http://www.datasheetcatalog.org/datasheet/stmicroelectronics/7646.pdf

I found this chip in my drawer, and decided to use it for a little mono amp...

But, I only had 8ohm 3W speaker nearby which I don't want to fry.

So, my question is, what is the maximum voltage I have to hook this chip on so the output power doesn't exceed 3W?

 

I vote for 9V.

 

Can it be calculated?

 

Sure we can. In datasheet we can find that for Vcc = 11V and RL = 8Ω Po = 7W.
So the voltage across RL is equal to Vo = √P*R ≈ 7.5V
So amp has Vast = 11V - 7.5V 3.5V saturation voltage.
You want Po = 3W_max
So
Vo = √3W*8Ω = 4.9V

So

Vcc_max = 4.9V + 3.5V = 8.4V

 

Great, so it'll be even portable with 9V battery, hooray.

Thanks mate

 

Great, so it'll be even portable with 9V battery, hooray.

Thanks mate

If you are talking about the classic 9volt battery then that will only supply 100mA at most before it's voltage sags to less than 7 volts so you will have to find something which can provide more power . perhaps an 8.4 volt Li-ion or Li-po