I've been working on a Raspberry Pi internet radio. The audio is spliced from the audio jack into the radio where it then goes into the radio's amplifier and speaker. Additionally I have the RPi powered off the radio's AC supply through a DC-DC step down transformer into the GPIO pins.
After I connected the transformer and RPi to the radio there's been a high pitched squeal coming from the radio. To troubleshoot the problem I cut the positive wire going to the transformer and the squeal stopped.
Does anyone know a solution to this? Is their a component I could put in to filter out the squeal?