Hi, First post on here!
I have a quad ch output using a teensy 4.0 and two audio shields stacked on top of each other. I am experiencing the same issue as described in this tread with a – to my ears not very faint – noise when reading sounds from the sd card inserted into one of the audio shields. The four line out outputs all have this noise when a sound plays.
I noticed a peculiar thing not described in this thread before, and this is the fact that in my setup, the noise is not there at all when replacing the 4.0 with a teensy 4.1 and reading from the sdcard inserted into the reader at the back end of the 4.1 instead of the one on an audio shield.
To me, this does not seem like a ground loop issue, the "sandwich" i made is placed in a aluminium box, with the case and case mounted rca outputs ground connected to a single ground point at one of the audio boards line out.
Another thing I noticed is that the headphone outputs does not have an audible disturbance. Could this be because of the separate ground for the headphone output?
I tried several different power supplies, and like others in the thread, this has no effect. Unfortunately I can not fit a teensy 4.1 in my case, so I have to go about solving it in another way.
If anyone has suggestions I would really appreciate it! As of now the only possible solution would be to use the headphone outputs instead, but would prefer another solution that removed the line out noise...