tomb
Member of the Trade: Beezar.com
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I don't know - I think this still proves that the buck converter is causing the issue. Just because you could hear no hum through the Pi that you had connected to the buck converter, that doesn't mean it wasn't causing noise - because it wasn't amplified. Once you connect the buck converter so that it's going through the MAX, you hear it. "Hum" may be a relative term here, too. Is it really 60Hz hum or is it something higher like 120Hz ripple or oscillation on some other frequency? I've had an amp design that was causing a spike at 180Hz. It sounded for all the world like ripple, but it was actually fast Schottkys oscillating as the rectifier. Some snubber capacitors around the Schottkys removed the noise.
There could be a lot of different types of noise coming from the buck converter.
I am not sure your measurements mean anything. Ground at the wall outlet (your "earth") is not connected to your project, because of the two wire AC output from the walwart. Output ground on the amp, on the other hand, is referenced to the DC-negative ground plane on the MAX board, which is also not connected to your "earth."
There could be a lot of different types of noise coming from the buck converter.
I am not sure your measurements mean anything. Ground at the wall outlet (your "earth") is not connected to your project, because of the two wire AC output from the walwart. Output ground on the amp, on the other hand, is referenced to the DC-negative ground plane on the MAX board, which is also not connected to your "earth."