A couple of days ago, when hooking up my laptop to my PC’s line input, I was met with an unacceptable noise that I found was coming from a ground loop. Disconnecting the laptop’s own ground indeed solved the issue, but grounding the laptop through the audio cable obviously isn’t ideal.
While looking instead for digital USB to USB solutions, which do not seem to exist on the market, I did come across the ADuM3160, a USB isolator.
What does this do, and why is it important? Very simple. It isolates all the lines of a USB connection, including the power and ground. That immediately solves the ground loop. This image from the manufacturer’s website illustrates it well.
And luckily, there exist a ton of cheap USB isolator modules with this chip. So I got myself one of those to try it out, plus a USB audio adapter for less than €1.
There’s an interesting twist to these off-the-shelf devices. They also add an isolated DC to DC converter into the module, which is what allows them to actually provide power from the host to an attached device, without requiring a separate external power source.
I can confirm that this solution works perfectly. After reconnecting the laptop’s ground, there is absolutely no noise on the audio anymore. Although, the sound quality of the audio adapter isn’t much to write home about…
Perhaps I could isolate the USB audio card of the desktop instead, and then connect the laptop’s internal audio to its line input. That would ground the desktop’s USB audio card through the audio cable to the laptop’s ground instead, which isn’t too bad.
While this works well enough, having to go through analog, especially using the cheap audio adapter, isn’t ideal. Unfortunately, there doesn’t appear to be an off the shelf USB to USB solution, and neither have I found any ICs with a dual USB device controller.
Another approach might be hooking up two FT932s. The FT93x is a USB device controller. But, rather than the USB isolator on one end, using an SPI isolator between them. They’d then have to be programmed to appear as a speaker output on one end, and a line input on the other end. This solution would allow for a plug-and-play digital audio connection between two devices, without requiring any special software configuration.