Universal power adapter problems with my Nintendo NES?

I bought a universal adapter and set it to 9v as it shows on the old Nintendo system and everything turns on. Screen is clear but the sound has buzzing. I'm guessing because of the power? I've tried this adapter on three nintendos and the same thing happens. It has to be the universal adapter because I know... show more I bought a universal adapter and set it to 9v as it shows on the old Nintendo system and everything turns on. Screen is clear but the sound has buzzing. I'm guessing because of the power? I've tried this adapter on three nintendos and the same thing happens. It has to be the universal adapter because I know my nintendo very well and that doesn't happen with the original power adapter. The universal adapter has different settings.. tried some of them.. same thing happens for 7.5v. There is an option to put it on 12v but I haven't tried it yet. Is it safe to try? Does anyone know why the buzzing sound comes on when a game is being played?
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