I'm building a pedals board for an electric guitar. In order to have better integration, I replaced the wall wart power supply that was doing the job by this (BTW, the wall wart power supply is also a Chinese one) one packaged like this:
The issue is that the power supply generates a kind of whistling in the speaker. What surprised me is that I plugged my oscilloscope on both power supply (with and without pedal) and I can't see any difference. The voltages seems to be pretty stable.
So my question is: what kind of measure / modification can I do to improve the system?
Edit: I tried a bead like this and I did not observe any change: