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» InCar Expert » System Planner » Subwoofer Amplifier

Subwoofer Amplifier

Choose the power amplifier to drive your subwoofer(s), if applicable.

WHAT'S WATT
How loud your system will sound is mostly down to the power output of your amplifiers. Although you may see speakers with "200W Max" on the box, it doesn't mean they will go that loud. Infact the ratings used on loudspeakers is another important thing to watch out for, and you can find more on that under the Speakers category. But it's the power amplifier that will really make your sound system rock or not!

Firstly, when comparing the claimed power output of one amplifier against another, always go by the RMS figures. Don't take any notice of MAX and Peak ratings as they don't conform to any particular measurement standard and are made up by the men in suits.

If it's included in the blurb, always take a note of the THD (Total Harmonic Distortion) figure quoted alongside the power output figure. It's a percentage and shows how much distortion will be produced at the claimed output. For example, two amplifiers might both say they give 100W RMS, but one might say 1% THD and the other might say 0.005% THD. In practice both will sound okay at 100W, because around 1% distortion isn't going to sound bad, but the first amplifier is likely to reach 10% THD much sooner than the second one. So actually the second one will give more good sounding power.

Things get more complicated because an amplifier that says it will give 100W RMS might only give that across a certain band of frequencies, say around 1,000Hz (1KHz). Most of the well known name amplifiers are going to perform pretty well across the entire audio band (20Hz-20,000Hz), but cheaper ones may perform badly in the bass frequencies. This is because they are driving into a speaker that's changing its electrical characteristics as the music plays, depending on the frequency and volume level. The speaker becomes harder to drive, generally at low frequencies, and the amplifier has to work harder to keep delivering the same volume level. It's a bit like driving your car up hill. Some amplifiers don't have enough in reserve (technically speaking they are not able to deliver enough current). The result is that the bass will sound lighter than it would do with a superior amp.

How can you tell which amp is beefier? Well, often you can't easily tell, unless you can actually compare them for real. Of course the marketing guys will put things like "high current" on the box, but you can't rely on that. This is where the advice of an experienced sales person, or a good magazine or web site review, becomes so useful.

Efficiency is another important factor. This means how much of the power going in (from the car's battery) gets converted into audible power. Car audio amplifiers are typically what's known as Class AB amplifiers. Class AB is an electronics term for the type of design these amplifiers use, and it gives a good balance between efficiency, sound quality and reliability. However, with Class AB, about half of the input power gets wasted - instead of being turned into audible power it floats away as heat.

There are other Classes of amplifiers that claim to give some improvement in efficiency, but the one that really delivers savings is Class D. Sometimes referred to as 'digital' amplifiers because they use a technique that's a little like the method for turning analogue audio signals into digital, Class D amplifiers can provide efficiency of around 80%.

Why does this matter? Well, your car's battery provides a nominal 12 volts output. Actually, when the engine is running it's usually more like 13.5-14.5 Volts. The power amplifier takes this low input voltage and increases it, thanks to some clever circuitry and the fact that a car battery is pretty good at delivering high amounts of current, and current can be used to give more volts. But there's a limit to this, and that results in a limitation on how much power your amplifier can produce.

Also, if you are using a really powerful Class AB amplifier, you'll need big thick power cable to transfer all that current, and if you run the audio system without the engine running, you'll soon flatten the battery.

Because Class D amplifiers are more efficient, they need less current to give the same output power. That means the power cables can often be less chunky, and there's less strain on the car's battery and alternator.

Great! So it has to be Class D then... umm, well, not necessarily. Class D amplifiers are good for bass, which is where Class AB amps often need and draw huge amounts of current. But because of the technique used in Class D, they usually (though not always) give a nasty sounding treble (high frequencies). So generally you'd use a Class D model for the sub-bass speakers (infact many have a built-in filter so they'll only handle the bass) and a Class AB for the midrange and high frequency speakers, then you get the best of both worlds.

Finally some quick do's and don'ts:
Don't install amplifiers upside down - the big metal heatsink should always be facing up or you risk it burning up inside.
Always leave plenty of space for air to circulate around the amplifier.
Don't mount the amplifier under a carpet or inside any closed area.
Never wire a power amplifier into the car's original wiring.
Always mount a fuse in the power cable going to the amplifier, and place it close to the battery positive terminal. The amplifier may have a fuse but that's to protect the amplifier - the one next to the battery protects your car and you!
Of course if you visit an InCar Expert they'll ensure these and many other things are covered.

And finally, it IS possible to destroy a mega subwoofer with a 20W amplifier. Here's how.

You're cruising after installing the BASS BOMBER 20 you just bought at the market, along with a nice 12 inch sub you bought from a pukka shop. You keep turning up the bass and ... sheeeeek, phaffff. Nada. You stop and can smell something nasty from the speaker.

It's got to be a faulty sub you reckon, 'cause the amp is only 20 watts. So you go back to the shop and they calmly explain (for probably the 632nd time in all) that overdriving a small amplifier causes it to go into extreme distortion, where it goes from putting out a nice smooth AC signal to a nasty DC signal that has a great way of toasting woofer voice coils. And guess what, it ain't covered by the warranty.
So, don't fall into that one, plan your system well or get pro advice.

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