I have tried with many (working) 230V - 9V DC wall adapter power supply (of different brands, some of them coming with an electronic Yamaha device, etc.):
they all show a higher voltage when measured in "DC mode" by my multimeter, like 12V - 14V
they even show a higher voltage when measuring in "AC mode" of the multimeter: ~ 30V! This doesn't really make sense: the output of the power supply should be DC, so a theoretic AC voltage should be 0V. Ok, I understand the output of such an adapter is never "perfectly" DC, and there might still be a small sinusoidal component... However, why 30V?
What could be the reason of this higher than normal voltage?
Could it be a ripple after the rectifier?
PS: I don't own a oscilloscope to test the output of the wall adapter, but I will try to borrow one!
PS2: if it was only with 1 such adapter, it could just be a faulty one. But now I've tested about 6 different (working) adapters of different brands...
PS3: it's a cheap multimeter (link) but efficient. All other voltages that I measured have always been correct.