Laptop computers are often not grounded via the mains power connection, so attaching multiple USB audio devices to them can easily lead to the formation of a ground loop. Thankfully, it’s usually easily solved!
I’ve introduced a Line 6 HX Stomp into my pedalboard (it’s really good!). For editing with the Helix Edit software (preferable) you must be connected via USB. Trouble is, the USB connection is introducing a noticeable ground hum when connected. I’m outputting to a Fender Twin Reverb. How do I solve this? DI box or isolator on the HX output to the amp? Something else?
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: The first step is to identify the real problem. An obvious implication of your description is the creation of a ground loop via the USB connection — but that requires a second ground path, usually through the mains wiring earths. Hopefully the Fender Twin is grounded via the mains, and if the computer is also grounded to the mains that’s probably completing the loop.
Desktop computers are normally grounded through the mains, but laptops usually aren’t, so some confirmation regarding the computer grounding would be helpful. If that is the source of the loop then your two options are: a USB isolator; or an audio line isolator. I’ve had success in similar situations with both, but you need to pick the isolation devices carefully.
For audio line isolators I’m a big fan of the ART Pro Audio DTI box... It’s still my go‑to problem solver.
USB isolators are available at 1.1 and 2.0 speeds, and some brands seem better than others so I’d advise checking for user feedback before buying. For audio line isolators I’m a big fan of the ART Pro Audio DTI box. I reviewed it back in SOS February 2012 and it’s still my go‑to problem solver. ART also make the CleanBox II (reviewed in SOS August 2005), which is also good, but not as versatile. Other manufacturers offer similar devices, of course, with better or worse sounding transformers, but the ART boxes strike a great balance between price and performance.
Both the USB and audio isolators use transformers to provide the galvanic isolation needed to break a ground path, and thus prevent a ground loop. But remember to keep the audio isolation transformer well away from any mains transformers — otherwise you’ll kill the ground loop hum but create an induced magnetic hum!
Hopefully this will solve the problem, though it’s worth mentioning that there might be ground loops elsewhere via the USB connection. For example something else on the pedalboard might be connected to the computer. So do check for anything else that might be grounded.