The power adapter that comes with it provides 9.6 volts, but it works just fine with 9. I have used its own and a One Spot with no problems. With mine there is one weird situation though: a few times there is a high-pitch sound coming from it, even when its own adapter is being used. After plugging in and out the power source a few times, that sound is (mysteriously) gone. It’s not instantly powered, but it looks as if the pedal “decides” over a period of one or two seconds whether it’s going to… whine about it or not. I’ve also tried this with a One Spot and others in its daisy chain – the problem disappears by doing the same thing (plugging and unplugging – the adapter, not the jack).