Rob French (KC4UPR)
Trying to visualize this in my noggin... so you're saying, a possible source of the hum is based on the following:
- Teensy IS at ground potential of the laptop, which IS NOT (probably) at earth ground (I will note, I was using the laptop plugged in).
- Teensy and laptop are isolated from uBITX.
- So somewhere along the line, hum is getting into the Teensy/laptop combination, which is then transmitted as audio through the transformer into audio output circuit (I'm using the uBITX LM386 for output).
- Then, when I plug in the Raduino to USB, it brings the laptop, Teensy and uBITX all to the same ground potential, and my hum goes away.
At some point, I'm going to have an internal (to the uBITX) 5v supply for the Teensy (I know there's a trace on the Teensy I need to cut when I do that), which will be tied to the incoming power DC ground for the uBITX. Should I expect that to help?
I suppose another check I could do is to temporarily tie the Teensy and and uBITX grounds together on my breadboard, and see if that solves the problem... though, that makes my whole isolation transformer scheme seem a little useless...