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post #7 of (permalink) Old 13th September 2006
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Originally Posted by thunderbird2
OK then, there is no such thing as a simple speaker spec.

"Peak" is the largest input you should give a speaker. The reason is that you blow the coil like a fuse if you exceed it. or else do mechanical damage.

"RMS" Is a theoretical constant figure. It will overheat if you drive it harder than that for long.

Now, sound is sound so don't listen to anyone who says that the phisics is different for cars.

What REALLY kills drivers is using an under powered amplifier too hard. If you do that you start feeding the driver a distorted (not audibly) signal. Part of that signal is square wave - a cross between music and DC volts. You can't hear DC at all but your coils work like heater coils .

Do you really want to listen to a 750W sub? I assume that you have at least a 150W/chan system for the mid/high.

I run a 1400 watt P.A. rig but I never use all of it.

The answer is to buy an amp twice as big as you need (1500W RMS) but never turn it beyond 50%. No one in ICE believes that, but professional sound reinforcement guys treat that as sacred.

P.S. did you know that 750W is only twice as loud as 75W?

Sorry Thunderbird, but some of that isn't actually true.

Peak power is a theoretical maximum output that the amplifier could produce, if it was given the absolute maximum input voltage it could handle, and just before the blade fuse popped. It is primarily used - and this will sound harsh - to sell low-end equipment to people who don't bother to do their research beforehand. You can safely ignore this figure, with one exception - if yuo see an amp with 2000W MAX written on it, then avoid it and buy something else.

RMS power (Root Mean Square) is the real-world output of an amplifier being driven at stock alternator voltage (14.4v) into a supported load. It is usually calculated when playing a sine wave at 0dB. When playing music, you'll never actually get the true RMS output of an amp (unless you like listening to tones at full volume!).

You're right about underpowering an extent. The effect you describe is known as clipping, and this isn't especially good for speakers, although most subs have huge magnets, so they can cope with a bit of clip as they have the thermal dissipation ability to deal with it. Clipping smaller drivers (i.e. mids or tweets) is a bad idea.

I would also recommend buying an amp with more RMS power than the sub is rated at. This allows you to drive the amplifier with plenty of headroom, so you'll get the best performance from the woofer, but without straining the amp. I wouldn't go for twice as much power though, somewhere in the 25-30% region is sufficient. For a 750w RMS sub, I'd be looking at an amp which can drive around 1kW into whatever configuration of voice coils the woofer has - remember, a class-D monoblock amp will produce more power as you reduce the load, so ahile it may well do 1000w RMS into a two ohm load, it'll probably only give 600w into a four ohm load (give or take) - make sure the sub can be wired to give the impedance you need.

The Decibel scale is logarithmic, but it works on factors of three. 103dB is theoretically twice as "loud" as 100dB, although for the human ear, you need to have about 6-10dB before you'd consider it "twice as loud". Just the way the ears work.

The easiest way to increase by 3dB is to either double the cone area for the same power, or double the power for the same cone area - so a sub being given 500w might produce 130dB...give it 1kW and it might give 133dB.

Going from 75w to 750w should net you an increase of between 9dB and 12dB - almost a four-fold increase in sound pressure.

Remember too that the design and build of the sub enclosure will have a marked effect on the sound you hear. A sealed box will usually sound tighter, and more controlled, but they usually play 3dB down on a ported enclosure. Ported give you a bigger "rumble" and will usually drop lower, but at the expense of some musicality. You should also use a subsonic filter if you're running ported, as if you aren't careful, you can see the blox play well below its tuning frequench (determined by the port you choose) and "unload" which can damage the sub.

EDIT: Also - PLEASE don't use the gain control as a volume control! The gain is used to tell the amplifier what sort of output it can expect on the input RCAs. If your HU is capable of driving 4v down the preouts, set the amp gain to 4v. If its capable of 2v...set it to 2v. If its still not loud enough, buy a more powerful amp!

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