Amp Smarts:
Amplifiers are devices that take a teeny tiny audio signal from a pre-amp, some power from your battery, and pass it on as a big big audio signal to drive speakers.
The most important piece of advice I can give to you is to buy lots of power. To get the cleanest sound, it is absolutely essential to buy as much power as you can afford.
Here are some terms that you'll need to know when evaluating car audio amplifiers.
Watt: A watt is a measurment of power. Power is the ability to do work. In the case of amplifiers, the work is to move a speaker. Amplifiers come in all power ranges, from 20 watts per channel up to thousands per channel. When it comes to amplifiers, watts are described two ways:
RMS: An amp's power rating might say: 75W RMS x 4 CH @ 4Ω 14.4V
This means that the amp is capable of 75 watts into 4 channels with 4 ohm speakers, and 14.4V of voltage. This amp is capable of providing that power if all of those conditions are met at all times. The truth is, even if you design your system to meet all of those conditions, it will not at all times. Speakers' impedences (measured in ohms, or Ω) change constantly while operating. Amps don't necessarilly put out the same power all of the time either, for example when a bass track is playing versus a much quieter track.
WHEN BUYING AN AMP, LOOK AT IT'S POWER RATING AS A MARKER FOR YOUR AMP'S CAPABILITY RATHER THAN WHAT IS LIKELY TO PUT OUT AT ALL TIMES.
However, I must also say that an amp's RMS rating is the one piece of information to look for to determine an amp's power.
CEA Compliant Power Output: This is (hopefully) going to become the industry standard for determining an amp's power output. This allows you to accurately compare two amps to each other, because you know that they were tested under the same conditions. To know why this is a good thing, click here...
MAX:
Right under an amp's RMS rating, you'll see an amp's "MAX" rating. It is often double the RMS rating. The MAX rating is pure junk. Never pay attention to the max rating because it only shows what the amp is capable of in a laboratory setting. That kind of environment cannot be reproduced in the auto, and therefore the rating is misleading. An amp's max rating is proably about 50% more than the RMS. Never buy an amp based on it's Max rating. Ok, and lastly, because this makes you look like a total jerk, never ever ever say your amp is a 1000 watt amp because it has a max 1000 watt sticker on it, and ESPECIALLY if you bought your amp at the swap meet. 1000 watt amps are BIG and HEAVY.
Anyway, in summation, never buy an amp based on it's MAX rating.
CEA Compliant Power: (CEA=Consumer Electronics Association) This spec (if your amplifier includes it) is a new rating. The goals of this rating are to give a useful reference for amp comparison. In the "bad old days", amp manufacturers could pretty much claim whatever power output they wanted without specifying the testing methods. The CEA rating changes that. The CEA compliant power output specifies what power output you can expect when the following criteria are met: 14.4V, 4 Ohm speaker load and 1% distortion. You can now fairly compare two amplifiers to each other because you know they were tested in the same manner. Make sense? No? OK, look at it this way: If an amp has a CEA compliant output power of 100 watts per channel, you know that the amp is essentially equal to another amp with the same CEA rating. HOWEVER, you cannot compare this amp to another amp that is rated at 100 watts/channel RMS.
I am a big fan of this rating. FINALLY, there is a uniform standard for amplifier power output ratings. If an amp doesn't include this spec, then the manufacturer is probably trying to hide something.
Bridging: Many stereo and 4 channel amps allow you to "bridge". What you do with bridging is combine two channels into one, or four channels into two. This allows you to allocate more power to a single speaker, like a subwoofer for example. This is a handy feature for someone who has a stereo amp lying around, but they want to use all of its power to drive a single speaker. There's nothing wrong with bridging, and is often used for subwoofers.
Crossover: Many amps have a simple crossover built in. Basically, it will be a switch that will have three positions: high pass, low pass, off. High pass basically allows all of the higher frequencies to pass. This is handy if the amp is powering tweeters and mids. The low pass allows only the lower frequencies to be reproduced. This is handy if the amp is powering a subwoofer. Off means that all frequencies will be sent to the speaker.
Bass Boost: Some amps have a circuit that when switched on gives a boost to your bass. It is not essential, and I usually don't use it in my own system.
Frequency Response: This specification tells you what frequencies that the amp responds to. Higher frequencies are treble, lower frequencies are bass. The human ear can perceive from 20hz to 20,000hz (aka 20khz). Most amps will reproduce this frequency band with ease. Some will go above and beyond. Don't worry about that because usually CDs don't contain data out of that range, and even if they did, you wouldn't hear it! Some people talk about harmonics, and I don't know if it's true or not. But whateva. Read about this debate elsewhere.
Channel: Amps have channels that basically tell you how many loads it is built to drive. Loads=speakers, so a two channel amp will power two speakers.
Classes:
Different amps have different designs that all achieve the same thing (amplifying a musical signal), but achieve this in different ways. The way in which the amplifier is designed is refered to as its "class". There are 4 major types of classes, and an occasional oddball. I don't really know the technical differences between them, but it doesn't matter. We're just installing amps, not designing them!
A - Very good sound quality, very inefficient, uses boatloads of power, and wastes most of it in the form of heat.
B - Good sound quality, moderately efficient, uses a lot of power, wastes about half of it in the form of heat. Some "crossover distortion" present
A/B - Combination of classes A and B. At low volumes, uses the class A portion of the amp. At higher volumes, uses the class B portion of the amp.
D - So-so sound quality (but reproduces bass very well), highly efficient, uses minimal power, wastes very little of it.
Other classes: Whatever. They exist. I don't know if they sound better. Your ears are the best judge.
THD: This stands for "total harmonic distortion", but the D is all you have to pay attention to. Obviously, the lower this number, the better. You don't want your amps producing distortion and sending it to your speakers! It causes noise and speaker damage. The experts say any THD rating below 0.1% is inaudible, meaning you can't perceive it. But remember that distortion travels through all components, and is amplified by each one. So, having a low distortion amp, and other high distortion items in your system will reproduce distortion.
S/N Ratio: A Measure of how an amplifier silences backround noise. This spec is measured in db, and the HIGHER the s/n ratio, the cleaner your sound.
Speaker Level Inputs: This feature allows you to connect the amp to your system through speaker wires (instead of RCA cables). This feature is handy if you're adding an amp to your factory stereo system.
Preamp Outputs: This feature allows you to connect an RCA cable to the amp to supply a second amp with the musical signal. This feature is handy if you want to add a second amp without running a cable all the way from your head unit to the amp. Also, it allows you to add an additional amp to a head unit that may have only one set of RCA pre-outs.
Remote bass control: This feature allows you to control the bass output of the amp without having to adjust it on the amp. You can mount the control wherever's clever, and adjust the bass remotely.
HOW TO DETERMINE WHICH AMP IS BEST FOR YOUR SYSTEM:
How many channels is your system?
How to hook up those channels.
Features you want
Crossover:
Inputs:
speaker (hi level)
rca, pre-amp (lo level)
How many channels are you going to drive? We already discussed the ranges of stereo systems, so you need to determine how many speakers you're driving. Say you want to build an intermediate system with 6 speakers. You figure you want to amplify all 6 of those with a "real" amp. You can go a few routes: You can buy a 6 channel amp, or a 5 channel amp, and power your whole system. That makes life easy if you want to keep installation clean and simple. You could also power your speakers with multiple amps: One mono, one 4-channel, you could even use three stereo amps. If you want to get really fancy, you could bi-amp your component speakers, bringing your amp count to 3, 4, and 5 amps!
One way that you can get creative with your installation is through bridging. If you want to power 2 or more subs on a stereo amp, you can bridge it, or combine the two channels into one. From there, you can wire your subs in parallel or series to obtain the proper ohm load. It's an easy way to power multiple speakers on one amp.
To save money, you can always opt not to install an amp for the rear speakers. I often times just power them off of the head unit. If I am listening to music without rear passengers, I can just fade the head unit to the front, leaving me with the amplified front speakers.
Do you need an amp to have a crossover?
Crossovers come in handy when you are powering subwooers. Most amps can reproduce the entire range of human hearing. However, when we install subs, we only want them to reproduce bass. If the amp has a crossover, you can turn it on, and essentially tell the amp to only send bass to the subwoofers. Since subs can only reproduce bass, why have the amp waste energy sending the subwoofers frequecies it can't reproduce? LUNACY I TELL YA!
Inputs
speaker: These are called high-level inputs because the signal is already highly amplified. Get an amp with high-level inputs if you are integrating an amp with a factory head unit.
rca: These are called low-level inputs because the signal is not highly amplified. These inputs are RCA style, and are preferred because they produce lower noise. This is the best option if you are using an aftermarket head unit.
So, now you're really educated about amps. Don't worry. You can thank us later!
-Honest AEB
