actually that the problem i can't buy anything from ebay that why i have to make a circuit on my own. btw my laptop power supply can output 65w output i saying 60w to make thing easier
the reason i want to use the buck converter because my audio amplifier can power from 12v 3A to 18v so it drain 50w max but the power supply exceed the rating which cause my amplifier to overheat

 

It sounds like you do not need tight regulation, and that makes the circuit much more simple. One power Darlington transistor (or equivalent) plus a couple of resistors and capacitors. With the output set to 16 V you will dissipate only 12 W. Can you buy the TIP series of power transistors? TIP41, TIP100, TIP120, etc?

ak

 

can u give a schematic.i still learning about electronic so i don't know much about transistor and stuff.

 

It sounds like you do not need tight regulation, and that makes the circuit much more simple.

But won't a buck converter still be needed, given the amp's power requirements?

 

Being as the amp can run on 18V, maybe just add a bridge rectifier wired up as 2 lots of parallel diodes in series with the 19V supply to get about 17.5V. A power bridge is an easy way to get high current diodes in a handy mount.
Connect the +19V to the bridge rectifier -ve terminal, and the amp +ve power in to the bridge +ve terminal.
Don't worry connecting anything to the AC pins.


Some suitable bridges.

 

I was heading toward a voltage divider and a darlington, but two bridges in series would get you four series diodes. That should be enough to keep the output below 18 V even at low load currents. Add a 1000 uF cap to lower the transient output impedance and I think you're done.

ak

 

But won't a buck converter still be needed, given the amp's power requirements?

With the updated requirements in post #21, the desired output current is not greater than the supply output current, so no magnetics are needed.

ak