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Apart from the regulation there's also the issue of ground loops if the wall-wart has a mains earth and this is shared with the DC -ve. I have a few like this and two of them are very nice medical units with no measurable ripple, but the buzz like hell when powering a stomp box due to the earthing. Randomly selected switched supplies can suffer from mild shocking effects when powering guitar gear due to the anti-EMI capacitor-coupling between primary and secondary (not to be confused with capacitive coupling of the windings). My preferred DIY approach is to use an unearthed, unregulated Class II wall-wart (12v or 15v) and feed this into a small aluminium enclosure that's just big enough to contain a 7809 regulator and associated caps/diodes.
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