Amplifiers take the signal from the head unit and makes it large enough to be able to drive your speakers. It is preferable to use separate amps for high and low frequencies but it is not necessary. The problem with using one amp for all frequencies is that you cannot adjust the levels among different frequency ranges as easily as you can adjust outputs of separate amps. Many people start their system with an amplifier for the low frequncies (bass) and use their head units built-in power to drive the higher frequency speakers. This is adequate but the built-in power in a head unit is usually not strong enough for high volume listening and not clean enough for the discerning ear. There are many options when choosing an amplifier.
Power?
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. Most reputable manufacturers use the RMS rating.
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 ask for 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 heatsinks). This alone can't be enough to determine if the amp is good or not. Watch out for companies that use bigger heat sink than needed, giving the idea of a more 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 maximum possible RMS power.
How much power do I need?
For mids and highs, anywhere from 30 to 50 watts (RMS) per channel would be a minimum. 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 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. Get the biggest amplifiers you can afford and your car's electrical system can handle. More power means louder sound, but most importantly, cleaner sound.
What Else to Look For in an Amplifier
It is a good idea to get an amp with a built-in crossovers, 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 a rare breed. 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 (less than 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 and expensive choice.
Class B amplifiers are the most common by far. They use two output transistors. One for the positive 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 (typically 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.
An increasingly 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. Although there are full-range class D amplifiers available, most high-end manufacturers are designing amps for low frequency applications. These amps are capable of over 1000 Watts. Efficiency is much higher in class D amplifiers (~80%).
Amplifier Power Specifications!
This page is intended to explain amplifier power specifications in more detail. I have a BS in Electrical Engineering so I do not know how much of this the average Joe is going to understand. I am also human so there may be mistakes below.
Amplifier power ratings are important in determining whether an amp will satisfy your system's needs or not. It is necessary for the amp manufacturer to give out a power specification which clear and complete. Otherwise you are just guessing. An example of a good power amp spec for a 4 channel amp is:
"50watts X 4 RMS all channels driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
Every part of that spec is important and without any part of it the power rating is virutally meaningless. Many times amp manufacturers do not give this much information but you have to judge for yourself whether they are hiding anything. Head unit power ratings are notorious for being very misleading. Now I'll go into what each part of the spec means and why each is important.
"50watts X 4 RMS all channels driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
The "50watts" part is the one we notice first and everything else qualifies how that "50watts" was measured. Having enough power is what most people look for in an amp. However, other things come into play. If the you are going to run a load less than 4 ohms, then the current capability of the amp is definitely important and most specs do not give a current capability. A power rating into 2 ohms can help though. If the power doubles into 2 ohms then you know that the amp is built strongly enough that it can deliver enough current to drive a 2 ohm load. You may think that this is not important if you are not going to drive 2 ohm loads but it is important. Speakers (woofers, midranges, tweeters, etc) are not purely resistive. They have capacitive and inductive properties as well. Depending on the music and your setup, the impedance may dip well below 4 ohms for a nominally 4 ohm speaker.
Whether you amp can supply current fast enough to reproduce the music faithfully depends partially on the amp's slew rate (how fast its output can change), its damping factor (how easily it can control the speaker) and its current capability. For these reasons 2 ohm power is important even when driving 4 ohm speakers. Slew rates of 100V/microsec and damping factors above 100 (referenced with a 4 ohm load) are good but that information is usually not given out by the amp manufacturer. I hope it is clear now that the number of watts an amp can produce is only one factor in determining whether an amp is capable of the performance you desire.
On a final note on this part of the spec, most head units use IC (integrate circuits or chips) for the built-in amp's output stage. Those chips rarely can provide adequate current which is why even most novices know not drive subwoofers from a head unit. Real amps often have ICs in them as well but the output stages are almost always discrete, meaning they are built from transistors, resistors, capacitors and not integrated together inside tiny ICs. Advances in IC technology always making them better though.
"50watts X 4 RMS all channels driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
The "X 4" implies that the amp has 4 output channels. The "RMS" stands for "root mean square" and is a method of measuring an AC waveform. More importantly here it implies that the power rating is not just a peak rating but continuous. "all channels driven" means that the power measurement was made with all channels of the amp driven to their maximum level at the same time. This means that the power supply is strong enough to allow all 4 output channels to produce 50watts at the same time.
This is a common place where head unit specs "cheat." They leave off the "all channels driven" and measure only 1 channel at a time which often gives a higher number. I've seen head units claming "30x4" which is meaningless but most people take it to mean that the head unit produces 30watts each into 4 channels. That's 120 watts from a head unit. No amp is 100% efficient so let us say it draws 150 watts to do this (80% efficiency which is still high). With a 12V power input, the head unit amp's power supply would be drawing 12.5 amps. I guarantee you that it is not easy to design a power supply that fits into a head unit leaving enough room for everything else (including the amp stages themselves) for any reasonable price that can deliver that kind of power. That is one reason why I say not preferable to use the head unit's power.
"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
"continuously" implies that the measurement was made using a continuous (probably sine wave) test signal and not just a quick burst. An amp capable of producing higher power for short amounts of time will have a higher power rating if they measure power with short bursts instead of a continuous input.
The argument can be made that continuous power is not as important because music by nature is dynamic and therefore the peak power is what we really should concentrate on. My response to this is that there is no standardized burst input which all amp manufacturers would use to measure "peak" power. In the end to make their power ratings look higher they would use extremely short pulses which would not represent the amp's performance with music. Because no standard currently exists for peak power we must rely on continuous power ratings for consistancy and to be able to compare amps with each other.
"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
"into 4ohms" means that the power measurement was done using a dummy 4 ohm resistor as the load. This is not the same as a 4 ohm speaker but provides a standard which everyone uses to measure power. Sometimes (but not very often) amp manufacturers will measure power specs into 2 or 3 ohm loads and not say "into 4ohms" only to make the power rating look bigger than it actually is but this is rare. 4 ohms is what car audio amp manufacturers almost always give their power ratings for.
"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
"with less than 0.1% THD" tells something about the distortion the amp is producing at this power level. Most amps have an intrinsic distortion that occurs at a near constant level for most of its power range and then when the amp starts to get overdriven the distortion rises quickly. THD stands for "total harmonic distortion" which is one way of measuring distortion that is standardized.
Often a power spec without the THD number was made with the amp driven until the THD reached 1% or more. This gives a higher power rating but you probably would not want to use the amp at that level because it would be distorted. This is a common ploy used when you see a 400watt amp for $50 at a flea market or discount store. This is often another way that head unit amp specs are inflated.
"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
"from 20Hz to 20kHz" tells us the frequency range into which this amp can produce its rated power. Some amps have power curves that fall off at low and high frequencies. Having this part of the spec present gives you reassurance that the amp can produce its power anywhere in the normal audio range. A power spec that says "into 1kHz" or leaves it off could be inflated. Many amps just put the frequency response as a separate datum on the spec sheet and not with the power rating. It should be with the power spec as well. A "+/- 1dB" or something similar should accompany the frequency response so you know how flat the frequency curve is.
That is it for amplifer power specs and be careful with incomplete specs. Even the best manufacturers put out incomplete specs and then it is up to you to figure out whether the amp is well designed or not but it should not be too difficult. You get what you pay for but look at the construction and "feel" of the amp as well to help make your decision. Also, keep in mind that these explanations are valid for home amplification equipment as well, although the FTC has more stringent requirements for power claims of home audio equipment.
Calculating Subwoofer Impedance Loads
This calculator is intended to help you determine the impedance of a speaker or subwoofer wiring configuration. There are many ways you can connect multiple speakers together. The most common and best way is to connect them in parallel. You do this by connecting the positive speaker wire from the amp to the positive terminal on the first subwoofer. In the picture below, one channel is shown.
Then, you connect the positive from that first subwoofer to the positive of the next subwoofer. You can connect as many speakers together like this as your amplifier can handle. Before you attempt this, be sure of what your amplifier can handle. If it says it can handle 2 ohms stereo, don't connect more than four 8 ohm woofers or two 4 ohm woofers per channel. If you ignore this warning, you may destroy your amp/speakers, or cause a fire, so unless you are absolutely sure your amplifier is capable of handling lower impedance loads, don't try it!
The formula above is for multiple subwoofers with the same impedance.
Example: You have three eight ohm woofers. Inserting the variables in the formula gives you the ratio 8/3. Eight divided by 3 gives you approximately 2.66 ohms. If your subwoofers were four ohm versions, you would get an answer of 4/3 or 1.33 ohms. This would cause a problem for most amplifiers. There are some high current competition amplifiers that can handle these lower impedances, but in general most amps won't run with this load. If you are not sure, contact the manufacturer of your amplifier. This also the same way you wire an amplifier in mono except in a mono wiring schematic you usually use the positive of one channel and the negative of the other channel. When you use a mono configuration, the impedance load at the amp is half as much.
Explanation of Terms
Rt is total impedance.
Amplifier Terms!
RMS Power: The power output of an amplifier should be roughly matched to what the amp will be used for and what speakers it will be driving. Oddly enough, the most common problem with matching speakers and amps is using an amp that is too weak to power the speaker. When an underpowered amp is used to power a speaker, the listener tends to turn the volume up higher in order to get more output of the amplifier. Eventually the amplifier runs into its limit and begins to distort. This distortion can cause the output from the amplifier to become DC for short periods of time and DC signals of even low power can destroy a speaker. Underpowering a speaker in this way can be more dangerous than overpowering it! Also more power is usually necessary when powering subwoofers because of their large size and excursion. Do not plan on using an amp of less than 75watts per channel to drive a subwoofer. The converse holds true for higher frequencies (midrange and treble) only 25-50watts per channel are necessary to drive speakers in those frequency ranges, however more power will not hurt, it just probably will not be used. Another factor in power output is stability in low resistance loads. Sometimes you can wire mutiple subwoofers to a single channel on an amplifier but the amp will have to work harder to drive this kind of load. Many moderately priced amps can drive loads as low as 2 ohms or less, with 4 ohms being the typical load of a single speaker.
Power Supply Regulation: The power supply in an amplifier converts the 12volt DC that is available in your car's electrical system to something the amp can use to produce more power. Several designs are employed by manufacturers today. Two classifications are regulated and unregulated. A regulated supply produces the same power regardless of whether your car's electrical system voltage sags (which a capacitor will help prevent). An amp using a stiffly regulated power supply will be able to supply full power even when the input voltage dips below 12volts. However, it will not gain any power if the input voltage goes above 12volts. An unregulated supply's power output depends directly on the input voltage. This causes changes in the maximum output power with changes in the car's electrical system. I recommend getting an amp with a regulated power supply so power output will be constant regardless of input voltage changes. This changes if you have a stiffening capacitor or another regulation device (Accumatch) to smooth out your car's electrical system. In this case, buy an amp with an unregulated supply. Some cheap amps use unregulated supplies to save money but provide none of the benefits of a typical unregulated supply. One way to determine whether an amp has a regulated supply or not is to view the power output specs for 12volt and 14.4volt inputs. If they are the same then the amp probably has a regulated supply otherwise it has an unregulated one.
Tri-Mode: Some amps can play in what is called "tri-mode." In this mode, 2 channels are used to drive a pair of high frequency speakers and one subwoofer. The subwoofer receives power from both channels. This is a very efficient way to use an amp for more than one purpose. A special crossover is required to separate the two ranges of frequencies and it should have a way of adjusting the output level between the high frequency speakers and the subwoofer. This can be a nice way to save money on your system although it wastes a little bit of amplifier power because of the crossover and it can be more difficult to adjust the relative level between the high and low frequency outputs.
Other Specs: THD (Total Harmonic Distortion) is a spec that often shows up with the power output spec. An example would be "45wattsx2 @ 0.01% THD" This spec says that at an output level of 45watts into each channel the THD will be no more than 0.01%. Sometimes manufacturers will quote the power spec at a THD of 1%. Be wary of this, 1% THD is poor and either implies that the amp is not very high quality or that the manufacturer is artificially inflating the power output spec by running the amp into a higher distortion region where it does produce more power but more distortion as well. Either way it is a sign of a poor amp or marketing that decieves. Anything less than 0.1% is negligible.
Built-in Crossovers: These allow you to use the amp to only amplify certain frequencies and dedicate the amp to a subwoofer or some other specialized speaker. By using an amplifier's built-in crossover you eliminate the need for a separate one which can save you considerable money. There are sophisticated amps on the market today that combine multiple channels and built-in crossovers so that you can use them in place of multiple amps and a separate crossover. They are expensive but often cheaper than buying separate components.
Pre-amp Outputs: Some amps have pre-amp outputs which allow you to "daisy-chain" multiple amps together without splitting the pre-amp output from your head unit. Also, if the amp has a built-in crossover, you can use it to drive another amp. For example if you have an amp you are going to use to drive a subwoofer with a built-in crossover at 90Hz, you can use its built-in crossover to set the amp to only amplify signals below 90Hz for the subwoofer and then have a pre-amp output that only has frequencies above 90Hz which you can connect to an amp that does not have a built-in crossover. That amp can then be used to power the high frequency drivers.
Input Sensitivities: I have received a number of questions about input sensitivities and their importance especially as to why 4 volt outputs on a head unit are better. Here's what an amp does: it takes its input and makes it larger so it can drive speakers. How much larger it can make the input signal is set by the input sensitivity and the maximum power output of the amp. You can turn the input sensitivity all the way up but that does not make the amp put out more power than its max, it just gets to that max level with a smaller input voltage. To see why 4 volt head units are better lets say we have 2 head units, model A puts out a 1 volt signal and model B puts out a 4 volt signal max. We're connecting these head units to a 25 watt amp. The amp puts out 10 volts.
Power = Voltage^2/Resistance = 10^2/4 = 25watts.
To get maximum output from head A, the gain needs to be 10 (10volts out per 1volt in, 10/1 = 10). Now let's say there's 0.1 volt of noise in the signal. With our gain set at 10 with our input sensitivity control we have amplified the noise to 1 volt. Consider what happens with head B. The gain needs to be only 2.5 to get full output. We still get 10 volts of output but the noise is only 0.25 volts. This noise level is 4 times lower than with head A. By using a higher voltage head unit you can set the gain on your amp lower and thus amplify less noise. Also lets say you left the input sensitivity set for a gain of 10 and you used 4 volt head unit at its max. If this did not make the input stage distort it would try to make the amp put out 40 volts (10*4) which would be 400watts! Obviously the amp can't do that and just hits its 25watt limit. To set your input sensitivity, turn you amp's input sensitivity almost all the way down. Now start with your head unit at its lowest volume and turn it up until you hear distortion and then back off some. Some head units will let you go to full volume without distorting the pre-amp level outputs. Now with your head unit putting out its max clean voltage, turn the input sensitivity up until you get to the loudest your system will play without distortion or the loudest you ever care to listen, whichever is lower. Now your amp is set to amplify the least amount necessary to produce full volume making it amplify noise the least.
How To Install An Amplifier!
Remote Turn-on Wire
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 multiple 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.
Power Wiring
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) or less. 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. Here's a table to help decide what gauge wire to use, based on total current draw and length of wire:
RCA Wiring
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.
Mounting
Amplifiers produce a lot of heat and need to receive plenty of fresh air. If the amplifier is to be mounted under a seat, upside down, in a rack or enclosed, a fan or two might need to be used to increase air flow.
To avoid noise problems, it is good practice to mount the amplifier itself to a piece of wood or other non-conducting material. That way the only ground it gets is from the ground wire and not the mounting screws.
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