Amplifier FAQ


There are different ways in which power is measured by amplifier manufacturers, to make people think that their amps have more power than others. Laws of physics tell us that Power can be obtained by multiplying Current and Voltage. For example, if your amplifier gets 12 volts, and it draws 20 amps, then power would be 240 watts, right? Not exactly. In the real world, amplifiers waste 50% or more of the power in the form of heat. That leaves you with only 120 watts. 

Things get more complicated than that. There are different ways to measure power. Power can be measured for top to bottom of the signal (Peak, or Max, etc). Another way to measure power is From the zero-level to the top half (usually called music power). The most accurate way to measure power is RMS (root mean square) watts. The RMS value is obtained by squaring the value of the signal, taking the average, then the square root. This is the equivalent of the actual power delivered. 

To get RMS power from peak or max power just divide by three. Music power is just half of peak power. For example, an amplifier is rated at 100w (peak) per channel. The so called Music power would be only 50w per channel. The RMS power would be 33w per channel. Big difference, isn't it? Be careful when checking specifications of amps before buying, to see what you are really getting. Always check the RMS power of an amplifier.

Confused enough? There is more. Some companies rate their amplifiers using unrealistic conditions, for example calculating power at 15 volts, under 2 ohms, at 10% distortion, etc. Make sure you see the actual test voltages and loads. 

How to tell if I am getting a good amp? 

Shop for reputable brands. Look at the size, weight of the amp. The more power the amp puts out, the more wasted heat, and the bigger area it will need to dissipate that heat (bigger heat sinks). Some exceptions are "digital amps" (i.e. Infinity). This alone can't be enough to determine if the amp is good or not. Watch out for companies that actually use a much bigger heat sink than needed so that it fools consumer, giving the idea of a much powerful amp. 

Look at the fuses that are either plugged into the amp, or specified by the instruction book. If you see a 400w amplifier with a 5-amp fuse, you should be suspicious. Remember what was said above, multiply size of the fuse by around 6 (12v at 50% efficiency), and that will give you a rough idea of what you are dealing with in terms of RMS power. 

How much power do I need? 

For mids and highs, anywhere from 30 to 50 watts (RMS) per channel would be decent. For subs you would need at least 80 - 150 watts (or more) per subwoofer. There should always be more total power going to the subwoofers than to the rest of the speakers, since human ears are more sensitive to higher frequencies than lower. For example, if you have 4 x 50 watts going to all your mids and tweeters (total=200 W), then you should have at least 200 W or more going to your subs. 

A lot of people wonder if too much amplifier power can burn up the speakers. What damages speakers most of the time is distortion, not power. If the speakers have the proper crossovers and are not distorting, then it is really hard to blow them. A bigger amp just gives you the opportunity to go to higher volumes without distortion.

What Else to Look For in an Amplifier? 

It is a good idea to get an amp with a built-in crossover, so that you don't have to spend extra money later on crossovers. If you are going to be using multiple speakers, make sure the amp is 2-ohm stable (or less). A bridgeable amplifier could come in handy in the future if you are planning to upgrade. Overheat, short-circuit, overload protections are good features that any good amplifier should have. Look for a low THD (total harmonic distortion) rating.

Amplifier Classes 
There are different amplifier designs: Class A, A-B, B and D 

Class A amplifiers are the most sonically accurate. On the other hand, they have some drawbacks that make them not be the most common choice. Class A amplifiers use only one output transistor that is turned "on" all the time, giving out tremendous amounts of heat. Class A amplifiers are very inefficient (~25%). More heat means more heatsink area, so even though most class A amps have built-in cooling fans, they are big. Class A amplifiers are usually expensive. 

Class B amplifiers are the most common and use two output transistors. One for the positive part of the cycle and one for the negative part of the cycle. Both signals are then "combined". The problem with this design is that at the point when one transistor stops amplifying and the other one kicks in (zero volt line), there is always a small distortion on the signal, called "crossover distortion". Good amplifier designs make this crossover distortion very minimal. Since each transistor is "on" only half of the time, then the amplifier does not get as hot as a class A, yielding to a smaller size and better efficiency (~50%). 

Class A-B amplifiers are a combination of the two types described above. At lower volumes, the amplifier works in class A. At higher volumes, the amplifier switches to class B operation. 

A not very popular kind is the class D amplifier (known as digital amplifier). These amplifiers are not really digital (there is no such thing), but operate similarly in the same manner as a digital-to-analog converter. The signal that comes in is sampled a high rates, and then reconstructed at higher power. This type of amplifiers produce almost no heat and are very small in size, but really expensive. Efficiency is much higher in class D amplifiers (~80%). 

Amplifier Installation

The remote turn on wire goes to the head unit. When the radio is on, it puts out 12 volts that turn the amplifier on. If you are using a factory radio that does not have a remote turn on (or power antenna wire) you can tap into, hook it up to the ignition, so that the amplifier does not remain on when you turn the car off. If you are using more than two devices (amplifiers, crossovers, equalizers, fans, etc), you might have to add a relay, since typical turn-on wires in a radio can't handle more that 300mA.

Even though amplifiers are easy to install, a lot of things could go wrong. The most important thing to consider is where to get the power from: Straight from the battery. ALWAYS put a fuse as close to the positive battery terminal as possible. If the wire going to the back of the car shorts out, then the fuse will blow. If you don't install a fuse or breaker and the wire shorts out, then the wire will carry so much current that the insulation will melt and could catch your car on fire. The size of the fuse should be the same rating as the fuses used by the amp(s). The ground (-) should be hooked-up to a metal part of the car. It is not necessary to run a ground wire all the way to the battery. 

It is not essential to spend a lot of money in getting 99.999999% copper 0-gage wire and gold connectors unless you are installing a competition system. 

When running power wires to the amp, keep them as far away from the RCA wires (see alternator noise section for more info), ideally on the other side of the car. It is OK to run the turn-on wire from the radio along with RCA's, since it carries very little current. 

Electrical System Upgrades

If your car's electrical system does not have enough power to keep up with your amp's current demands, then some changes have to be made to your car. If you feel that your amps can't really handle transients (going instantaneously from low to high volumes), the first thing to do would be to add a stiffening capacitor. What a capacitor does is act as a secondary battery that reacts very quickly to sudden current demands. A rule of thumb is to have 1 farad of capacitance for every 1000 watts of power. Capacitors should be installed as close to the amplifiers as possible, with the positive lead going right into the positive lead of the amp, and the ground to a metal part of the car (not to the ground terminal of the amp). When a capacitor is first installed, it has no voltage in it, so if it is hooked up to a battery, it will act as a short circuit and draw a LOT of current (not a good sight). Capacitors need to be "charged up" first. This is simply done by either putting a resistor (1k-ohm or more) or a test light between the positive post of the battery and the positive terminal of the cap (with ground hooked up to metal). After a few minutes, the capacitor will be charged up, and it can be connected. Every time the capacitor is "drained" i.e the lights were left on in the car and battery is dead, the capacitor should be DISCONNECTED, battery charged, and then capacitor has to be recharged and reconnected. 

Another upgrade would be to get a high output alternator, and to add a secondary battery to keep up with higher system's demands. This should be done by someone who has a fairly good understanding of a car's electrical system, since computers are designed to control the factory components. Always keep in mind that when a car is running the batteries become loads that take power away from your amps. The advantage of secondary batteries is that when the car is off, you can listen to your stereo for longer periods of time. Adding more batteries will not make your stereo perform any better when the car is running. In fact, a second battery will steal power from the electrical system when the car is running. A battery isolator should be used to avoid batteries draining each other