Hi. Im kind of new to electronics, but im building a stereo amp, and i want to add a led to indicate main power. how do i do that simplest?

The setup is: one 12v lead battery, one poweramp (12v) and one tone adjustment circuit (12v)

thanks in advance

Well the first thing I do is a google search for “led calculator” which brings up: http://led.linear1.org/1led.wiz

Fill in the power supply voltage (12V in your case).

Use a typical LED forward voltage of about 2V (unless you know the exact value for your LED)

Fill in a typical LED current of about 15mA (unless …)

The web program calculates the resistor value you need and even shows you the color code to help you find it.

Note that it specifies a 680 Ohm 1/2 watt resistor in this case. If you only have 1/4 watt resistors, just tie two 1.2 kOhm 1/4 watt resistors in parallel to get 1/2 watt 600 Ohm (close enough).

Now just build and test the circuit. If the LED doesn’t light, you most likely have it backwards. Otherwise you either have too high a resistor value or too low. (the LED will burn out very quickly if the resistance is not big enough!)
have fun!