, then into my audio interface.  That seemed to prevent he sort of issues you are describing, except for a weird humming we used to get from my buddies Telecaster.  Not sure if we ever totally solved that one, but Rob's suggestion of a DI box might be the answer.

I've never had to invest in a DI, but my understanding is that you would want "active" for anything that needed a pre-amp.
 
Since your Moog should be outputting at instrument level, passive would most likely work.

Since you do have several external pieces of gear, you might consider investing in a good used hardware mixer to connect all hardware modules outboard of the audio interface. 
 
This compact mixer has very flexible routing options.  I have an ancient version of this that served me well, back in the day.  It has a second stereo bus with the Mute/Alt 3-4 buttons in the channel strip.  This is routed to a separate pair of outputs, so you really can have four outs at a time (Main 1-2 & Alt 3-4).  Plus with control room outs, you can run directly to a set of monitors or an amp without needing a computer to play live.
 
For example this Mackie 1202 new is only $270. 
https://www.amazon.com/Mackie-1202VLZ4-12-Channel-Compact-Mixer/dp/B00EDHVQN6/ref=dp_ob_title_ce
 
On eBay, pre-owned seems to run about $100-150.
https://www.ebay.com/p/Mackie-1202-VLZ-Pro-Mixer-1202vlz-1202VLZPRO-12-U076692/1519581409

I had a frustrating similar problem a few years ago. I could make it go away by running a wire metal chassis to metal chassis between my synth and my computer, essentially grounding them together. Another "fix" was when I discovered that several  unused inputs on my Roland  interface had their gain turned up for no reason. Turning down the gain on the unused channels made the hum disappear, even without grounding the synth to the computer. My assumption was that the loop was still there but no longer being amplified. For some reason it was not audible on the channels that have the synth plugged in no matter what the gain on those channels.
 
I am not sure if grounding devices to each other is safe or not. The effect is similar to lifting the ground on one of the devices, though (not an expert by any stretch here) it seems the devices would still be grounded.
 
Brian

i'd say it was more related to plug sockets (or adapters) that have that bigger voltage, than any pc parts.
 
you need to eliminate various aspects. e.g. if it's a laptop try without PSU.. if you use a gang socket , cheapo ones here cause issues (first place I looked when finding them for others) , to narrow it down to 1 thing.
 
depends on where you are, as US is obviously different from the UK.
have a read
 
https://support.native-instruments.com/hc/en-us/articles/210293145-How-to-Resolve-Ground-Loop-Issues-Humming-and-Buzzing-Sounds-