Hallo Jani,

the poor answers show, that you posted a good question! And I'm absolutely shure, that you are not the only 89C51 user, from time to time needing an easy power amplifier in combination with 89C51.

I think this very simple, cheap and highly available TDA2030 will do the job! Have a look at:

http://62.236.156.23/ideaport/d/tda2030.pdf

Keep in mind, that minimum supply voltage is 12V.

Do not make relevant changes to recommended circuits of datasheet. Especially do NOT decrease the gain, because you might not need any gain with your set up. The reason is, that this amplifier achieves its highest stability with the gain recommended by manufacturer!
So, if you have a big signal voltage, decrease it by the help of shown potentiometer, or any other voltage divider at input of TDA2030. TDA2030 is very reliably working. Maybe you must connect a good cooling fin to its case...

By the way, if you only use this amplifier to produce 2kHz signal, you might think, that electrolytic capacitor at output can be decreased. But keep in mind, that this capacitor must be able to tolerate your output current of about 1A. And this can only be achieved by a big electrolytic capacitor! Have a look at a typical datasheet of electolytic capacitors.

Good luck,
Kai