During my new design I came across a problem that I can't solve. The system is an intercom consisting of 2 terminals that are connected with 2 cables. Each terminal is connected to a 12/24Vdc power supply. Each terminal outputs an audio signal which produced from an onboard microphone circuit. Also each terminal has an input to receive the other terminal's audio signal and then it outputs it to an 8R speaker via an audio amp. The speaker is not onboard.
While I increase the audio amp. gain (sound volume) the system get in a coordination state making both 2 speakers to sound at ~5Khz. Using an oscilloscope I can read the coordination signal in each audio input. Just note that before each audio signal outout there is a cap in series (onboard) to make the audio signal AC (from DC).
Also, during the PCB design I placed ground copper between the tracks.
Can anybody help me to remove or even reduce the coordination from the system?
Also I would like to ask if ground copper is allowed in an audio design ?