Is that because of the inefficiencies of the amplifier? Or was I missing something with the equation?

 

Actually, that assumes a 100% amp efficiency. Assume something a lot closer to 50% (reality for class-AB designs, but it depends on the amplifier topology) and you're up to 134A+.

 

So why is it so much higher than the number we get using calculations I used? Is there a different way of calculating it? I'd really like to learn the proper way to estimate the current draw of an audio system.

Thanks MacGyver

 

From the looks of it, I really screwed the pooch on my initial in-my-crazed-head calculations... <sigh>

The two equations any engineer should memorize...P=I*V and V=I*R. You can actually come up with the answer using several convolutions of these two equations.

Your amp is rated to output 950 Wrms at full tilt. The speaker has a resistance of 4 ohms (I'm neglecting Imaginary value of impedance for simplicity, so we'll call it resistance). Plugging the two equations above together, we get P=I^2 * R, or I=sqrt(P/R).

I=sqrt(950/4) = 15.4A

Class-AB amps are (theoretically) 50% efficient, which means a true current draw from that amp of around 31A. An efficiency closer to 40% is more reasonable for a pull of around 40A.



I REALLY need to start typing out my answers in full...some of the first shots are out and out wacky! I'm not a complete idiot...I'm still missing a few parts

 

Hey don't worry about it!

I haven't even looked at or thought about most of this stuff since high school physics! Although now I wish I paid more attention!

Your calcualtions bring up a point I never considered, the resistance of the speaker being driven. This I assume can make a big difference in the current draw of an amplifier considering many people run DVC 4 ohm subwoofers with a resulting 2 ohm total resistance. In Natedoggs2k example he is running his sub at a 1 ohm load, however it is most likely a D class amp which from what I understand is closer to 90% efficient. So if I understand correctly:

I = sqrt (700/1) = 26.5A @ 90% efficency = 29.4a
+
I = sqrt (240/4) = 7.7A @ 50% efficiency = 15.5a

For a total current draw of 44.9a

Which if we assume about 44a of headroom from the alternator and the fact that his sytstem won't be drawing a steady 44.9a when playing music, he shouldn't have a problem with over-taxing his electrical system. Something of note is that if he was using a A/B class amp at a 2ohm load (which seems to be somewhat common) the current draw would be closer to 53a.

Obviously a D class amp running a subwoofer can help reduce your system's total current draw, but are there any advantages to running an A/B class amp instead of an D class amp for a subwoofer?

Do my calculations look correct? Thanks to everyone for helping me understand this.

 

So based on your calculations guys, you are in the same ballpark as I was. 44A. I just rechecked the stuff I calculated on top, and that was for 50% effieciency.

So basically, you can go 40 or so A safetly *i think*.....

Remember, you arent drawing 40A constantly, only on the heavy long hits.

 

Yeah, this is only for times when you really have the volume turned up to near max and leave it there...but you should always account for worst case scenarios.

In the case of subwoofers, there's no advantage (and some disadvantages) to running a class-AB over a class-D. Class-D amps are extremely efficient and work well at low frequencies, and offer no issues with noise pollution as long as they are designed and shielded correctly...just don't buy a cheap class-D, you'll regret it later.

 

So in my Subaru the way I had it was
75x4 running the fronts and rears (75 is a bit underrated though)
640x1 running my two 8ohm subs @ 4ohm imp (640 is also underrated)

I=sqrt (640/4)=12.65 @ 50% =25.3
I=sqrt (300/4)=8.66 @ 50% = 17.32
OR 21.31 @ 50% = 42.62

Is my math correct (and the reasons why my subaru's electrical system is now in the crapper)?

 

Not *too* likely.

Whats the Amperage of the Subies alternator....I would guess no more than 90A?

 

The math looks right.

I don't know if that would cause your electrical system to be "in the crapper". Depending on the rating of the alternator in the subaru and the current draw from the stock electrical system, I would think you would only see things like the lights dimming or maybe a dead battery, I don't know if it would kill it (unless maybe the load was so much it fried the alternator?).

Of course I'm merely taking an educated guess, Macgyver and others would definitely have a better idea.