I am having an issue. I have an old, old Sony receiver amp. It does not have a dedicated subwoofer port, so I currently have an 8" sub hooked up to its "center". I do a lot of movie watching, and I have to go to "phantom" setting (everything but center) because the sub doesn't do well with voices. Now, I dont want a center speaker. I listen to my music, and I listen to it LOUD and proud, and with the sub switched on it takes some of the pressure off the other 6 channels. I found a way to run a powered Logitech 2.1 system to my "center" channel wires and get a sub AND mid-range for center. The only problem, the Logitech has a ground wire and without that touching L or R the 2.1 system gives off a lot of hum and feed back. Now, if you touch the ground to either channel that mid-ranged speaker will work and the sub will, but the other speaker won't work. The amp doesn't have a ground wire, only left and right (of course). Could I go to radio shack and buy a female 3.5mm jack and wire it to the center left and right then wire the ground to the casing of the jack and expect both L and R speakers to work on the 2.1 powered logitech system?
I would suggest connecting the sub in parallel to your left and right front speakers or the receivers' connection for them (whichever is easier). This will give you bass from the sub all the time.
You could try grounding the Logitech to a screw on the case of the receiver or if it has a phono input there will be a ground terminal near that you can use. Since the center speaker is only one channel and the logitech is 2 channel you need to connect that channel to both left and right inputs. Just run two pairs of wires from the center channel +/- to the Logitech.