guy help: Anyone knows audio electronics? - Help.com

120 Watts is just the amount of power that the speakers CAN handle. It actually makes good sense to have the capability of the speakers exceed the maximum power output of the amp so you don’t risk damage to the speakers…no worries about using the old speakers.

Speaker impedance is another issue entirely, but if you are designing the amp to run 8 ohm speakers it won’t be a problem.

so you mean size doesn’t matter if its 8 ohms speaker I’m using?

Absolutely! It’s entirely possible to use five watt speakers with your amp…but if you crank the volume too high, then you will be sending too much power to the speaker and it will blow. That’s why it’s good to have speakers that can handle more power than your amp will deliver, which is why the situation you outlined of using 120 watt speakers with a 100 watt amp is so ideal.

thanks for that advice!!!

Speakers come in several ranges of impedance from 1.6 ohms to 32 ohms. The main reason for these different values is to match the output impedance of your power amp. Mismatching your amp output can cause anywhere from distorted output to overloading your output transistors enough to blow them. In your case the output is rated as 100 watts across 8 ohms. This was a very important factor with the old vacuum tube amps but, generally transistor power amps are not as sensitive to output impedance and can usually range from 4 to 16 ohms. A few things you should know, if you bridge two speakers of the same value of impedance the resultant will be to divide the total impedance to half. Example 2 16 ohm speakers in parallel will equal 8 ohms. Two 8 ohm speakers hooked in a chain (in series) will appear as a 16 ohm load. So extending that fact four 8 ohm speakers two in parallel and two in series will equate to 8 ohms.

The power of a speaker is controlled by the size of the coil and the magnet that it is suspended over. As a voltage is applied to the coil the speaker cone is forced either outward or inward according to the polarity of the voltage. The larger the wattage the heavier the coil wire and the strength of the magnet. The speaker wattage is calculated just before the coil reaches the end of the cone travel (bottoming).
So if you don’t have a 100 watt speaker you can always use two 50 watt speakers. Since your speakers exceed the maximum wattage of your amp they should work fine. There is only one caveat here. Make sure that the rating of both the power amp and speakers is the same. Peak power is not the same as true or RMS power. A 100 watt RMS power amp can produce almost 140 peak watts.

Hope this helps