Total posts : 45366
I run into that all of the time around here. Amps don’t like the same ground on the input and many times, as mentioned above, the input (positive) is passed through electrolytics to the circuit that can pass D.C. (Which is usually the issue)
I install 600-600 ohm transformers on the input(s) and all is cured. It isolates both ground and any chance of things sneakin’ back through the hot lead of the input. Most all of these transformers will pass good audio fidelity. Rad Shack used to sell some nice, small ones but not no more.
I do alot of stereo audio, operating from a central battery. I must install these to keep from fryin’ stuff! Some amps also have a floating ground and ground isn’t always ground..
I fried a really nice TV and it took a week for it to happen. I took the earphone output to a small amp to push some external speakers I had mounted to the ceiling. Smelled smoke and the TV was smoking big time, an so was my amp! I noticed that the audio was somewhat muffled when I first connected it but I didn’t think it was a big deal. It sounded better than the speakers in the TV. Had a few other issues after that before I figured it out..
The transformer also adds a load to the curcuit on both ends which could help with any noise around the cabling. Causes two loaded, isolated circuits..
I did a quick search. I’m sure you can find something leaded..