Rob French (KC4UPR)
Evan, thanks for the input.
Regarding the feedback into the headphones... that totally makes sense. I will try the dummy load test. My breadboard is a bit of a spider's nest, and even has a couple of wires floating in space at the moment (the mic input to the Teensy, which I don't actually have hooked up yet, since I'm using USB audio for my transmit audio). I also noticed that moving my hand in various places around the system seems to impact the sound of the feedback audio--muffle it, change the tone a little, change the amount of hum, etc.
I'm using a laptop computer.
I don't understand how I'd be missing a connection to ground, however. USB carries a connection to ground, so the Teensy (and the Teensy side of the audio transformers) should be referenced to the same ground as the computer, via the USB cable, right?
I wonder if I should try switching USB ports?