Mains Hum Problem

Hi,
I have an Interenet Radio that runs off a 7.5V Regulated power supply.
I feed the line output into a FM transmitter, so that I can here the internet radio on any other radio. The FM TX is battery powered.
My idea was to run the FM TX from the same PSU as the radio. Using a 7805 Regulator, I powered the FM TX and so I only needed one PSU for both. The FM TX is silent (as heard on an FM radio), when the line output of the radio is not connected. However, when I connect up the line output, I get a bad hum over the signal, that I can hear on the FM radio. There seems to be some kind of a loop problem. Is there any way I can get over this ?

It sounds like you have a ground loop.

You could:

• Connect an audio isolation transformer between the output of the Internet radio and the input of the FM transmitter or power.
  
• Power the Internet radio and FM transmitter of isolated power supplies. Either use a separate 5V mains adaptor to power the FM transmitter from or an isolated DC-DC converter to power it from the 7.5V supply.
  
• Use a ballanced differential amplifier to buffer the signal between the FM transmitter and the Internet radio.

Many thanks !