Why does radio buzz when plugged into the wall but not when running on battery?

Answer 1

Check to see if there is a switch to go from ac (wall socket) to dc (battery).

AnswerIf the radio is connected to anything else you could have a ground loop. A ground loop is when a ground connection between two components (Usually the shield in a RCA cable) forms a second path for current to flow between the appliances, along with the ground line to the breaker panel. If there is even a minute voltage difference between the two components, it will cause a fair amount of current to flow through your ground wire. This current induces a signal into the signal wire and causes a hum.

If not, it is probably just a cheap radio and you are hearing RF interference from its power supply. If it is a poorly designed switch mode supply, it can create significant interference across many bands. (Most switchers cause abrubt current changes in a wire: a square wave off a fet. While this is mitigated by the switching inductor, it can still cause traces to act as antennas sending out interference. Remember, a square wave is a sum of many, many sine waves. The sharper the square wave, the more sine waves, the more frequencies to resonate a trace. Designers want a sharp square wave to keep the fet out of its ohmic range to keep it from dissipating energy in heat. And there is your lesson on switchers for the day.)