Thanks Audioguru, your name makes me feel sure what you're talking about.
It seems like math has an important role in this project.
Actually i want to keep that speaker (due to its cost) and because it is a great quality loudspeaker.
Having said that i suppose i cannot get 100W from that speaker with that darlington.
In case of 0V loss i would have 13,8V as peak voltage on the speaker, correct?
That would mean 11,9W. Not that difference...
Now i suppose the siren i was talking about doesn't use all the 100W of my speaker.
No way to gain some extra watts from my project?
I noticed that using MJ11015 instead of BDW84C increases the loudness but i've been told that so many amperes would blow the speaker filament.
I'm kinda confused.
Added after 24 minutes:
Thinking about what you said i checked the siren pcb again.
Checking the solder points i marked where it may be possible that a round transformer could have been used to rise the voltage around 45v to reach 100W and that would bring it to your theory.
Ok so let's say that i want to do something like that... beginning to the circuit i made... how can i convert 12V to 45V (easily) and amplify the signal with that voltage to the speaker?
Thanks a lot, that solves part of my mistery on this circuit.