I'm hoping that somebody knowledgeable may help to shed some insight into a situation I've recently encountered.

I have a Teensy development setup in an environment with known (intentional) ground loops that are used for testing.

A laptop PC + audio interface + USB hub are fed from one AC circuit, and a mixer is fed from a different AC circuit entirely. By inserting audio gear between the PC and the mixer, I can reliably elicit ground loop noise in audio gear that is susceptible. Switching in an EBTech isolation transformer pair before the mixer reliably abolishes ground loop noise and permits A-B comparison.

Now I have a project that was originally prototyped using Teensy 3.2 connected to an Adafruit AudioFx board. In that case, ground loop noise does not appear so long as the analog and digital grounds are kept separate.

When I substitute the Teensy audio board, ground loop noise is back at unusually high levels. This is not AC hum, it's several different patterns of "digital" noise that appear on the headphone output only when a ground loop is present. Switching in the isolation transformers instantly cures the problem, but of course the users of the intended device will not have isolation transformers. Many of them will not have ground loop problems, either, but for those who do, the device as it stands would be unusable.

There is no particular dependency on software: the noise is present on the headphone jack at high levels in every example from the Audio Workshop.

In my first pass I have simply stacked all pins between the teensy and the teensy audio board. Now I wonder whether anybody has found a way of connecting the two that does not result in such high propensity to ground loop noise. I can un-stack the boards and begin playing with connections, but if anybody has specific suggestions or insight I would greatly appreciate the guidance.


-- Craig