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Switching power adapters & noise?

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I just bought a GEM RP-X piano module, which has amazing acoustic piano sounds, but I noticed, when I'm not playing, there is a background buzz that phases on a 1-2Hz cycle.

I tried all the usual experiments to determine if something in my environment was causing the problem: different interconnects, house circuits, amps, etc...: no difference.

I then noticed that I heard the noise even if the unit itself was powered off--until I unplugged its wall-wart type 12VDC power supply. Only then did the noise stop completely. I hooked my 'scope up to the end of the wall-wart and could see a sawtooth waveform in the audio range.

The adapter is rated 2.2 Amps and is the switching type. I've never had big problems with wall-warts before, but I think most of the lower-current ones are linear, not switching.

Do you think I got a bad adapter, or are they all this way? Will I have to build my own supply?

David Anderson



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