Whats the thing connected to the heatsink on a amplifier or a hifi stereo? ?

[JoseFirstNameG]

I seen those things and the color is black and very small. Why does It turns hot? Whats is that thing called thanks!!

 

[zqert988]

Power output transistor.

http://www.germes-online.com/direct/dbimage/50284137/Power_Transistor.jpg

Or

http://www.goldmine-elec-products.com/images/G13076B.jpg

 

[DavidW]

It's probably the final amplifier, and it's on the heat sink because it gets hot. It gets hot because a lot of electrical power passes through it.