Thread: Power draw of amplifiers

Rocky388's got it...all I'm looking for is numbers and a general idea of what your system is composed of. Anyone?

One question, why do you need this? I have consistently said I don't see the point in this but you have never retorted with a reason. Perhaps if people could understand your need for what seems to me as arbitrary data that holds little value, they may be more willing to take the time out to check. For example people have measured the current draw of their car pc's during standby to see how long they could leave their car without draining the battery fully.

http://www.bcae1.com/wire.htm

Use these calculators;
Nobody is going to go out and start ripping their setups apart just to give you an average power usage.

Originally Posted by Linus

My computer speakers draw 20-22W AC continuously no matter how high or low the volume is - I found this curious considering they're rated at 130W for the sub and 35W for each satellite.

Welcome to the wonderful world of ratings. ;-) Those are ratings are almost certainly the instanteanous peak ratings of the speaker drivers themselves, and likely even somewhat exaggerated there.

For example, my Dual amp claims "340 watts" of power, but the fine print says that is peak power. The specs rate it at 70 watts/ch RMS (140 watts total) which is probably a reasonable figure.

Originally Posted by Linus

Sorry Gnomad, I hadn't seen your post when I wrote this. Thanks for passing on what you know, even though it's hard to get too much out of it without knowing exactly how much your battery drains in how much time. Assuming the Chinese amp is the only power load, that equates to something less than 172W - not surprising given the max rated output is 140W

I've been meaning to put a meter on the amp to see what it really draws, but I've just been too darn busy. And even my time quotes are somewhat off, since I'm typically running a blender from the battery as well. ;-)

-p.

Originally Posted by StrataG

One question, why do you need this?

Okay, I'll be straight with everyone. I'm interested in this because I'm hoping to design a computer audio system for my house that runs off a 12V battery. For a variety of reasons (run-time, environmental views, pride in my own creations), I am very interested in maximizing efficiency. As such, I would like to build a system with as few AC-DC/DC-AC conversions as possible - I'd like to avoid inverters. The computer's power can be supplied by pretty much any of the DC-DC PSUs mentioned here on MP3Car. My only option for amplification seems to be car audio amps.

In a car, power draw doesn't really matter all that much - the car is almost always on when you're playing music and the alternator generally supplies plenty of power. When you want to get maximum performance out of a single battery's worth of power, it matters a heck of a lot more. What I'm trying to figure out is how amp power draw relates to its RMS power rating so I can estimate the power draw of a given amp and use that estimation to size the battery I'll use.

Maybe I should have included all of this in the first post - I just didn't find it necessary for you all to answer my questions. Maybe it was. If so, I apologize for taking up everyone's time.

So I guess I should restate my question: Does multiplying an amp's RMS power rating by its estimated efficiency produce a number reasonably close to its measured power draw?


And by the way...

Originally Posted by pancit175

And the funnier part is that he doenst understand that he is being mocked.

Of course I know I've been mocked - I'm familiar enough with the Hitchhiker's Guide to know what Will Albers was getting at. But what's the point in responding to that kind of post? I don't log in to MP3car to be entertained by making fun of people; I do it to learn things I may be able to put to use.

Originally Posted by Linus

So I guess I should restate my question: Does multiplying an amp's RMS power rating by its estimated efficiency produce a number reasonably close to its measured power draw?

Most certainly it should. Keep in mind though I don't think you will be using the full RMS output at any given time which was my point about getting those figures. Opinions are relative, so one person's idea of loud is different from another's. Also a home environment is very different from a car environment as far as air space obviously. Everyone sets their gains differently, have different pre-out voltages, etc. No one else's figures will do you much good. If you are generating 100 watts on an amp that puts out 100 watts RMS, or that same 100 watts on a amp that puts out 1000 watts RMS, they will draw a very similar amount of power if their efficiency is the same. I'm sure there will be slight variations, but unless the amps are severely over/under rated as far as efficiency, the draw should not be significantly different. Going with a decent quality amp, i.e. no ebay specials, you should get results close to the amps' ratings.

One other keypoint is that many amps are rated at 13.x-14x volts rather than 12V. This is just something to take into consideration when purchasing the amps. Also as your battery voltage drops due to drain, you will constantly need to raise the volume. It won't be sudden drops, but will be audible over the length of a 2 hour movie depending on your battery capacity. Perhaps creating a circuit that controls volume in relation to battery voltage would be a good idea. Mosfet circuits help with this problem, but aren't full proof.

What I imagine is you are creating a UPS system of some sorts. Go with a battery designed for boat use as they generally have more Ah. Since you will not need the battery start a car (drawing a lot of instantaneous power), I would go with a deep cycle battery as they will sustain repeated power drains better.