Basic Electronics / Transformer Question

[theCoder]

I'm working on a small lightweight bartop cab and am trying to squeeze every ounce of weight out of it.  Unless I can figure this out, I'm looking at puting in 3 different voltage transformers, one for the laptop (18vdc), monitor + fan (12vdc), and sound system amp (9vdc).  I'd like to pull this off with fewer transformers than 3.  I suspect the speakers don't draw too much current and could possibly share the 12vdc transformer.  Any recommendations on how to go about this?  Resistor in series?  parallel?  What about the value.  I suspect I need to measure the current of the amp in action?

Any assistance would be appreciated.

[SirPeale]

Okay...if you're using a laptop, why are you using a seperate transformer for the monitor?  Unless you're not using the one from the laptop, of course.

[theCoder]

Correct.  I'm using a slightly bigger and much better monitor.  Unfortunately the transformer for it takes 12vdc and the laptop takes 18vdc.

[MonMotha]

Run the laptop, and only the laptop, off the specified brick.  They tend to be spec'd right at the edge.

Generate a 9V rail from the 12V rail using a LM7809 regulator.  Check the current draw on your speakers.  If it's less than 350mA, you don't need a heatsink.  If it's more, get a TO-220 heatsink at radio shack or whatever and attach it with a little thermal paste.  You'll be good to about 1-1.5A with the heatsink (basically, the device just has to remain below 125C).

Of course, make sure your 12V brick can source enough current for both the monitor and the speakers.  There should be a current rating on everything.

[Level42]

What MonMotha says.

Here's some instructions and schematics how to do it. Everywhere where you see 7805, think there is 7809 instead....

http://www.tkk.fi/Misc/Electronics/circuits/psu_5v.html
http://www.iguanalabs.com/7805kit.htm
http://www.play-hookey.com/digital/experiments/logic_ps.html