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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:

enter image description here

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:

enter image description here

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  • \$\begingroup\$ Good question! It comes from the speaker. I will edit the post. \$\endgroup\$ – Julien Dec 9 '19 at 11:15
  • \$\begingroup\$ Guitar pedals are notoriously sensitive to noise on the power rail. Maybe try a linear power supply. \$\endgroup\$ – Dampmaskin Dec 9 '19 at 11:25
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Guitars, guitar effects and amps catch all available noise due non-balanced signals, high input impedances of the devices and ground loops. In addition the quitar itself has a pickup coil.

You have at least your amp and power supply for effects connected to mains AC. "they are isolated!" you can say. They aren't, there's very likely a strong capacitive connection inside the power supplies.

You have a good HF noise transmitter in your power supply box. Test it with an AM radio. I bet you haven't any metal shield inside the plastics. Check also, is there any real input filter which protects your mains AC. I have seen (=purchased) a good looking power supply where the input filter was decorative only. The ferrite core was plastic. The transformer fortunately was functional. There was no metal shield around the circuit board. All certifications were printed well. One could easily find the right stamps for every application.

Run your effect with a battery and find what actual connections are needed for the power supply to start the noise. You may be able to filter them. But if you are in a hurry, get a linear (=non switch mode) regulated power supply as already suggested.

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