I'm thinking about designing a circuit with a built in amp. Not sure what class of amp I'd be using yet because I don't really know the difference between them, but based on some brief research I suspect Class D will end up being the easiest to build and the audio quality will be sufficient. This amp will need to be powered by 8 D cell batteries at most, and drive at least 2 large speakers, and maybe a small one. To this end I have been researching what ohm ratings and wattage really mean, and I think I have the basics down, but I've still got some questions.
So here's what I think I know, and what I'm still confused about:
1) The lower the ohm rating, the more current the speaker will draw. A 4ohm speaker will draw twice as much current as an 8 ohm speaker. 2) Most speakers list their peak wattage. The RMS wattage is 1/2 peak. 3) I don't see a voltage rating for any of the speakers I've looked at, but I suspect if I use ohm's law I can calculate that based on the peak or RMS wattage and the speaker's ohm rating. 4) I suspect a speaker like this one with "dual 4 ohm voice coils" will draw as much current as a 2 ohm speaker: http://www.walmart.com/ip/Boss-Audio-CX154DVC-Chaos-Exxtreme-15-DVC-Subwoofer/16829704 5) Assuming said speaker behaves as a 2 ohm speaker, given it's listed rating of 1800 watts at peak, this speaker should be rated for a peak of 60v. 6) That being the case, can I assume a car amp must be able to step 12v up to 60v?
Finally, I'm wondering if anyone has any recommendations for an amplifier chip to use for this application. To answer that, you probably want to know how loud I want to make this. Unfortunately, I can't give you an answer in decibels. But I can say that this will be used at noisy conventions, and that those eight D cell batteries should probably last around an hour.
A D cell, accoridng to wikipedia, has around 12000mAh, so divide that by 1A and that gives 12 hours, but I don't know how accurate that is since the voltage will drop as it discharges. Looking for some graphs with discharge curves now.
I estimate if I were to drive two 8 ohm speakers at 12v then I would need to put 3A through them, but that may be the wrong way of going about my calculations if the amp actually steps up the voltage. I suspect I am incorrect because 4 ohms at 12v is only 36 watts which is well below what even the cheapest Walmart speakers seem to be rated for.
Can I assume that that 3 way speaker behaves as three 4 ohm speakers, resulting in a mere 1.3 ohms of impedance, and that the 700W rating is the peak for all three combined? Cause I get 30 volts needed to max those out with 23 amps. Obviously I don't plan to generate that kind of power from eight D cells. Though if it does turn out the batteries are kinda weak it might be possible to run the amp off something like a NiMH pack. I want to keep the option open to run off alkaline though.