The 9v bit of the adaptor usually has a power connector that plugs into the pedal. This has a central terminal and an outer terminal. Polarity is which of these is positive. Most pedals and adaptors are centre positive but some are outer positive.
Take a look at the pedal and there should be a symbol on it indicating which is the positive terminal. The adaptor you buy should have the same symbol on it.
You're talking about one of those power supply boxes, a wall wart as they say or something.
Power supplies typically involve a jack or something that basically has 2 contact points. One is negative and one is positive. But which is which. That can vary from one wall wart to another. And the equipment being plugged into may be expecting a specific contact to be negative or otherwise, or vice versa.
Wall warts are typically stamped with little pluses and minuses to indicate which contact is which, or anyway they should be. They're not always marked.
Around here they sell "universal power suppies" with little collections of plug in jacks to use with them, and typically the jacks can be plugged in to suit either polarity.
Buy can buy adapters which have a switch on them to change the polarity. They also usually have a switch to change the voltage, and four different size adapters on it. You should be able to buy one in any decent electrical shop
Im going to buy an adaptor for a guitar pedal, which needs to give out 9V and 500mA, and i was told "the polarity must be correct" in order for it to suit my adaptor. what does this mean? how can i know if its the right polarity? thanks
