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Amplifier Power Ratings
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.

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.

The "X 4" implies that the amp has 4 output channels. The "RMS" stands for "root mean square" and is a method of measuring an AC waveform. More importantly here it implies that the power rating is not just a peak rating but continuous. "all channels driven" means that the power measurement was made with all channels of the amp driven to their maximum level at the same time. This means that the power supply is strong enough to allow all 4 output channels to produce 50watts at the same time.

This is a common place where head unit specs "cheat." They leave off the "all channels driven" and measure only 1 channel at a time which often gives a higher number. I've seen head units claming "30x4" which is meaningless but most people take it to mean that the head unit produces 30watts each into 4 channels. That's 120 watts from a head unit. No amp is 100% efficient so let us say it draws 150 watts to do this (80% efficiency which is still high). With a 12V power input, the head unit amp's power supply would be drawing 12.5 amps. I guarantee you that it is not easy to design a power supply that fits into a head unit leaving enough room for everything else (including the amp stages themselves) for any reasonable price that can deliver that kind of power. That is one reason why I say not preferable to use the head unit's power.

"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.

"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.

"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.

"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.

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