Parallel circuit POWER division ?

[TanguyClt]

Hello,
I want to build a speaker system composed by one sub(50W), one midrange(20W) and one tweeter(25W).
I will make my own crossover so I will take a single way amplifier.
What power do the amplifier have to issue ? Should it be the sum of each speaker's power or something else ?
And I thing the amp impedance should respect 1/R=1/R1+1/R2+1/R3 so if all my devices have 8Ohms (whitch is the case) the amp have to be settled on 1/(1/8+1/8+1/8)=8/3=2.6 Ohm so a standard of 3 Ohm. Am I right ?
If it's true what must I do to increase the impedance ? (because the minimum I found is 4 Ohm and a 3Ohm system will damage the amp).
Thanks for reading

PS: excuse me if my english is bad, I am french

 

[rayma]

the amp impedance should respect 1/R=1/R1+1/R2+1/R3 so if all my devices have 8Ohms
(whitch is the case) the amp have to be settled on 1/(1/8+1/8+1/8)=8/3=2.6 Ohm so a standard of 3 Ohm.

For a parallel crossover design like yours, each crossover section will present to the amplifier
a high impedance when out of band for its driver. In parallel, the multiple crossover sections
will sum to a roughly constant impedance that is about equal to that of the individual drivers
in their own frequency bands, especially if you use Zobels. If each driver is 8 ohms, the system
will be 8 ohms. Your calculation would only be correct for identical drivers in parallel, without crossovers.

Power needed is largely determined by the woofer's requirements.

 

[TanguyClt]

Thanks a lot for your very fast and clear answer. It really help me. I will so use a 50W and 8ohms mono amplifier for these components:
Subwoofer: Monacor SPH-210, 8 ohm 50W (100W max)
Woofer : Faital PRO 3FE22, 8 ohm 20W (40W max)
Tweeter: Audax TW010I1, 8 ohm 25W (50W max)
Did I understand you well ?

If there is one, what margin of error is allowed about the 50W amp ?(because you said that it is "largely" dertermined by the sub power)It will permit me to choose the sub among a larger choice.
Thanks again

 

[rayma]

what margin of error is allowed about the 50W amp ?

It's hard to have too much amplifier power if you use it carefully, but the minimum needed
would be enough to drive the woofer well. Midrange and tweeter drivers will not demand
so much of the amplifier in a home system. That said, double the power is just +3dB in loudness,
only a small increase. For twice as loud, roughly +10dB is needed, which is about ten times the power.
You can increase the loudness for free (and improve the room acoustics) by sitting closer to the speakers,
instead of using a very high power amplifier.