[David]

The problem is hum in the sets do not understand but it happens. Using a polarized plugs can make the hum a bigger problem.  Old houses with poor or no ground system and no ground wire big trouble. One old wiring standard from 1930 to 1970 a house was wired for 110 volts only the fuse box had 2 sets of fuses one for each leg. If you took a voltage reading from each leg to ground 55 volts. Have a army signal corp generator from 1952 that does 60 volts to ground on each leg the army finally got rid of them in 1977. Have worked on a few habit houses the electrical system were different some had no ground rod.