[757labs] 20,000 Watt Car Amp CL Ad, follow-up post
nathan at meltphace.org
Sat Feb 18 17:09:46 EST 2012
For Harold: http://www.youtube.com/watch?v=RRDA64OegJY
So if we're talking about exhibitions systems, 1200 amps is pretty insane but I've seen full size SUV's with custom mount brackets for 3 and 4 GM AD244 (220 amp rated) alternators on the engine (and now thanks to youtube I've seen even more). Ethan has talked about systems that use a hydraulic drive system to run a sufficient number of alternators to power the stereo continuously. You'll also see plenty of them that run on banks of batteries in the back (usually specialty ones sold for audio use) for an hour or two and then have to be shut down so the batteries can recharge because the automobile's electrical system is not sufficient to run the audio system at full power. Both the "bank of batteries" and the "array of alternators" approaches seem to be popular still in the stuff I saw at SEMA this past year. Granted these are completely unreasonable for anything but a competition or show car, but they are certainly possible and pretty commonly seen at car shows. One afternoon at SEMA
and you'll have seen probably a few hundred thousand combined watts of subwoofers, if not a million or more.
It's also a mistake to assume 4ohm impedence. That is still the standard with car door speakers, but car subwoofers (which is where you see all the big wattage numbers, who the hell needs >250w to run 6" door speakers) commonly run lower impedances for exactly that reason. The little 10" one I'm using now is dual 2-ohm voice coils, so it can be configured for 4ohm or 1 ohm by wiring them in parallel or series. Most decent subwoofer amps are stable down to 1ohm load, and the high end big-shit are stable down to 1/2 and 1/4 ohm loads (but you may not like the distortion numbers unless you're buying some *really* expensive gear).
It is true that most of the 'max power' numbers are bunk and always have been. There are stories in the 80's of running amplifiers (meant for cars) at double rated voltage to get those numbers, or hooking them to a near-0 impedance load to read a peak number before the amplifier smoked itself. Any reputable manufacturer now adheres to the CEA-2006 standard testing methodology which specifies a 14.4 volt source, 4-ohm load and some other metrics to try to keep the measurements applicable to real life (http://www.crutchfield.com/S-heWiN3E2ntn/popups/cacomp.html). It's not perfect, but it's a far cry from the '1500w MAX POWER' stuff from the old days.
In the end, I agree completely that the aforementioned craigslist bonehead couldn't possibly have a 20kw audio amplifier for a car. At least, not a single amplifier. The overly indignant response just rubbed me the wrong way in that the person didn't seem to know what they were talking about in reference to modern car audio. Their assertion that it's impossible to exceed 250 watts from a 12vdc system (which is actually 14.4vdc in most automobiles) is just plain wrong.
On Saturday, February 18, 2012 at 11:28 AM, Harold Scadden wrote:
> Here is the bottom line ... let us look at some figures to start with.
> If this mystical car could pump 20 KW output lets assume that they have the best
> damn amplifiers in the world and they are pulling about 95% eff. ratings ... then
> lets assume that the amplifiers themselves have to burn up some power in order to
> do all of this magic ... so add at least 10% more on top of the 20 KW so ...
> 95% efficient ... to get 20,000 out you need to pump 21,000 ... then we add the
> 10% loading for all the electronics and that is another 2,100 watts of power being
> burned off this wonderful 12 VDC from hell system.
> Now we are up to 22,100 watts from a 12 volt source. Wow that is wonderful, how
> many amps would that take?
> On the small amount of 1,841+ amps of current from the car. Gee, that is a pretty
> bad ass alternator system you got there and I am sure your battery could supply
> that for about a microsecond.
> Let us not forget the size of the fargin bus bars you are running your power
> on!!! I build 440 VAC 600 Amp cables at work and they are 500 MCM cables. Think
> of a cable about 1" in diameter for 600 amps of current. Hell the hundreds of
> pounds of copper you are adding into the car just to carry this amount of raw crap
> all over the place is going to be ridiculous not to mention, where they hell are
> you going to put it?
> As the commercial says ... "Show me the Carfax .." I want to see this bastard if
> it is "real"
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> If we knew what we were doing it wouldn't be research.
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