. Summarizing: I'm experiencing a loud buzzing noise in my stereo, accompanied by interference from local FM radio stations. I've done more investigation and have eliminated my power as a (direct) cause of the noise.
The original post has a more detailed description/history of my problem. After some investigation with an oscilloscope, I ended up with this image
of my AC power superimposed over the noisy preamp output (close-ups here
). My working theory at the time was that the hiccups at the top and bottom of the sine wave in the AC output were causing the corresponding jumps in preamp output (which would conveniently explain 120Hz buzzing). In order to test this theory, I got my hands on an AC power regenerator
and tried powering my preamp with that. The noise didn't go away (and didn't really change). In the course of scratching my head at this development, I also discovered that the noise doesn't go away when I physically unplug the preamp and allow it to run for a few seconds from only the power left in the power supply reservoir capacitors. Of course, the noise goes away when the energy in the capacitors is used up.
So now I'm left with this: 60/120Hz buzzing and FM radio interference when everything is physically isolated from my AC power (ie: preamp has been totally unplugged, driving the inputs of a laptop running from batteries). This leads me to believe that the cause of the noise is RFI/EMI, and that the hiccups in my power are not the cause of my issues, but are instead also caused by whatever is causing the noise in my preamp, which would explain why they are temporally coincident. I'm still inclined to believe that this is somehow power-related, as the hiccups and my AC power are in perfect synchronization, but nothing as simple as the line-noise-sails-through-power-supply that I had initially hoped for.
I'm an engineer, but not an electrical one; I've managed to debug this problem to this point, but I'm pretty much out of my element here. I don't understand enough of the principles at work to figure out this issue past this point or what I can do to try and fix it. Suggestions, please!