I use a universal power adapter for various things - the one where you can set the voltage 1.5, 3, 4.5, 6v etc. and set the polarity. In the past the devices would state the polarity required i.e. centre pin negative or positive. However I have bought a couple of items recently with a dc input which states the required voltage but not the polarity. Is this because there is a standard or is it more likely to be one than the other?