This is a general post intended to provide some helpful information to builders who may be experiencing audio noise in their audio panel/intercomm systems through inputs from accessories such as music systems or handheld GPS units.
Audio frequency noise can be injected onto the electrical bus from many different types of airborne systems. Strobe lights are only one such source, but many other systems can also generate audio frequency noise. DO-160 requirements are written such that it is the responsibility of each piece of equipment to filter out and reject audio frequency noise on the power supply input, but there are many devices that do a poor job of this.
Two types of equipment that I am aware of that can pass power supply audio noise through to their audio outputs as common mode noise are handheld GPS units and music systems. Some panel mount GPS units also seem to suffer from this problem.
Audio noise can be identified by the fact that it does not come into the audio panel via the radio antenna (does not break squelch and is still present when the antenna is removed or radio is off). It can typically be isolated to one or two audio channel inputs that when disconnected stop the noise from being heard. This problem does not just plague builders, many certified airplane manufacturers have run into this problem and they had to fix it by doing this:
What is shown are two 600 ohm audio transformers that are used to provide ground isolation between the audio output and the audio input. Only the differential audio signals are passed, common mode noise and "ground bounce" noise are rejected. The transformer part number listed is available from Digikey.com and they are not very expensive.
So, if you suspect you have an audio ground loop noise problem, try disconnecting audio inputs until you find the source of the noise. Using audio isolation transformers may solve the problem.
I just helped an LSA manufacturer with this issue today (music system was the culprit), and thought I should share it here as well.
Eliminating audio noise from ground loops
Linear Mode
