Current (Amperes) is, in simple terms a measurement of how many electrons flow through a device. In the water tank analogy, current would be water flow rate.

Resistance (Ohms) slows down current flow. The higher the resistance of a circuit, the lower the current will be. Resistance would be equivalent to pipe size. If you have the water tank at a high level, but the pipe is very small in diameter (high resistance), not much water will flow. If you use a big pipe (low resistance), then the water flow rate will be larger.

Knowing the relationships between voltage, current and resistance brings us to ohm’s law: “Current is proportional to Voltage divided by Resistance”. This equation can be manipulated to obtain any value knowing the other two. For example, by measuring the voltage and current of a circuit, resistance can be calculated by dividing voltage by current. When a circuit is open (disconnected), the resistance is infinite (zero current). The formula also shows what happens when a circuit is shorted (resistance = 0): The battery will put out as much current as it can instantaneously (not a good sight).

DC vs. AC Circuits

On a DC circuit, current flows in one direction only. Voltage can remain at a level or change, but it always has the same polarity. A car’s battery produces DC voltage.

AC circuits are a bit more complicated to understand. The voltage supply reverses its polarity switching from positive to negative. The current produced goes in one direction while the voltage is positive and then flows in the opposite direction when voltage is reversed. AC circuits have a frequency associated with them. The frequency (Hertz or Hz) is how many times per second (cycles) the current (and voltage) switch from positive to negative and back. The higher the frequency, the faster the circuit will switch polarity. AC voltage in the car is produced by the alternator, which is converted to DC voltage to charge the battery (even though some of the AC energy from the alternator remains in the electrical system, this is what causes alternator whine when the car is running). The audio signal that comes from the head unit, gets amplified and drives the speakers is also an AC signal.

A Typical DC Circuit

Electrons flow in a circuit from the negative side of the battery to the positive side of the battery (that is why physicists will argue with the direction of the current in the circuit). Engineers represent current in the opposite direction of electron flow, as in the diagram. It does not matter what convention you follow for current direction. The important thing to keep in mind is how much current flows through the circuit, and that you stick to only one of the models when analyzing a circuit.
For a circuit to have current, there has to be a path (i. e. wire) and a battery. A circuit also has a resistance, which slows down flow of electrons.
If the path is broken, current can not flow. The battery supplies the voltage. The top portion of the circuit in a car is represented by the positive battery cable going to the fuse box and to all the accessories (radio, wipers, lights, etc). Each accessory has a resistance. As more accessories are added, the resistance drops, and more current flows through the circuit. To save money, car manufacturers use the car metal for the bottom part of the circuit, instead of running a ground wire to every device.

Common Engineering Notations

To represent very high or low values, zeros or decimal points are represented by letters. These are the most common used in car audio:

(from internet)

Ohm’s law: Current = Voltage / Resistance or I = V / R
You can rearrange this formula in a couple ways: V = I * R and R = V / I

Power equation: Power = Voltage^2 / Resistance or P = V^2 / R
Rearranging this formula gives us: R = V^2 / P and V = SQRT(P * R)

Using Ohm’s law we can derive some more power equations: P = I^2 * R
Rearranging gives us: I = SQRT(P / R)

Abbreviations used on this page are:
I : current, measured in amps
V: voltage, measured in volts
R: resistance, measured in ohms
P: power, measured in watts

With that out of the way we can get down to business. As stated earlier, ohms are a measure of electrical resistance. It helps in this discussion if we consider amplifiers to be perfect voltage sources.

Consider this 25 watt amp:

P = V^2 / R

Most amps are rated into 4 ohm loads so we now have:

25 watts = V^2 / 4 ohms

Solving for voltage we get: V = SQRT(25 * 4) = 10 volts

So our 25 watt amp can be considered here to have the ability to produce up to 10 volts output.

Now let us consider what happens when we connect two speakers in parallel to the amp. At this point I am going to introduce some more formulas. We are considering speakers to just be simple 4 ohm resistors for this discussion. There are formulas which dictate what happens when you combine resistors in various ways. Before that we need to explain what is meant by parallel and series ConneXions.

Each channel of an amplifier has a positive (+) and negative (-) connection. The amp develops a voltage between these two terminals and this voltage is what drives the speakers. The equivalent resistance of what you connect to the amp is referred to as the load.

Normally when connecting a single speaker to a single amp channel you merely connect the “+” terminal of the speaker to the “+” terminal of the amp. Then do the same thing for the “-” terminals. Things get more complicated when you are connecting multiple speakers to a single amp channel. In a parallel configuration you connect both “+” terminals of the speakers to the “+” terminal of the amp. Then do the same thing for the “-” terminals.

A series connection is a little more complicated. First, you connect the “+” terminal of the amp to the “+” terminal of one of the speakers (let us call it speaker A). The next thing you do is connect the “-” terminal of speaker A to the “+” terminal of the other speaker (speaker B). Lastly, you connect the “-” terminal of speaker B to the “-” terminal of the amp. You can see in this connection that power from the amp goes through both speakers one after the other, hence the name “series.”

Finally, here are the formulas that tell you what resistance load you end up with when wiring multiple speakers:

For two speakers in parallel:

1 / Rt = 1 / Ra + 1 / Rb

where Rt is the total equivalent resistance or load and Ra and Rb are the resistances of the two speakers. You can see that you can use speakers of different resistances but there other implications of doing that which are usually undesirable because the power will not be spread evenly between the speakers.

Working through the math if you put two 4 ohm speakers in parallel you get:

1 / Rt = 1 / 4 + 1 / 4 = 1 / 2, Rt = 2 ohms

The equivalent resistance is exactly half of what we started with. We will look at the implications of this a little later. Let us do another example first with three 4 ohm speakers in parallel:

1 / Rt = 1 / 4 + 1 / 4 + 1 / 4 = 3 / 4, Rt = 4 /3 = 1.33 ohms

You can see that as you put more and more speakers in parallel the equivalent resistance will drop further.

Next lets look at the equivalent resistance for speakers in series:

Rt = Ra + Rb

Now that is easy! You just add the resistances for each speaker so putting two 4 ohm speakers in series will you give a single load of 8 ohms.

With that background out of the way we can look at what effect these different wiring combinations have on the amplifier. Going back our 25 watt (10 volt) amplifier with a single 4 ohm speaker we have:

Current = Voltage / Resistance = 10 / 4 = 2.5 amps

So when this amp is producing maximum power (25 watts) into a 4 ohm load, the load will draw 2.5 amps from the amp.

Now let us look at what happens when we connect two 4 ohm speakers in parallel (which gives us a 2 ohm equivalent load) to this amp:

Power = Voltage^2 / Resistance = 10^2 / 2 = 50 watts

This is seems great! Our 25 watt amp is now producing 50 watts but there are some complications. Let us see the current now:

Current = Voltage / Resistance = 10 / 2 = 5 amps

Even though our voltage is still the same (10 volts) our current has now doubled from 2.5 amps to 5 amps. If the amp has the capability to produce this much current and dissipate the heat that this will generate then everything will be fine. One way to determine if your amp is capable of this is to look for power ratings that are given into 2 ohms in addition to the normal 4 ohm rating. Further if the power doubles into the 2 ohm rating then the amp has ample current capacity. Another clue to tell whether the amp will work with 2 ohm loads is look for the phrase “2 ohm stable.” Being 2 ohm stable only means that the amp will function with 2 ohm loads; it does not necessarily mean that the amp will produce more power into 2 ohms. If you attempt to use a 2 ohm load with an amp that cannot handle it a well designed amp will shut itself off or blow a fuse and a poor one could be permanently damaged.

Next let us look at a series connection with two 4 ohm speakers. This will give us an 8 ohm load and we will use our 25 watt (10 volt) amplifier again:

P = V^2 / R = 10^2 / 8 = 12.5 watts

Our amp is now producing only half its power rating! And the current is:

I = V / R = 10 / 8 = 1.25 amps

The current is 1/2 its original value as well. Series ConneXions are not used as often as parallel ConneXions because they reduce power. However, they are easier for your amp to drive since they draw less current.

Another option that is often available is to bridge an amplifier. This process takes 2 amp channels and combines them to act as a single more powerful amp channel. How to do this to an amp and wire everything varies so please do not ask me how to bridge your amp. I can explain the effects of it though.

What typically happens when you bridge an amp is that the voltage it can produce doubles. Our 25 watt (10 volt) amp can now produce 20 volts. Let us look at how that affects power:

P = V^2 / R = 20^2 / 4 = 100 watts

This is 4 times the original power of the amp but let us look at the current situation:

I = V / R = 20 / 4 = 5 amps

So now we see even with a regular 4 ohm load the current is already double what the normal value (2.5 amps) was. For other reasons the power usually does not usually quadruple when you bridge an amp but will typically at least double. Connecting a 2 ohm load to a bridged amp raises the current requirement even more. However, if your amp can handle it then you will be squeezing a lot of power out of the amp.

In fact, many people use that kind of setup in competition where the classes are judged by the power rating of the amps in the system. A 25 watt amp can produce many times more power when bridged and driving low resistance loads so the competitor gets more power than what appears on the surface. Zapco and Phoenix Gold make amps which are able to drive such low resistance loads (sometimes as low as 0.5 ohm!)

Some final notes:

* On this page I have considered amplifiers to be perfect voltage sources. They are not though and they have some internal resistance which lowers power output slightly.
* I have also considered speakers to be perfect 4 ohm resistors. In actuality the resistance of the speaker depends on the frequency the speaker is playing. For example, a speaker may have a 3 ohm resistance at 80 Hz and a 9 ohm resistance at 300 Hz. If you were to make a plot of resistance versus frequency you would get what is called the impedance curve of the speaker. Also, speakers act in some ways as inductors and capacitors so a true model of a speaker must include those components as well. How does this affect the sound you ask? Well if you have an amp that has very weak current capability it may work fine into perfect 4 ohm loads but when you connect our real speaker which has an impedance dip at 80 Hz the amp may have difficulty and smear sounds that have 80 Hz components. These are minor but audible effects. This is why it is good to get a 2 ohm stable amp even if you never plan on running 2 ohm loads.
* Placing speakers in parallel and bridging amp channels are effective methods for increasing the power in your system assuming your amp can handle the increased demand.
* Use a series configuration when you need to raise the effective resistance of the load. This occurs more often when you are using dual voice coil speakers.
* Dual voice speakers have two speaker ConneXions on them. This typically increases power handling capability and gives you more wiring options. For example, if you have two dual 4 ohm voice coil speakers you can get a single 4 ohm load which is suitable for connecting to a bridged amp. You would do this by connecting the voice coils on each speaker to each other in series. This would give you two 8 ohm speakers. Next you put those two 8 ohm speakers in parallel and this will give you a single 4 ohm equivalent load.
* Be aware that using lower resistance loads and bridging produces a greater load on the amp. Well designed amps that cannot handle the demand will either shut themselves off or blow a fuse. A poorer designed amp can permanently damage itself. Also, even if an amp works in these configurations it will probably generate more heat so ventilation is even more important.
* As mentioned earlier ohms are a measure of electrical resistance. You should be able to understand why resistance changes affect the amp power as it does. If you raise the resistance the amp is not able to drive as much current through the load and thus you get less power. If you lower the resistance of the load math says that more current will be drawn from the amp. Assuming the amp can handle this you get more power.
* To understand why series and parallel configurations have the effect on resistance that they do consider this. When you connect speakers in series current must flow through both speakers and so it hits the resistance of both speakers. When you have speakers in parallel, the current has multiple paths since it can go through either speaker so the equivalent resistance is always lower than that of either speaker alone.

(from internet)

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.

(from internet)

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 “30×4″ 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.

(from internet)

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.

(from Internet)

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.

Click to View

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%).

(from internet)

An inexpensive subwoofer loaded in the proper enclosure will sound much better than an expensive one in the wrong box. If you choose the wrong type of box for your speaker, the speaker may sound bad or suffer mechanical damage. There are some general rules to help you decide which alignment is the right one for your subwoofers. You need to compare the Thiele-Small parameters of your driver to the guidelines below.

Sealed box (and sealed bandpass) woofers should have the following characteristics:

* Qts => 0.40
* Fs <= 35 Hz
* Xmax => 4 mm

Vented or ported box (and ported bandpass) woofers should have the following characteristics:

* Qts <= 0.40
* Fs <= 45 Hz
* Vas should be relatively low (less than 4 cubic feet unless enclosure size is not a factor).

An additional calculation to help you determine the proper enclosure is the efficiency bandwidth product. This is figured by dividing the Fs value by the Qes value. If the result is close to one hundred, a vented enclosure is the right choice. Closer to fifty means this driver will function best in a sealed alignment.

The best results (widest passband) for bandpass subwoofers are obtained when the Fs value divided by the Qts parameter is ninety or higher.

(from internet)

Check with manufacturer or dealer for appropriate box volume and design. Some subs can’t be used in certain types of boxes, and have very small tolerances for box volume errors. If a sub is installed in a box larger or smaller than what is supposed to, it will sound bad and could be destroyed. Boxes can be built in any shape, but it is difficult to calculate volume for complex shapes.

Materials

A box has to be very rigid. Most common building materials are 5/8″ or thicker particle board or medium density fiberboard.

If building a box with Plexiglas, do not use anything less that 1/2 inch thick.

A common material used to mold complex shaped boxes is fiberglass, but it is a real pain to work with, and several layers need to be applied for a solid finish.

Gluing, Sealing

Use glue at all joints (cheapest and most used product is Liquid Nails). Make sure there are no holes. Any leaks will degrade the performance of your subs, not to mention the annoying noise air makes when being pushed out of a small hole.

Let glue cure for at least 24 hours before mounting the woofers. The fumes of some products will eat up rubber and other materials subs are made of.

Holding Joints Together

Screw joints (use 2″ – 2-1/2″ screws) every four inches or so. Pre-drill about 3/4″ deep, so that screws do not split the wood at the edges, especially when working with particleboard.

A box for Each Sub?

Even though it is not necessary to have two separate chambers for two subs, it is best to take this approach for two reasons: First, if one of the subs dies, then the volume of the box will be “twice” as big, as seen by the sub that is still working. This could cause problems and even damage the other sub. The second reason is bracing. building a box with a divider in the middle will be much sturdier.

Making Ports

There are several way to build ports. If a pre-made port is not available, the most common material is PVC tubing. PVC tubing is very rigid, comes in different diameters, and is easily found at any hardware store.

Cut the tubing at the desired length. Consider the volume the port takes up when calculating the box volume. Cut a hole in the box. Make sure the hole is as perfect as possible to minimize gaps between the box and the tube. A couple wood braces can be added for screwing the port top the box. Seal the gaps using plenty of Liquid Nails or similar product.

Bracing

Boxes that are more than a foot on width or length or height, should be braced (use a piece of wood maybe 3 or 4 inches wide across the box, so that box does not flex). It is a good idea to put wood blocks on the corners for reinforcement. Always consider that blocks, braces, neon lights, etc. inside a box take up space and should be accounted for when calculating internal volume.

Damping/Filling

It is advisable to put damping material inside a box. Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin. This increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil. Polyfill also “fools” a sub into thinking it is in a bigger box. Play around with different amounts of polyfill until you get the desired results.

Finishing the Box

Add wood filler to holes and sand the box to make a smooth surface. If you are painting the box, It is a good idea to apply primer under the paint.

It is not necessary to sand the box if you are using carpet or padding under vinyl, since the thickness of the material will cover any small imperfections. The best way to cut carpet or vinyl is with a good quality carpet knife. Blades wear out pretty quickly, so buy a handful. Cut a piece of carpet (or vinyl) big enough to cover the whole box. Apply adhesive to both box and carpet (spray 3M adhesive 77 or 90 works great). Wait about a minute and place the fabric over the wood. For a good fit, stretch the fabric when applying it. The fabric should wrap around and end in a place of the box that will not be seen. Do one side at a time, cutting excess carpet. If possible, add staples to hold the fabric at the ends.

(from internet)

An electronic crossover takes a full range signal and divides it into different frequency ranges. The most common types are 2-way or 3-way. A 2-way crossover divides the frequency range in half at some cutoff frequency. All signals below the cutoff frequency are routed to a low pass pre-amp output and the rest are sent to a high pass output. These outputs can be connected to amps to dedicate those amps to producing only those frequency ranges. A 3-way crossover is similar but splits the signal into 3 parts. You can get a single box that has an equalizer and crossover. Electronic or active crossovers act on pre-amp level signals. They use the pre-amp level output of your head unit as input and their outputs go to your amp(s). By doing this you keep the amp from trying to amplify frequencies that you do not want (like high frequencies for a sub amp). On the other hand, a passive crossover acts on the signals after they have been amplified, they are connected after the amp and before the speakers. Usually these are just simple high pass or low pass units. You connect a high pass crossover to a speaker to block bass to that speaker. Some people call these bass blockers. You use a low pass crossover with a woofer so it only plays “lows.”

Number of Bands in the EQ:
The number of bands in an equalizer tells you how fine an adjustment you can make. A 10 band equalizer breaks up the audio range into 10 parts and you can adjust the levels of any of them. The Q of an equalizer tells you how wide a range each adjustment makes. Let us say a specific band is labeled as 100 Hz. A high Q high equalizer will only boost or cut frequencies right around 100 Hz and not really affect signals at say 70 Hz. A low Q equalizer generally affects a wide range of frequencies even though it may be centered at one specific one. Typically, the more bands in the EQ the higher the Q so the different bands are not affected by each other. Simple bass and treble controls have the lowest Q. Equalizers with only few bands are good for making general adjustments but bad for fine tuning. A 30 band equalizer is great for making specific adjustments and tailoring the sound exactly how you want it. A tool called an RTA (real time analyzer) is used in setting those equalizers. It gives the system a flat signal (pink noise) and shows the user what the system returns. The user adjusts the equalizer until the RTA shows the desired response. The desired response is rarely flat because a flat setting results in dull, bass shy sound that is hard and edgy. Working with an experienced installer is key here.

Slope of the Crossover:
When any crossover splits the frequencies it is not a hard split. At the crossover frequency in a 2-way crossover both outputs will have this frequency in the output albeit at a lower level. How fast the crossover transitions from one output with rising frequency to another is called the slope of the crossover. There are many interchangable terms for crossovers. A 1st order crossovers transitions at 6dB/octave or 10dB/decade. A 2nd order one will transition twice as quickly. For tweeters a minimum of a 2nd order crossover should be use in order to prevent the tweeter from seeing any bass frequencies. 4th order crossovers are common and digital crossovers of any order are possible but expensive. Use at least a 2nd order crossover to be safe. For an example of tweeter safety, say we want to use a high pass crossover frequency of 2kHz. With a 1st order crossover (or filter) at 1kHz the level is only down by 6dB and only down by 12dB at 500Hz. 500Hz is way too low for a tweeter to play so this will probably cause the tweeter to distort or blow up. Using a 2nd order filter would have the output down by 24dB which would be a signficant improvement. 3rd and 4th order filters are even better but expensive. Also analog crossovers change the phase response so try wiring your tweeter out of phase to see if it makes the sound better or worse and leave it the way it sounds better to you.

Crossovers can also be made to have different types of response near the crossover point. Butterworth filters have smooth but slow response. Chebychev filters are quicker but have some overshoot. Details of these filters is beyond the scope of this page.

No head unit typically has more than about 60watts of total output power because more would require a real DC-DC power supply (which does not fit in a head unit easily). Using the head unit power can be a temporary solution until a separate amplifier can be purchased, just make sure you do not try to power any subwoofers or insensitive component sets with the head unit’s built in power. According to Car Audio and Electronics magazine, most head units use the same chips for the internal amplifiers so they all produce about the same low power. The best they have measured is about 14watts into 4 channels at 1% distortion. Their power level at a better lower distortion figure (like 0.1%) is significantly lower.

Note about using factory head units:
Many people ask me about using the factory head unit that came with their car with external aftermarket amps. Typically you cannot get a clean signal from the head unit because factory heads do not have line level (RCA) pre-amp outputs to drive an amp. You can use a speaker level to line level converter but the sound is still going through the factory head’s internal amps. Some people are willing to sacrifice some sound quality in order to keep their factory head. Also, if your factory system uses an external amp you may be able to find an adapter so you can use an aftermarket amp instead.

Usability:
Your head unit is the part of the car stereo that you interact with most so it is important to get one that “feels” good to you. Always look at a head unit in a store display and use it for awhile. Try to flip through radio stations and tracks on a CD to see if it is quick and easy. If you have problems with small buttons, imagine what it will be like when you are driving! Since many models in the same price range are similar in features and sound quality, usability is often the deciding factor between models.

Power:
Even though I just said not to use the built-in power of a head unit I know sometimes it is necessary. Bear in mind that the power specifications given by most manufacturers for head units are not accurate. They often use terms like “music power” or “peak power” which have little real meaning because there is no standard definition of those terms. If the power is quoted in “RMS” terms then it is usually accurate. However, there is still one other place of misconception. Often manufacturers will quote power as “30watts x 4 RMS”. The “RMS” seems to mean it is a true indication of power but they are implying that all 4 channels can produce 30watts rms AT THE SAME TIME. With a head unit, this is almost always not true. Because of the small power supplies in head units they can rarely output more than 15-60 watts TOTAL. This means that the power to each channel at maximum loading would only be 1/4 of that total. Some manufacturers are better than others about giving accurate specifications and a few models are available with sophisticated power supplies which have higher power output but they are VERY expensive. If you’re paying less than $800 for a head unit (and most of us are!) then your head unit will not put out much power. I have written a more comprehensive explanation of power amplifier specs as well. Speakers which are not producing bass do not draw nearly as much power so you can get away with using the head unit to power them but use passive high pass crossovers (bass blockers) and they will play even louder and cleaner. Bear in mind that the distortion may be higher from the head unit than an external amp however.

Cassette vs. CD:
This choice is mostly a matter of preference. If you do not have many cassettes then an in-dash CD player is probably right for you. If you need the capability to listen to cassettes and CDs then a cassette head unit with changer controls should be adequate. Be aware that many in-dash CD head units can control a CD changer as well so you can use both. An in-dash CD is convenient for changing discs quickly while on the road. Because of size of most CD changers they are usually mounted in the trunk or under the seats although there are some newer models which are small enough to fit in glove compartments. Under the seats or in the trunk are not easy places to get to while you’re driving!

Theft Protection:
Detachable faces are the most common theft prevention scheme in head units today. There are two flavors, fully detachable and partially detachable. With a fully detachable face all the controls on the front come off leaving behind a blank panel, whereas a partially detachable face leaves some features on the head unit but the head unit is still useless without the face. Fully detachable faces are larger and bulkier to carry around than partially detachable ones but leave nothing behind to be seen. Another option is Eclipse’s ESN system. With these head units when you first apply power to them you must supply a CD which the unit remembers as the “reference” CD. Thereafter if the unit ever loses power you must insert the “reference” CD before it will work again. Only you know what the “reference CD” is so the head unit is useless to a thief. Eclipse also tracks the units they repair. More than once a stolen head unit was returned to them for service because it was not working. Upon verifying the head unit was stolen they can apprehend the thief as the person who returned the stolen head unit for service. I still wouldn’t count on the thief to know that Eclipse does this though so I stick with a conventional fully detachable face. A new twist from Kenwood flips the face around when you turn off the power so the thief can’t see the head unit. I think it would work even better if the face then went back into the head unit, giving the appearance that the unit is a detachable face head unit with its face removed.

Pre-amp outputs:
These are must for any serious head unit. These outputs allow you to run an amplifier directly without need for any conversion. This is the cleanest output of the head unit. Some units have multiple outputs and sometimes ones that are crossed over. Look for the amount and type that you need for your system but keep in mind future expansion. One is sufficient but having two allow you fade, or adjust the levels of multiple amplifiers right from the head unit. Some head units now offer 4 volt outputs instead of the usual 1-2 volts. This can be very beneficial since cars have a lot of electrical noise in them. The 4 volt output is less susceptible to noise, however, you must be certain that the amplifier or crossover being connected to the output can handle 4 volts or you will not be able to use the extra voltage. If your head unit does not have pre-amp level (RCA type) outputs you can buy an adapter which will convert your speaker level outputs to line level. They range in price from $12 on up but since I have not used them I do not know how much difference there is among them. Another option is to use an amplifier that accepts speaker level signals directly but those are not as easy to find.

Other features:
There are many other minor differences in features between head units. Choose the one that appeals to you most. Switch able illumination is nice if you want the head unit’s display to match the other instrumentation in your car. Dolby Noise reduction and full logic tape controls are nice as well. Finally, a remote control can be useful or can be a waste depending on whether you use it. A remote control mounted in the steering wheel can be very convenient though. Some CD heads come with a buffer to minimize effects from bumps. This can be useful but in my experience if you mount the head unit securely it will not skip much anyway and using the anti-skip buffer can have a slight negative effect on sound quality because of the way the buffer is implemented.