I have a camper trailer that has a built in 12 volt power converter/ battery backup system. I wish to install a car stereo in my trailer and run it from the 12 v DC supply, however the power supply is very old and its line filter has failed. There is AC hum in the line, which doesn't bother the DC lights. But I'd assume would damage a stereo. Without tearing the power box out and replacing the capacitor inside, how can I clean up that hum so it will run a radio without damaging it? This is a 12 volt supply @ 15 amps. Can I insert a capacitor across the line I will be using for the stereo? What value cap would I need?

 

Putting a cap across the leads out of the power supply is the easiest solution. Without knowing how much ripple you're talking about I'd start with 1000uF.

 

Have you removed the battery from the trailer? A big wet cell battery is a pretty good filter, in itself.