resistors
//content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gifdont try it unless u know what you are doing
And a part of this it the ability to disperse heat.it has to do with the size/tolerance/voltage of the output transistors I think. I'm no EE, but that would be my educated guess