Please tell me how can an Amp have a same power at 2 and 4 ohms


Bronze Member
Username: Baba


Post Number: 18
Registered: Mar-05
recently was comparring the alpine PDX 4.100 & alpine 4.150,

saw a rating of an amplifier as such...
Per channel into 2 ohms : 150W x 4 (<=1%THD+N)
Per channel into 4 ohms : 150W x 4 (<=1%THD+N)

usually at 2 ohms the power goes more. But still some say this sort of a rating is better.

Platinum Member
Username: Glasswolf

Wisteria, Lane USA

Post Number: 11925
Registered: Dec-03
some amplifiers like this one and the JL Audio slash series use what's called a "regulated output stage" where between a given load range, like 1.5 to 4 ohms, the amplifier uses a variable duty cycle not just for the input stage (PWM tightly regulated power supply) but also for the output stage, thus attenuating the output of the amplifier to maintain the same power rating regardless of the load it's presented.

Bronze Member
Username: Baba


Post Number: 21
Registered: Mar-05
makes sense,
but is this always a good thing, to tightly regulate supply, Is it not better to get more power /output from your amp, why choke it...

Silver Member
Username: Sinful_systems70


Post Number: 674
Registered: Nov-06
Very well put GlassWolf!!!!

Silver Member
Username: Killswitchjd

Post Number: 428
Registered: Apr-06
check another site. typo's are very common. I have never seen an alpine with that, only the jl. maybe tho

Gold Member
Username: Nd4spd18

Southeast PA

Post Number: 1645
Registered: Jul-06
Regulated power supplies on the input stage are a very good thing. An amp with one will not reduce power output when the inputs voltage drops, it will just draw more amperes.

Platinum Member
Username: Glasswolf

Wisteria, Lane USA

Post Number: 11933
Registered: Dec-03
understanding how amps and power supplies work will help a lot in understanding the pros and cons of various designs.

I can repost my amplifier fundamentals paper if you want to read about the basics of amp design.

Gold Member
Username: Nd4spd18

Southeast PA

Post Number: 1650
Registered: Jul-06

Glass, I'd like to take a look at that, is it posted in the FAQ section on CAC .net?

Platinum Member
Username: Glasswolf

Wisteria, Lane USA

Post Number: 11935
Registered: Dec-03
yes it's posted there under amplifier fundamentals parts I and II, with additional information posted under "amplifier specs"

Audio Power Amplifier Fundamentals Introduction

The term amplifier is very generic. In general, the purpose of an amplifier is to take an input signal and make it stronger (or in more technically correct terms, increase its amplitude). Amplifiers find application in all kinds of electronic devices designed to perform any number of functions. There are many different types of amplifiers, each with a specific purpose in mind. For example, a radio transmitter uses an RF Amplifier (RF stands for Radio Frequency); such an amplifier is designed to amplify a signal so that it may drive an antenna. This article will focus on audio power amplifiers. Audio power amplifiers are those amplifiers which are designed to drive loudspeakers. Specifically, this discussion will focus on audio power amplifiers intended for DJ and sound reinforcement use. Much of the material presented also applies to amplifiers intended for home stereo system use.


The purpose of a power amplifier, in very simple terms, is to take a signal from a source device (in a mobile system the signal typically comes from a head unit) and make it suitable for driving a loudspeaker. Ideally, the ONLY thing different between the input signal and the output signal is the strength of the signal. In mathematical terms, if the input signal is denoted as S, the output of a perfect amplifier is X*S, where X is a constant (a fixed number). The "*" symbol means" multiplied by".

This being the real world, no amplifier does exactly the ideal, but many do a very good job if they are operated within their advertised power ratings. The output of all amplifiers contain additional signal components that are not present in the input signal; these additional (and unwanted) characteristics may be lumped together and are generally known as distortion. There are many types of distortion; however the two most common types are known as harmonic distortion and intermodulation distortion. In addition to the "garbage" traditionally known as distortion, all amplifiers generate a certain amount of noise (this can be heard as a background "hiss" when no music is playing). More on these later.

All power amplifiers have a power rating, the units of power are called watts. The power rating of an amplifier may be stated for various load impedances; the units for load impedance are ohms. The most common load impedances are 8 ohms, 4 ohms, and 2 ohms. The power output of a modern amplifier is usually higher when lower impedance loads (speakers) are used (but as we shall see later this is not necessarily better).

In the early days, power amplifiers used devices called vacuum tubes (referred to simply as "tubes" from here on). Tubes are seldom used in amplifiers intended for mobile use. Modern amplifiers almost always use transistors (instead of tubes); in the late 60's and early 70's, the term "solid state" was used (and often engraved on the front panel as a "buzz word"). The signal path in a tube amplifier undergoes similar processing as the signal in a transistor amp, however the devices and voltages are quite different. Tubes are generally "high voltage low current" devices, where transistors are the opposite ("low voltage, high current"). Tube amplifiers are generally not very efficient and tend to generate a lot of heat. One of the biggest differences between a tube amplifier and a transistor amplifier is that an audio output transformer is almost always required in a tube amplifier (this is because the output impedance of a tube circuit is far too high to properly interface directly to a loudspeaker). High quality audio output transformers are difficult to design, and tend to be large, heavy, and expensive. Transistor amplifiers have numerous practical advantages as compared with tube amplifiers: they tend to be more efficient, smaller, more rugged (physically), no audio output transformer is required, and transistors do not require periodic replacement (unless you continually abuse them). Contrary to what many people believe, a well designed tube amplifier can have excellent sound (many high end hi-fi enthusiasts swear by them). Some people claim that tube amplifiers have their own particular "sound". This "sound" is a result of the way tubes behave when approaching their output limits (clipping). The onset of output overload in a tube amplifier tends to be much more gradual than that of a transistor amplifier. A few big advantages that tube amplifiers have were necessarily given up when amplifiers went to transistors. First, tubes can withstand electrical abuse that would leave even the most robust transistor completely blown. Also, tube amplifiers use an output transformer to interface to the speaker; such a device provides an excellent buffer (protection to the speaker) in the case of internal malfunction. Modern amplifiers (with no output transformer) occasionally fail in a way that connects the full DC supply voltage to the speaker. If the amplifier does not have adequate protection circuitry built in, the result is often a melted woofer voice coil.

Power amplifiers get the necessary energy for amplification of input signals from your car's alternator and battery. If you had a perfect amplifier, all of the energy it took from the alternator would be converted to useful output (to the speakers). However, in the real world no amplifier is 100% efficient, so some of the energy from the alternator is wasted. The vast majority of energy wasted by an amplifier shows up in the form of heat. Heat is one of the biggest enemies to electronic equipment, so it is important to ensure adequate air flow around equipment (especially so for those units using (passive) convection cooling).

Many amplifiers have a number of features to help monitor the status of the amplifier and also to protect speakers (and the amplifier itself) in the event of an overload condition. Some features include power meters, clipping indicators, thermal overload shutdown, over current protection, etc. Features vary from manufacturer to manufacturer. In addition, there are many variations in how protection circuits are implemented and how much "safety margin" they allow. For example, I tested the clipping indicator on one particular amplifier. The clipping indicator did not come on until there was a substantial amount of clipping actually occurring (as viewed on an oscilloscope). In this case, I did not notice a significant degradation of the sound quality despite the clipping. The manufacturer in this case chose to "allow a little more volume" before actually lighting up the warning light.

A well designed amplifier in the 200 watt per channel class may be a better investment than a marginally designed 500 watt per channel unit.

What are the functional blocks of an amplifier?

All power amplifiers have a power supply, an input stage, and an output stage. Many amplifiers have various protection features which fall into a category we'll refer to as housekeeping.

Power Supply: The primary purpose of a power supply in a power amplifier is to take the 12-16VDC power from the alternator or battery and convert it to a DC voltage of a higher value, in a good amplifier, consisting of at least three times that of the amplifier's output. Many different types of power supplies are used in power amplifiers, but in the end they all basically aim to generate DC voltage for the transistor circuits of the unit. The very best of stereo amplifiers have two totally independent power supplies (they do share a common DC power cord though). Such amplifiers are really just two monaural (monoblock) amplifiers mounted in a single case.

Input stage: The general purpose of the input stage of a power amplifier (sometimes called the "front end") is to receive and prepare the input signals for "amplification" by the output stage. Balanced inputs are much preferred over single ended inputs when interconnection cables are long and/or subject to noisy electrical environments because they provide very good noise rejection. The input stage also contains things like input level controls (input sensitivity, or gains) Some amplifiers have facilities for "plug in" modules (such as filters); these too are grouped into the input stage.

Output stage: The output stage of an amplifier is the portion which actually converts the weak input signal into a much more powerful "replica" which is capable of driving high power to a speakers. This portion of the amplifier typically uses a number of "power transistors" (or MOSFETs) and is also responsible for generating the most heat in the unit(unless the amplifier happens to have a very bad power supply design). The output stage of an amplifier interfaces to the speakers.

What are Amplifier Classes?

The Class of an amplifier refers to the design of the circuitry within the amp. There are many classes used for audio amps. The following is brief description of some of the more common amplifier classes you may have heard of:

Class A: Class A amplifiers have very low distortion (lowest distortion occurs when the volume is low) however they are very inefficient and are rarely used for high power designs. The distortion is low because the transistors in the amp are biased such that they are half "on" when the amp is idling. As a result, a lot of power is dissipated even when the amp has no music playing! Class A amps are often used for "signal" level circuits (where power is small) because they maintain low distortion. Distortion for class A amps increases as the signal approaches clipping, as the signal is reaching the limits of voltage swing for the circuit. Also, some class A amps have speakers connected via capacitive coupling.

Class B: Class B amplifiers are used in low cost, low quality designs. Class B amplifiers are a lot more efficient than class A amps, however they suffer from bad distortion when the signal level is low (the distortion is called "crossover distortion"). Class B is used most often where economy of design is needed. Before the advent of IC amplifiers, class B amplifiers were common in clock radio circuits, pocket transistor radios, or other applications where quality of sound is not that critical.

Class AB: Class AB is probably the most common amplifier class for home and mobile audio and similar amplifiers. Class AB amps combine the good points of class A and B amps. They have the good efficiency of class B amps and distortion that is a lot closer to a class A amp. With such amplifiers, distortion is worst when the signal is low, and lowest when the signal is just reaching the point of clipping. Class AB amps (like class B) use pairs of transistors, both of them being biased slightly ON so that the crossover distortion (associated with Class B amps) is largely eliminated.

Class C: Class C amps are never used for audio circuits. They are commonly used in RF circuits. Class C amplifiers operate the output transistor in a state that results in tremendous distortion (it would be totally unsuitable for audio reproduction). However, the RF circuits where Class C amps are used employ filtering so that the final signal is completely acceptable. Class C amps are quite efficient.

Class D: The concept of a Class D amp has been around for a long time, however only fairly recently have they become commonly used. Due to improvements in the speed, power capacity and efficiency of modern semiconductor devices, applications using Class D amps have become affordable for the common person. Class D amplifiers use a very high frequency signal to modulate the incoming audio signal. Such amps are commonly used in car audio subwoofer amplifiers. Class D amplifiers have very good efficiency. Due to the high frequencies that are present in the audio signal, Class D amps used for car stereo applications are often limited to subwoofer frequencies, however designs are improving all the time. It will not be too long before a full band class D amp becomes commonplace.

Other classes: There are many other classes of amplifiers, such as G, H, S, etc. Most of these are variations of the class AB design, however they result in higher efficiency for designs that require very high output levels (500W and up for example).
At this time I will not go into the details of all of these other classes.

Why do Amplifiers have different power ratings for different "ohms"?

Power amplifiers are typically rated for "4 ohm" and "2 ohm" loads, and some also give ratings for "1 ohm" loads. If you have ever looked at a spec sheet, you probably noticed that the power output of an amplifier is higher when the load impedance (number of ohms) is lower. Important: a load with a low number of ohms is a more difficult load to amplify than one with a higher number of ohms! (that is, a 4 ohm speaker is harder for an amplifier to drive than an 8 ohm speaker). The performance of an amplifier with low impedance loads is closely related to the capabilities of its power supply.

If we had a perfect amplifier (and it was plugged into an outlet that had unlimited current capability), its output power rating would double each time the load impedance was halved. For example, let's say the amplifier puts out 200 watts per channel at 8 ohms. At 4 ohms, it would put out 400 watts per channel, at 2 ohms it would put out 800 watts per channel, and at 1 ohm it would put out 1600 watts per channel. For the perfect amplifier, one could keep going with this until the load impedance approached zero, at which time the amplifier output would approach infinity! On the other side, if the load impedance was 16 ohms, the amplifier would put out only 100 watts per channel. In this direction, one could keep raising the load impedance, and the power output would grow smaller and smaller.

The power supply of the perfect amplifier generates a DC voltage that does not change no matter how much current is demanded from it. This means that the perfect amplifier can drive an unlimited number of speakers. In the real world, amplifiers have real power supplies which do have limits as to how much current they deliver. For such typical amplifiers, the 2 ohm power rating is usually about 50% more than the 4 ohm rating. Amplifiers with exceptional power supply designs will do better than this, but eventually a limit will be reached (if by nothing else the alternator can only deliver so much current!). Lesser designs will "run out of juice" when driving the heavier loads. Stay away from amplifiers that have a 2 ohm rating that is less than 25% greater than the 4 ohm rating!

Amplifiers utilizing exceptional power supply designs will invariably be the more expensive units available, and possibly the (physically) heavier designs. This is because good power supply designs usually require heavier and better (low loss)"magnetics". All power supplies utilize some combination of transformers, rectifiers, capacitors,and in the case of so called "digital" amplifiers, switching components.

"Analog" Amplifiers: An analog signal is a continuous wave signal, a digital signal is an analog signal which has been converted to a sequence of numbers. Analog when spoken in terms of power amplifiers typically refers to the design of the power supply, and most analog amps are those with a straight Class AB design. A so called analog amplifier has a power supply which typically uses a large power transformer, and large capacitors. These two basic devices step up the DC voltage from the charging system to a higher voltage (more suitable for the internal needs of the unit), and filter and store energy. These types of power supplies have been around for many years; they are simple and reliable. The downside is that the power transformer is usually large and quite heavy (the transformer core utilizes a considerable amount of iron), and the capacitors (a minimum of two are normally used) are also large and bulky.

"Digital" Amplifiers: When the term digital is associated with a power amplifier, it often refers to the design of the power supply and that the power supply is of the switching type (sometimes referred to as a DC - DC converter). Also, digital amps are often of one of the more exotic classes (class G, H, S, etc). These classes of amplifiers use special switching circuits that change the power supply voltage to the output stage on the fly such that higher efficiency is maintained. NOTE: A digital amp in no way means that it is inherently better at producing sound from "digital" sources such as CD's and DAT's!!! I don't recall any manufacturers calling their amplifiers "digital", but I have heard salespeople use this term. What advantages do a switching power supply offer? They are able to use much smaller transformers and capacitors, and are therefore considerably smaller and lighter than an equivalent analog power supply. The concepts behind switching power supplies have been known for many years. However, until fairly recently the components necessary for switching power supplies were unable to be produced cheaply enough for consumer use. Advances in transistor technology have made the necessary devices available at a cost which permits their widespread use. (Note:ALL of the "super systems" heard in automobiles today are powered by amplifiers using switching power supplies).

On the minus side, switching power supplies are a great deal more complicated than their analog counterparts. They work basically by first creating a "crude" DC voltage. This crude voltage is applied to a circuit which uses a specially designed high frequency transformer. A control circuit monitors the output voltage of this stage and makes adjustments "on the fly" in order to keep the final DC output voltage as close to the design value as possible. So, the advantages of lighter weight and smaller size come at the expense of increased part-count(which ultimately might translate to less reliability if the parts are of lesser quality). Also, switching power supplies are harder to repair if they fail.

Many "digital" amplifiers also use a "multi-rail" power supply system. Such systems are more complicated than conventional amplifier designs, however they offer considerable improvements in amplifier efficiency. The amplifier selects a "rail voltage" based on the output demands of the amplifier. Higher efficiency is achieved by minimizing the voltage drop across the amplifier's output transistors. Since less of the amplifier's power is wasted as heat, the power supply and transistor heat sinks do not have to be as large as those in a "conventional" design. As before, the theory behind "digital" designs has been known for decades, but until recently components necessary to make unaffordable design were unavailable.

"Analog" vs. "Digital"... Which is better?

Many of the amplifiers on the market today are of the "digital" type, using switching power supplies and/or special power supplies that maintain high efficiency at high outputs. Some people believe that "digital" amplifiers are not so good at producing powerful bass notes. While it is true that there are probably some marginally designed "digital" amplifiers which do have less than ideal bass response, weak bass response is not a necessity of digital designs. The dominating factor in performance comes back to the ability of the power supply to provide adequate current; a solid design means adequate current is available for loud bass notes and/or difficult speaker loads. In addition, a second important factor is the adequacy of the charging system. Two well designed amplifiers (one of each type) operated on a DC voltage rail which doesn't "sag" should both provide excellent sound quality. Many of the higher power amplifiers available today are of the "digital" (switching power supply) design. But keep in mind that this does not necessarily make them better or worse. Stay with vendors that have proven track records of reliability and you should have few problems with either type of design.

Power Ratings

Two amplifiers with the same power rating put out the same power, right? Not necessarily. Manufacturers vary as to how conservatively they rate their amplifiers. As an example, I measured one particular amplifier, rated at 350 watts/channel, and found it actually was able to put out 450 watts/channel! Manufacturers often understate what their units will actually putout. It would be a bad idea to publish the "absolute maximum power" that the unit could put out, since a margin needs to be allowed to insure that all production units will meet published specs. In addition, a manufacturer may publish a very conservative 4 ohm rating in order to make the 2 ohm rating look better (a really terrible amplifier will put out LESS power into a 2 ohm load!).

Amplifiers are generally rated in watts per channel , at several load impedances, with both channels driven, over a frequency range of usually 20 Hz - 20,000 Hz, at some amount of total harmonic distortion. Most amplifiers will put out slightly more (but not a tremendous amount more) power when only a single channel is driven. This occurs because the power supply only has to provide power for a single channel, and its DC voltage doesn't sag as much. The exception is amplifiers which use dual independent power supplies (since each of their supplies only has to supply power for one channel anyway).

A word on speakers is in order. All speakers have a characteristic known as impedance (measured in ohms), with most speakers being either 8 ohms, 4 ohms or 2 ohms. Lower impedances represent more difficult loads for amplifiers to drive. Two 4 ohm speakers connected in parallel will result in a 2 ohm load at the amplifier. And, two 2 ohm speakers(wired in parallel) result in a 1 ohm load. In actuality, speaker impedance can vary by a factor of 10 or more over the audio frequency range. When a speaker is said to be 8 ohms, it is understood that this is a nominal or approximate rating (the same goes for 4 ohm speakers). An 8 ohm speaker could have an impedance as low as 2 or 3 ohms and as high as 50 ohms (impedance is frequency dependent)! Further, a speaker load is not the same as a resistive load, speakers are reactive loads. A reactive load is a load that has inductive or capacitive properties. Depending upon the input signal frequency, speaker loads may be resistive or resistive with an inductive or capacitive component. Without going into a ton of technical explanation, what this means is that speakers are often difficult loads for amplifiers to drive. Driving difficult speaker loads is where better amplifiers are separated from lesser designs.

Even though an amplifier may be rated for continuous use at 2 ohms, there are several reasons why this is not the best thing to do:

Paralleled speaker loads may be lower than you think: As stated before, the actual impedance varies and the minimum impedance may dip considerably below 2 ohms at certain frequencies. Lower impedance loads mean more losses and more heat dissipation in the amplifier (see next item).

Heat Considerations: Operating an amplifier with a low impedance load increases the heat dissipation of the amplifier (try it if you don't believe it!). This is because low impedance loads require more current, which taxes the amplifier's power supply more severely. More current means more losses(which translates to more heat). Excessive heat is unhealthy for electronic devices and should be avoided.

Increased Line Losses: As the speaker impedance is lowered, more of the audio signal is lost (in the form of heat) in the speaker cables! This can become significant if you run long cables. Speaker wires have resistance (the value depends on the thickness and length of the cable); if the speaker impedance becomes very low the resistance of the speaker wire may no longer be insignificant. To prevent this problem, the cross sectional area of the speaker cable conductor must double for each halving of speaker load impedance! In other words, running 2 ohm loads means using VERY heavy speaker cables.

Damping Factor degradation: Using super low impedance loads on an amplifier will degrade the system's damping factor (discussed in detail below). Degradation of damping factor means that the amplifier will have less "control" over the speaker system, possibly resulting in "boomy" bass response.

So, just because an amplifier has a super powerful 2 ohm rating, don't look for ways to wire up multiple speakers in order to "use" this power! Treat the 2 ohm rating as "headroom" and know that your amp has the ability to more easily handle the most difficult "normal" speaker loads that you are likely to ever encounter. If you need more power, get a second amp. Two medium powered amps are better than one monster (what if your one big amp dies? With two smaller amps at least you can still run!).

All amplifiers generate a certain amount of electrical noise. Generally, the more powerful the amplifier, the more noise. If you turn on an amplifier (with the input jacks disconnected) and listen to a speaker you can clearly hear a hissing sound. This pretty much represents the noise floor of the amplifier. For a powerful system, the noise might seem pretty obvious; however when actual music is playing the noise will be totally masked.

All electrical circuits generate a certain amount of noise. Better designs minimize the amount of noise, however no matter how good the design there will always be some. The noise is generated by the movement of electrons in the system and cannot be eliminated (unless you chill your equipment to absolute zero!). The noise floor of an amplifier by itself is usually not obviously audible in a typical car (unless you are sitting right next to a speaker). However, the remaining components in a system (preamp, equalizer, processors, etc.) each add in some noise. So, the total system noise (when no music is playing) might be objectionable. If this is a serious problem, a device called a noise gate can be used. Such a device is essentially a "squelch" which is wired in just before the power amps (or electronic crossover in multi-way systems). The device basically cuts noise from upstream components when no music is playing. Most noise gates have adjustable controls to set the threshold at which noise cut begins and also to set the amount of desired noise cut.

The noise floor of an amplifier is relatively constant, meaning it does not increase with increasing output signal (unless the amplifier has a poorly regulated power supply). In other words, the amplifier's noise floor is pretty much the same whether or not music is playing loudly or softly. So, when music is playing softly, the noise will be proportionally larger. When music is playing loudly, the noise is essentially "buried" or masked.

As stated, an amplifier with a poorly regulated power supply can create some additional noise. If the filtering of the power supply is marginal, the "smoothness" of the DC power supply voltage will be degraded when the amplifier is playing loudly. This will result in additional noise being added to the system (generally in the form of alternator whine). This type of noise isn't really part of the noise floor. Such noise is often inaudible when music is playing loudly. It can be clearly heard however when playing test tones at levels near the output limit of the amplifier (don't try this unless you are thoroughly familiar with testing practices... blown speakers will otherwise be the result!).


ALL amplifiers alter input signals, generally in two ways: they make them stronger (amplify them), and they add characteristics which did not exist in the original signal. These undesirable characteristics are lumped together and called distortion. Noise can be considered a type of distortion and was discussed in the above section.

Everyone is familiar with gross distortion, the sound quality that results when turning up a radio or boom box to "full blast". An excessive amount of amplifier clipping (see section below) results in hideous distortion that would be totally unsatisfactory for a sound system. However, not all distortion is blatant. In addition, there are several types, two of which will be discussed. Knowing what causes distortion will help you to prevent it from occurring. Knowing how to control distortion is important because excessive distortion can be detrimental to speaker systems (and your reputation).

Harmonic distortion: One common type of distortion is harmonic distortion. Harmonics of a signal are signals which are related to the original (or fundamental) by an integer (non decimal) number. A pure tone signal has no harmonics; it consists of only one single frequency. If 100 Hz pure tone signal was applied to the input of an amplifier, we would (upon measurement with special test equipment) find that the output signal of the amplifier was no longer pure. Careful measurements would likely show that several "new" frequencies have appeared. These new frequencies are almost certain to be integer multiples of the original tone; they are the harmonics of the original signal. In the case of a 100 Hz input tone, we might expect to find tones at 200, 300, 400, 500 (etc.) Hz. We would also probably notice that the odd harmonics are much stronger than the even harmonics (we will not go into the reasons why in this article). In a good amplifier, the harmonics will be much weaker than the original tone. By much weaker, we mean on the order of a thousand times for decent amplifiers.

All amplifiers are generally rated for Total Harmonic Distortion (or THD), usually at full power output over a given frequency band with a particular load. Good values are anything less than 0.5% THD. When an amplifier is measured for THD, a pure tone is applied to the input and the output is measured with special test equipment. The energy of the pure tone is measured, and the energy of the harmonics is measured. Those two values are compared, and a THD rating is calculated. A THD rating of 1% means that the total energy of all the harmonics combined is one one-hundredth of the energy in the fundamental.

Harmonic distortion (although certainly undesirable) is one of the more tolerable types of distortion as long as it is kept reasonably low. Distortion levels of 10% may be very tolerable with music so long as the 10% level is only "occasional." (10% THD on a pure tone can easily be heard by the human ear... but who listens to pure tones?) The reason that a seemingly high value of THD is acceptable for music is partially because many sounds in nature are rich in harmonics. Also, most decent cassette decks (which most people agree sound pretty good) have THD (off the tape that is) of several percent. Worse, even good speakers can have THD up to 10%, especially at low frequencies! All in all, the human ear can tolerate a fair amount of THD before it becomes objectionable.

Do two amplifiers with identical THD ratings sound the same, everything else being equal? Not necessarily (but differences will be subtle). The reason is that the THD specification states nothing about where the harmonics are in the frequency band. For example one amplifier could have a dominant harmonic at one frequency and a second amplifier could have a dominant harmonic at a very different frequency. Or, one amplifier could have a few "big" harmonics while a second has many weak ones. These situations could easily result in identical THD ratings. The variations could be easily measured with laboratory equipment. However do not be overly concerned. Minor variations in THD ratings will not cause major differences in sound when listening to music. With pure tones as input signals it might be fairly easy to discern which of two amplifiers was used (but again, who listens to tones?)

Intermodulation distortion: Intermodulation distortion is the second "major" type of distortion that is often specified for amplifiers. Intermodulation distortion is much more objectionable to the human ear because it generates non-harmonically related "extra" signals which were not present in the original. It is analogous to someone singing way off key in a choral group

Intermodulation distortion (sometimes abbreviated IM) is more complicated to test for and specify. Basically, two pure tones are simultaneously applied to the input of the amplifier. If the amplifier were perfect, the two tones (and only the two tones)would be present at the amplifier output. In the real world, the amplifier would have some harmonic distortion (as described above), but careful observation of the output signal (using laboratory equipment) would reveal that there are a number of new tones present which cannot be accounted for as a result of harmonic distortion. These "new" tones are called "beat products" or "sum and difference" frequencies, and are a result of the interaction of the two pure tones within the amplifier. No amplifier is perfect, all have some non linear characteristics. Whenever two signals are applied to a nonlinear system, new signals (in addition to the original two) are generated. For a good amplifier, the new signals are very small in relation to the two original tones. This is fortunate, since the ear can detect much lower levels of intermodulation distortion as compared to harmonic distortion.

It should be noted that distortion measurements on amplifiers are made with test tones. These tones are usually sine waves (pure tones), which represent the simplest possible test signal to measure and quantify. A music signal is an extremely complicated waveform consisting of many constantly changing sine waves. Since music has so many harmonics and frequencies present, quantifying how two different amplifiers will sound by using simple THD and IM specifications is extremely difficult. In other words, just because two amplifiers have the same published specs for THD and IM does not mean that they are equivalent. Fully and completely quantifying the technical performance of an amplifier would be extremely complicated and costly (and would probably have little benefit in the end). Most amplifiers available today (from reputable manufacturers) have THD and IM levels low enough to yield excellent performance (so long as they are not overdriven). This leads nicely into our next topic...

Platinum Member
Username: Glasswolf

Wisteria, Lane USA

Post Number: 11937
Registered: Dec-03
Clipping: What is this?

Clipping is a term which many people have probably heard, but may not fully understand. Very simply, clipping of an amplifier occurs when one tries to get a larger output signal out of an amplifier than it was designed to provide.

As stated before, all power amplifiers have a DC power supply which provides power to (among other things) the output stage of the amplifier. For most amplifiers, the power supply consists of a "plus" supply and a "minus" supply. The two voltages are often referred to as "rail voltages" or simply "rails". As an example, a 200 watt per channel amplifier (at 4 ohms) might have a power supply voltage (rails) of +/- 120 volts DC. This means that the output voltage which drives the speaker can never exceed + 120 or - 120 volts. If the amplifier is playing at near full volume, and someone cranks up the volume, the amplifier will attempt to put out more power. However, the power required to meet the sudden new demand for more volume cannot be met by the power supply voltage, which has limits of +/-120 volts in this example. The result is a waveform with the top portion (or peak) "clipped" off (hence the term "clipping"). Such clipping represents a distortion which is added to the waveform (and if it is severe enough it will be clearly audible). If a signal is severely clipped, the waveform takes on the shape of a "square wave", and the resulting sound will be absolutely hideous. Clipping can be easily observed using an oscilloscope attached to the amplifier output.

Clipping is not usually a major problem for amplifiers (unless it is extreme), but it can be very detrimental to speakers. Whenever clipping occurs, two things happen: (1) the spectral content of the music signal is altered (high frequency components are generated), and (2) signal compression occurs. If excessive clipping occurs, tweeters will be the first to blow followed by midrange drivers. Woofers are best equipped to survive clipping (unless the abuse is blatant or the subs are poorly designed.)

In general, clipping of an amplifier should be avoided. Use an amplifier that has clipping indicators, and pay attention to them! Occasional clipping is OK and probably not very audible. However if you find yourself clipping the amp most of the time, you should consider obtaining a stronger (or additional) amplifier.

Damping Factor... What is this?

The Damping Factor of an amplifier in general refers to the ratio of the amplifier's output load impedance (the speaker, nominally 4 ohms) to the output impedance of the amplifier. Ideally, the damping factor would be infinity (in other words, the ideal output impedance for an audio amplifier is zero ohms). Damping factor, like many amplifier specifications, is a function of many factors and is thus difficult to quantify with a single number. As such, "low end" manufacturers can have a "field day" with this spec, publishing fantastic numbers (however with no information as to how the measurement was made).

The damping factor of an amplifier depends greatly upon the speaker to which it is connected, the wire connecting the speaker to the amplifier, the signal frequency that the amplifier is sending to the speaker, and the power level at which the amplifier is operating, among other things. Damping factor is most critical at low frequencies, generally 100 Hz and below (i.e. frequencies that a woofer reproduces). At such frequencies, a high damping factor is desirable in order to maintain a "tight" sound. If an amplifier/speaker pair has a low damping factor, the bass response is likely to be "boomy", "uncontrolled", and "loose" sounding.

Specifying damping factor as a simple single number does not really tell the whole story. Damping factor is a ratio of two numbers, one of which (the speaker impedance) varies by a large amount depending upon frequency. This being the case, the damping factor will also vary considerably as a function of frequency. Most of the variation in damping factor is due to the characteristics of the speaker connected to the amplifier. The wire which connects the speaker to the amplifier has finite resistance which must be accounted for; basically it is lumped in with the impedance of the speaker. So, it is wise to use heavy speaker wire in order to minimize degradation of the damping factor.

As mentioned, the output impedance of an amplifier is ideally zero. In the real world, this is never the case. The next best thing would be a very low constant (non changing) impedance. Again, the real world does not allow this either. The output impedance of most amplifiers is relatively constant except for when they approach the last 10% or so of their voltage output. This is due to the nature of the waveform from which most power supplies obtain their energy (especially analog supplies) . What this means is that the output impedance of an amplifier tends to rise considerably as it approaches its output limit. As the amplifier's output impedance increases, the damping factor must decrease proportionally. In my opinion, if manufacturers specified the output impedance of their amplifiers, there would be a lot less ambiguity among the numbers.

High damping factor numbers go hand-in-hand with amplifiers that can drive very low impedance loads (these are amplifiers with power supplies capable of delivering tremendous current). If you want to "artificially" degrade the damping factor of your system (to hear the effects), a simple test can be done:
Listen to your system at a "healthy" volume (use a CD with lots of low, tight percussion type sounds); be sure to use a heavy gauge short length speaker wire. If you have a sound level meter, note the sound level at which you listened. Then, connect your speaker up through a 100 foot (give or take) wire with much smaller gauge (use #20 or higher). Play the same music as before, but make sure the volume (to your ears, not the volume control!) is the same (this is where the sound level meter comes in handy). The volume control on the amp will have to be turned up a bit to overcome the power loss in the smaller wire. You should be able to tell that the sound has changed (for the worse, in most people's opinion).

Do not be terribly concerned with damping factor when choosing quality equipment. Most of the good amplifiers and speakers available today will yield excellent sound when used together. To avoid degrading the damping factor of your system, simply follow these (easy) steps:

Don't load up an amp with multiple pairs of low impedance speakers

Use heavy gauge speaker wire, ESPECIALLY in long runs

Never wire resistors in series with your speakers (you can't change a 4 ohm speaker to 8 ohms by doing this!)

Use a heavy duty (i.e. 8 gauge or heavier) power cable wiring your amps.

Can I get a shock from the speaker connections on my Amp?

YES! Amplifiers in the 400 plus watt per channel range are not uncommon today. Such an amplifier will put out about 50 to 60 volts RMS to a speakers. While this is only about half the amount that comes out of a wall socket, it's definitely enough to be unpleasant if you are holding on to it!
Note: The US Military defines any voltage in excess of 30 volts as hazardous. Such a voltage can be generated by any amplifier in the 100+ watt per channel range.
Zapco's C2K series amplifier manuals actually state as a warning that their amps can produce over 120 volts AC at 60Hz, which is equal to the output of a wall outlet! Not the sort of thing you want to test with your tongue.

As a side note, it's not a good idea to plug in or unplug speakers when the amplifier is playing at high volume. The "make and break" of connectors can cause momentary short circuits, as well as voltage and current transients (none of which is healthy for the amp). The preferable procedure is to make all speaker connections (and disconnects) with the amp turned OFF.

What is "Bridging"?

Bridging an amplifier refers to configuring a two channel (stereo) amplifier to drive a single load with more power than the sum of the two original channels combined. For an example, a 100 watt per channel at 4 ohms amp may put out 400 watts(one channel at 4 ohms) after bridging.

There are important things to know about running an amplifier in the bridged mode:
An amplifier running in bridged mode has one output channel to which a load (speaker) can be connected. It is no longer a two channel (stereo) amp as far as input signals and loads are concerned.
If the amp you want to run in bridged mode does not have built in facilities for doing so, you should not attempt to use it in this manner (unless you are thoroughly sure of what you are doing).
If you run bridged amplifiers, you must pay close attention to speaker phasing (see next item). Otherwise, you may have "hollow" or "weak" sound.
You must pay close attention to speaker wiring. The manufacturer will state which terminal is really the "positive" connection when bridged.
The speaker output signals of a bridged amplifier are floating; such connections must never be connected to any grounded device (such as an external accessory power meter, for example). If you do make such an illegal connection, one amplifier channel is basically short circuited (worst case result is a blown amplifier!).
Amplifiers running in bridged mode are generally limited to speakers with impedance ratings of no less than 4 ohms (in other words don't use a 2 ohm speaker load unless the manufacturer specifically allows it).

Bridged amplifiers work basically as follows:
A single input signal is applied to the amplifier. Internal to the amp, the input signal is split into two signals. One is identical to the original, and the second is also identical except it is inverted (sometimes called phase-flipped). The original signal is sent to one channel of the amp, and the inverted signal is applied to the second channel. Amplification of these two signals occurs just like for any other signal. The output results in two channels which are identical except one channel is the inverse of the other. The speaker is connected between the two amplifier speaker output terminals. In other words, one channel "pulls" one way while the second channel "pulls" in the opposite direction. This allows considerably more power to be delivered to a single load.

If we had our perfect amplifier, upon bridging it we would have a single channel amplifier with exactly four times as much power as any one channel of the amplifier in "normal" stereo mode, assuming a 4 ohm speaker load. This is because the effective output voltage available to drive the speaker has doubled as a result of bridging. A doubling of voltage on a given load results in a fourfold increase of power delivered to that load. If we used a 4 ohm load on the perfect bridged amplifier, the output power would be a very substantial eight times the normal stereo single channel 4 ohm output! These numbers should give some clues as to why real world amplifiers cannot meet such expectations. Once again, we are back to limitations of the power supply. In reality, most amplifiers in bridged mode will put out about 3 times the power as any one channel of the amp in normal stereo mode. The fourfold increase cannot be achieved because the power supply is unable to provide the current required for such performance. With 2 ohm loads, the situation is compounded. The amount of current required to drive a 2 ohm load when in bridged mode will tax the amplifier's power supply to its absolute limits. Not to mention, the output stage may not be able to safely handle the extra heat that will be dissipated.
Bottom line: stay away from 2 ohm loads if you are running an amplifier in bridged mode!

Maximum Power Transfer Theory and Efficiency

Note: This section is intended primarily for engineering students or those with a deeper technical interest. The purpose is to provide a "real world" explanation of the Maximum Power Transfer theory and why it is NOT used in amplifiers designed for stereo systems.

Second year electrical engineering students have most likely covered the theory that basically states "maximum power is transferred to a load when the output impedance of the source is identical ("matched") to that of the load." The connection that some people fail to make is that maximum power transfer doesn't mean maximum efficiency! At best, if the maximum power transfer theory is used, efficiency will be only 50% (not such a good figure for an audio amplifier.) In other words, if an amplifier is designed for maximum power transfer to a load, fully one half of the energy required by the amplifier's output stage will be dissipated (i.e. wasted) in the source impedance.

For amplifiers used in stereo systems (audio amplifiers), the goal is to have the amplifier output impedance be as low as possible (ideally zero, but this is never achieved). If an amplifier were to have an output impedance of 4 ohms (a common value for speakers), maximum power transfer would occur. However two other bad things result. First, the efficiency of the amplifier is at best only 50%, meaning that the amplifier will generate a lot of heat. Secondly, the amplifier/speaker system will have a terrible damping factor. Damping factor basically refers to the ratio of speaker impedance to amplifier output impedance; high numbers are better. A low damping factor will not damage anything but it will tend to louse up the sound considerably. To maintain a "tight" sound, it is important to have the output impedance of the amplifier be as low as possible with respect to the speaker. Otherwise, the amplifier will not have as much control over the speaker. Speakers, being highly complicated electro-mechanical devices with reactive impedance properties, behave better when they are connected to an amplifier with an extremely low output impedance. Speakers tend to electrically "buck and kick" an amplifier when in operation; the best way to tame this behavior is to put a heavy "load" (i.e. an amp with a very low output impedance) on the speaker. An amplifier/speaker combination with a low damping factor will tend to have a "boomier" sound and poorer transient response, (such a sound is not always bad, some people actually prefer it!).

There is a quick test anyone can do to get a feel for what effect the damping factor has on a speaker system. Disconnect your speakers from the amplifier, remove the grille, and gently tap on the woofer cone. You will hear a low frequency sound, this is the "resonant frequency" of the speaker (in it's enclosure.) Note the characteristic of the sound as you tap the cone. Now, connect the speaker up to the amplifier, and turn the amplifier ON (but leave the volume at zero). Now tap on the speaker cone as before. You will observe that the sound has changed considerably. The sound will be much "tighter", and the cone will seem harder to move. This is because the amplifier has in effect "loaded" the speaker. The case where the speaker was disconnected from the amplifier represents the worst possible damping factor (zero).

Anyway, back to the topic of this section. Although there are many applications where maximum power transfer is desired, audio amplifiers are not one of them. Audio amplifiers generally deal with a considerable amount of power, so high efficiency is a more important design consideration. In addition, to maintain high quality audio, an audio amplifier ideally has an output impedance which is VERY small compared to the impedance of the speaker it will be driving. Note that using 2 ohm speakers on an amplifier will degrade the damping factor as compared to using 4 ohm speakers (total load.)

Platinum Member
Username: Glasswolf

Wisteria, Lane USA

Post Number: 11938
Registered: Dec-03
additional information on amplifier specs:

Slew Rate:

This is a term used to describe how quickly the output of an amplifier can track its input. Slew Rate is usually measured in V / usec. The higher the value (up to a point), the better the amp is at potentially reproducing the subtle nuances and dynamics associated with music reproduction.

Speaker Sensitivity and Efficiency.

Speaker sensitivity is a specification provided by all manufacturers of high-quality speakers. The sensitivity rating has no relation to sound quality, as some of the very best speakers have low ratings. Sensitivity ratings simply tell you how much sound a speaker will produce for a given power input.

Sensitivity ratings are given in decibels per watt at one meter, or db/Wm. So, with an input of one watt (usually white noise), a speaker with a sensitivity of 90 db/Wm will produce 90 decibels of sound at a distance of one meter. A sensitivity of 90 is considered average, with ratings of 87 and below considered low sensitivity and above 93 considered high sensitivity. To increase the volume by 3 db, you must double the power. So, using the example above, to make 93 db you would need two watts, and to make 96 decibels, four watts.

Most of the time your system is cruising along producing only a few watts. You need extra power for loud bass passages, crescendos in classical music, and other highly dynamic passages. Your speakers may need more than 10 times the average power to re-create these dynamic passages accurately, and if you are playing loudly to begin with, you may need an awful lot of power if you have speakers with a low sensitivity rating.

So, when you are buying an amplifier, consider your speakers, your vehicle size and how loudly you want to play. If you have sensitive speakers, you probably will not need as much power -- even 20 clean watts would probably be enough. If your speakers are only moderately sensitive, your vehicle is large or exceptionally noisy at highway speeds and you want to play loudly, you will need more power in order to faithfully reproduce dynamic passages.

"Sensitivity," which is expressed in dB, should not be confused with "efficiency" that is expressed as a percentage of power out relative to power in. Efficiency data for loudspeakers suffers from many problems such as failure to consider variations in frequency response.

Speaker efficiency is the ability of the speaker to do work or use power. The more efficient the speaker; the less power is required for the speaker to produce sound. Voice coil design, type and size of the magnets, speaker cone design and material, speaker size, etc. all play a critical role in determining speaker efficiency. However, speaker size is a good general method for guessing efficiency.

Typical speaker efficiency (for physicists) is about 5%. Meaning that for 100% power input, you get about 5% acoustical work back.

Keep in mind that when considering subwoofers, or any speaker that will get more than ~100 watts RMS of power, these measurements are affected by other factors that make this specification less than useful when choosing between speakers.

THD or Total Harmonic Distortion.

Back in the old days (1982) It was FTC mandated for the manufacturer to provide a comprehensive single criteria power specification. However, with the de-regulation craze of the 80's, this requirement was dropped. This left it up to manufacturers to determine how to advertise and display their product specifications with no commonly accepted standard for emasurement.
I'll attempt to explain how THD is measured.

Of a signal, the ratio of (a) the sum of the powers of all harmonic frequencies above the fundamental frequency to (b) the power of the fundamental frequency.

The THD is usually expressed in percent as distortion factor or in dB as distortion attenuation.

Measurements for calculating the THD are made at the output of a device under specified conditions.

Now, there are several ways to measure THD that are commonly used. One is defined by the FCC, and another by the EIA.

Years ago a number of papers were written on human hearing and harmonic distortion. What they found was that the human ear is very insensitive to harmonic distortion that is close to the main signal, and increases in sensitivity to harmonic distortion further away from the main signal. The second harmonic, which is an octave away from the main signal, is the hardest to hear, especially when you are driving a loudspeaker.
The best estimates that I can give you is that we can detect somewhere between 1% and 3% of second order harmonic distortion. Which is why you can't hear it. If the sum total distortions were farther away from the main signal you would be able to hear it. Some solid state designs can have pretty low distortion but they can get to be aggravating after awhile. That's because the distortion generated by the amp is further away from the main signal where the ear is more sensitive.

Without going into too much detail, there are many factors in how THD can be measured, including but not limited to:
is the signal being used to measure THD a notch frequency, of full 20Hz-20KHz at equal power?
Over what unit period of time is THD being measured?
What kind of signal is being used to measure the distortion?
Point being, very few manufacturers specify this data, so THD is helpful at times, but again, not something to base a purchase on.
Remember, some of the best amplifiers in the world have an advertised THD of between 1% to 10%.
It's commonly agreed that distortion below 1% is inaudible, and in a car, below 10% is inaudible to the human ear.

There are two widely accepted ways of measuring THD. One is mandated by the FCC, and is the best way to measure distortion for car audio amplifiers. The other method is defined by the EIA and is far less acceptable for accurate audio amplifier comparisons.

S/N or Signal to Noise ratio

reference links:

signal-to-noise ratio (SNR):
The ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time.

SNR is expressed as 20 times the logarithm of the amplitude ratio, or 10 times the logarithm of the power ratio.

SNR is usually expressed in decibels (dB) and in terms of peak values for impulse noise and root-mean-square (RMS) values for random noise. In defining or specifying the SNR, both the signal and noise should be characterized, e.g., peak-signal-to-peak-noise ratio, in order to avoid ambiguity.

The sSNR of car amplifiers today is below the threshold of human hearing, so this emasurement is of little use when comparing amplifiers. Factors such as slew rate, damping factor, and power supply voltage are more important in determining the quality of an amp.

Amplifier Damping Factor

Damping factor is rarely published with low to medium grade amplifiers but it is almost always published with high end American amplifiers. And even when it is published, it is rarely published correctly. The damping factor is the ratio between the load impedance and the amplifier's internal impedance (load impedance divided by internal impedance). Like output power ratings, the damping factor is an amplifier characteristic that cannot be represented by a simple number.

An impedance value is a complex number made up of a real term and an imaginary term. The real term comes from the resistance of the object being measured. For example, if you measure the resistance of an 8 ohm driver with an ohmmeter, you will find that it is around 6.3 ohms. Some times, this is referred to as the DC resistance, but this is being redundant because a resistance value by its nature must be taken at DC. The imaginary term is from the inductance and reactance of the object being measured. A driver's voice coil, for example, is made up of winds of wire. The resulting effect is an inductor contributing a significant amount of inductance to the impedance value. There is also a bit of reactance caused by inherent capacitance between parallel wires in the driver assembly but it is usually small enough in a driver that its value is negligible.

Those familiar with a complex value will know that its behavior is dependent on the frequency of the source signal. The impedance of a driver might be 8 ohms at say 100Hz but it could be 30 ohms at 1kHz. Thus any measurement taken that is dependent on the complex impedance of a driver will also be dependent on the frequency of the source signal. So the damping factor of an amplifier will be dependent on the frequency of the signal that the amplifier is generating, which is the reason why you can't just give a single number as the damping factor like most manufactures do.

As if that isn't complicated enough, keep in mind that the impedance graph is different for each driver, the amplifier's internal impedance is also complex, and you have to figure in the impedance contributed by the wires and connectors used in the signal path. In other words, it is impossible to accurately specify a damping factor. This is all fine, but what bothers me is that most high end amplifier manufacturers just publish a number with no indication to the uselessness of such a simple representation of what is a complicated relationship between the amplifier and the load.

Not all amplifier manufacturers are lazy so some come up with ways to specify the damping factor. One way manufacturers specify it is to limit the various conditions that the damping factor is dependent on. Thus they may specify a damping factor of "200 at 1kHz with a 4ohm impedance load at the amplifier output terminals". So in other words, if you put a load with an impedance of 4 ohms at 1kHz across the amplifier's output terminals and the frequency generated by the amplifier is 1kHz, the ratio between the load impedance and the amplifier's internal impedance is 200. Which means that the amplifier's internal impedance at 1kHz is 0.02ohms. The amplifier's internal impedance at 1kHz will stay the same but damping factor will fluctuate depending on the load impedance used and the wires/connectors used in the signal path. For example, if you use a driver with an impedance of 8 ohm at 1kHz instead, then the damping factor becomes 400. Conversely, if you use a driver with an impedance of 2 ohm at 1kHz, the damping factor will be 100.

For reasons that I will indicate later, the damping factor is mainly significant for amplifiers used to power sub woofers. Given that, a damping factor given at 1kHz is pretty much meaningless since sub woofers are usually limited to producing frequencies below 100Hz. How do you get around this then? Well some manufacturers publish damping factors as "greater than 200". What this means is that provided a load with a constant impedance of 4ohms across the frequency spectrum, the damping factor measured at the amplifier's output terminal is greater than 200. Which is the same as saying that the internal impedance of the amplifier will never rise above 0.02ohms from 20Hz to 20kHz. This is the best solution to the problem that I've seen so far since it specifies everything the amplifier's manufacturer can specify. The damping factor specified this way is only dependent on variables controlled by the consumer such as the driver, wire and connectors used. Usually, a damping factor of greater than 50 is considered adequate, though most high end amplifiers have a damping factor of greater than 200.

With that said, why should you care about the damping factor at all? If it is so complicated to specify, why would we want to know it in the first place? Well, the significance of the damping factor is twofold. First, and perhaps more obscure and lesser well known, the damping factor indicates the efficiency of the output device (transistors) used in the amplifier. Second, the damping factor indicates the amplifier's ability to control the motion of a driver.

The load and the output device of an amplifier makes a complete circuit and whatever current flows through the load also flows through the output device. Thus if the amplifier is putting out 2 amperes of current, then the same 2 ampere of current is flowing through the load and the output device of the amplifier. The total power dissipated in the complete circuit is then the current squared multiplied by the total impedance in the circuit. Lets assume a damping factor of 200 for a load impedance of 4ohms. Thus the amplifier's internal impedance is 0.02ohms. The total impedance in the circuit is then 4.02 ohms. Multiplying 2 squared and 4.02 together we get 16.08 watts. Of this power, 0.08 watts is dissipated by the amplifier's output device and the rest is delivered to the load. Thus about 0.5 percent of the power is wasted by the amplifier's output device. Because this percentage of wasted power is rather small compared to the overall wasted power in the whole amplifier (around 50 percent), it is rarely mentioned. But it is nonetheless indicated by the amplifier's damping factor.

The damping factor is most often used as an indication of the ability of an amplifier to control the motion of a driver. When a signal sent to a driver is suddenly stopped, the driver's cone continues moving back a forth for a short period after the signal has stopped. A driver with a cone that stops quickly is said to have a good transient response while a driver with a cone that does not stop quickly is said to have a bad transient response and thus is described as inaccurate.
I think it goes without saying that most people would prefer a driver with good transients and thus would prefer that the cone of the driver stop quickly after the source signal stops.
With tweeters and mid-bass drivers, this not a hard task since the cones of these types of drivers are relatively light and a relatively large motor structure can be used to control the motion of the cone. However, the cone of a low frequency driver is quite sizable and it is physically impractical to use a motor assembly large enough to obtain transient responses as good as that of a tweeter or a mid-bass driver. Thus low frequency drivers usually have relatively poor transient responses.
This is really not too much of a problem since humans are less sensitive to distortion in the low frequencies. In fact, THD of 3 to 6 percent from a low frequency driver is considered acceptable. Low frequency distortion only becomes objectionable when it gets close to 10 percent.
Since drivers are just electric motors, they become generators when their terminals are shorted. If you place an ammeter across the leads of a driver and push the cone up and down, you will see a current flowing through the ammeter. The higher that current is, the more difficulr the cone becomes to move. Thus, if a driver's cone is moving, the quickest way to stop it is to place a dead short across its leads.
The internal impedance of an amplifier is usually very small and in the absence of a source signal, it is like a short across the leads of the driver. The amplifiers with a higher damping factor will have a lower internal impedance so it will be closer to a short, thus the amplifier with a higher damping factor will cause the driver to stop quicker than the amplifier with a lower damping factor. Since low frequency drivers need all the help they can get to stop their cone from moving when the source signal stops, a high damping factor is desirable for an amplifier intended to power low frequency drivers. The damping factor is not as relevant when the amplifier is used to power mid-bass and tweeter drivers since those drivers already have pretty good transient response due to their relatively small cone size.

Amplifier Classes

There are five main amplifier designs: Class A, A/B, B, D, and Tube amplifiers. All of these but tube amplifiers are considered "solid-state."

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. Pure 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 mode. At higher volumes, the amplifier switches to class B operation.

The class D amplifier (known as digital amplifier) is the last of the solid-state types. These amplifiers are not really digital (there is no such thing), but operate similarly in manner to a digital-to-analog converter (DAC). The signal that comes in is sampled a high rates, and then reconstructed at higher power. This type of amplifier produces almost no heat and is very small in size. Efficiency is much higher in class D amplifiers (~80%).
The sound quality of a Class-D amplifier is much lower than that of other solid-state amplifiers, which is why Class-D amplifiers are only used for subwoofers in car audio. This is because the switching speed of the transistors, and lower sound quality are masked by the lower frequencies being reproduced by the subs, since distortion is harder to discern at low frequency.

other variations on a theme:

Class T: Class T (Tripath) is similar to class D with these exceptions: This class does not use analog feed back like its class D cousin. The feedback is digital and is taken ahead of the output filter, avoiding the phase shift of this filter. Because class D or T amplifier distortion arises from timing errors, the class T amplifier feeds back timing information. The other distinction is that this amplifier uses a digital signal processor to convert the analog input to a PWM signal and process the feedback information. The processor looks at the feedback information and makes timing adjustments. Because the feedback loop does not include the output filter, the class T amplifier is inherently more stable and can operate over the full audio band. Most listeners can not hear the difference between class T and good class AB designs. Both class D and T designs share one problem: they consume extra power at idle. Because the high frequency waveform is present at all times, even when there is no audio present, the amplifiers generate some residual heat. Some of these amplifiers actually turn off in the absence of music, and can be annoying if there is too much delay turning back on.

Class G: Class G improves efficiency in another way: an ordinary class AB amplifier is driven by a multi-rail power supply. A 500 watt amplifier might have three positive rails and three negative rails. The rail voltages might be 70 volts, 50 volts, and 25 volts. As the output of the amplifier moves close to 25 volts, the supply is switched the 50 volt rail. As the output moves close to the 50 volt rail, the supply is switched to the 70 volt rail. These designs are sometimes called "Rail Switchers". This design improves efficiency by reducing the "wasted" voltage on the output transistors. This voltage is the difference between the positive (red) supply and the audio output (blue). Class G can be as efficient as class D or T. While a class G design is more complex, it is based on a class AB amplifier and can have the same clean characteristics as well.

Class H: Class H is similar to class G, except the rail voltage is modulated by the input signal. The power supply rail is always just a bit higher than the output signal, keeping the voltage across the transistors small and the output transistors cool. The modulating power supply rail voltage is created by similar circuitry that you would find in a class D amplifier. In terms of complexity, this type of amplifier could be thought of as a class D amplifier driving a class AB amplifier and is therefore fairly complex.

Lastly we have tube amplifiers, which aren't often used in car audio. Tube amplifiers have about 50 to 60% efficiency.
Tube amplifiers are said to sound more musical. The reason is that tube amplifiers produce even ordered harmonics. Musical instruments give off harmonics in even orders. Transistor amplifiers tend to give off harmonics that are odd ordered. These harmonics are not pleasing to the ear as second order harmonics are. Modern solid state amplifiers have very low distortions but their distortions are less tolerated by the ear than even ordered harmonics. This means that when you hear someone say a Tube amp is "warm" sounding, they are actually talking about the second order distortion produced by that tube amplifier, which they find pleasing to the ear. A good example of this is in guitar amplifiers, which often pride themselves on their second order harmonics.
One should note that while most solid state amplifiers have very low distortions (Total Harmonic Distortion) for the left and right channel, other channels are often much higher as these specifications are rarely noted. Subwoofer amplifiers are particularly bad at creating odd ordered harmonics.
I believe that the best tube and solid state amplifiers sound amazingly alike. Bad tube amplifiers sound tubby and slow. Bad transistor amplifiers sound harsh, bright and strident.

Just like you can't judge a good book by its cover, you can learn very little about an amplifier without digging in and seeing what is inside. Generally speaking, the most important component of any amplifier is its power supply. Is it sufficient? Is it accurate? Is it fast? Unfortunately, almost no amplifier company talks about their power supplies or what transformers they use (An example of a good company would be Eclipse, who uses dual toriodal transformers in their amplifier power supplies.)
I think most manufacturers would prefer you not ask.

I hope this clears up some of the more frequent questions regarding amplifier classes, as well as tube versus solid state amplifiers.

Bronze Member
Username: Baba


Post Number: 24
Registered: Mar-05
thank u so much,

Silver Member
Username: Sinful_systems70


Post Number: 689
Registered: Nov-06
It's not unbelievable when you know GlassWolf. He is the most suggested "best ecoustics member" and I feel he is very knowledgeable.

Silver Member
Username: Oleg

Santa Monica, CA USA

Post Number: 968
Registered: Nov-04
Glass ripped this thread a new one lol

Bronze Member
Username: Pete_the_pup

Gaffney, Sc Usa

Post Number: 28
Registered: Feb-07
Who needs College!! Get the WOLF!!! PtP
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