Quoted from G-P-E:^^ what wayout said. You were getting the noise from rectified AC induced into an audio trace.
Plus if the board designer ran one or more of his audio traces too close to noisy power traces without guard tracks - you will get noise induced from trace to trace. I suspect this is why removing the FWB rectifier out of the active circuit and using a separate DC source cleaned things up.
The first prototype version of both the rectangle and square wms sound board I did had a nasty hum. Just changing the routing cleaned it up almost totally unless you get your ear right next to the speaker. Mostly with the ground fill around the amp seamed to help the most, but added extra caps and isolated the audio signals best possible.
The audio signal on this sound board takes quite a trip. From dac all the way over to speech board to get mixed in with speech then it comes all the way back to the amp.
-12v filter cap could probably be bumped up in value too.
Vista, CA
