Thought I’d give an update on this, it’s been a while.
After troubleshooting almost every part of my electric installation, I seem to have found the main culprit: shoddy PCB design. It’s a pity, I really enjoy my other Frequency Central stuff, but I cannot with good faith recommend the Routemaster.
Here’s the underside of the board, on which I have traced out the ground trace. Notice the long and winded trace which connects the +12V regulator section (top left) to the ground connector. Same thing for the IDC headers, no bueno.
So I used some 22AWG wire to create some shorter ground connections. And sure enough, the supply rails went from looking like this:
To looking like this:
The supply ripple decreased from 30mV to 6mV. This raises the next question, what is an acceptable ripple level? 6mV is an improvement, but it’s still three times more than the 2mV my FC microbus gave me. At least it isn’t audible on a DC output anymore…