- often by a single, common ground plane.
Actually, I recently bought a (wait for it...) a "3-Way Input Switch Box" t hat was on-sale very cheap from MCM (now Newark), and I see they don't carr y it anymore.
Here's one just like it on eBay:
It hums like crazy. Sources: PC Audio, Alexa Echo Dot, and iPod Nano.
It has a really nice metal case and reasonably good switch and RCA plugs (f or the money), but the internal wiring is absolutely horrid. A few minutes with the oscilloscope showed the problem was indeed all the internal groun ds tied together (very crudely, I might add).
Rather than try to fix it (picture a PCB with a board edge trace acting as the audio common, with the center RCA pins manipulated over that trace - UG LY and difficult to re-do), I decided I was just going to repurpose / salva ge the box.
So I went to Digikey and ordered a nice 1:4 dual-channel audio multiplexer and wired it up. Properly, with grounds that are actually switched, adding a headphone jack in the process. No more noise.