I was reading an article in PERFORMANCE AUTO & SOUND and thought it was interesting. So i decided to share it and maybe help a few noobs out... And its a good read. ENJOY!!!


"When talking about audio and specifically amplifiers and speakers, pretty much everyone is familiar with the terms "Watts"it is probably one of the most misunderstood and misrepresented terms in the audio industry. In this months column, I'm going to present some examples of why watts are so commonly misunderstood.

In the context of the audio world,Watts have traditionally been used as a measurement of audio power. The power could be amplifier output power, or the limit of the amount of power the specific speaker can handle safely. However, simply saying an amplifier has 100 Watts per channel is not exactly useful. To properly describe the power level of an amplifier, you need to know several other parameters used to make the measurement. Critical parameters generally include the level of distortion, the frequency of bandwidth used for the measurement and the impedance of the load being driven.

Using our 100 W example, it would tell me much more if the description said "100 W per channel, all channels driven, from 20 Hz to 20Khz into 4 ohms, with no more than 1% total harmonic distortion (THD). Without all the qualifying info, the watt figure is meaningless. Remember that next time you're shopping for an amp.

Over the years I have been asked how head unit, rated at 40 or 50 W per channel sells for the same price or more than a dedicated amplifier, which may only be rated at 25 W per channel. The difference lies in how the specifications were obtained. Many companies will measure the power of a head unit at 10% or even 30% THD. This much distortion creates an inflated power number, much to the joy of marketing departments . On the other hand, the dedicated amplifier rated at 25 W May use a more realistic distortion level limit of 1%, which more accurately describes that amplifiers actual useful power if we were to use a 1% THD limit on the amplifier built into the head unit, we would probably find that the actual power is now reduced to only 15 to 18 W per channel for reference most people can detect 1% distortion in a pure tone, but we'll except brief periods of high distortion in musical signals. However, No one would enjoy listening to music with 30% distortion!

Once we know the actual power at a given distortion level we can make more accurate comparisons. This is why the consumer electronics Association created the CEA 2006B amplifier standard, which evens the playing field and allows consumers to compare apples to apples, with meaningful specifications. All of our test reports follow these guidelines, so you can accurately compare the products reviewed.

But, it gets more complicated from there. To measure amplifier power on a TestBench, most engineers use high-power handling precision resistors this is okay because most amplifiers are measured this way but when we get into the real world and start connecting speakers to the amplifiers, things change. You see, the load resistors only provide a simple load impedance un like a loudspeaker, which presents a much more reactive type load to the amplifier.Because a speaker load includes resistance,inductance and capacitance, speakers are typically more difficult to drive then a resistor. For this reason, complex and expensive test equipment like the audiograph power cube is necessary to help engineers understand how an amplifier will react when driving a loudspeaker.

To those folks who believe they can derive amplifier power with a simple voltmeter and a current clamp, unfortunately you're wrong. I hate to burst your bubble, but unless you are also monitoring distortion as well as a difference in the phase relationship between the voltage and the current in the measurement, something known as power factor your measurement's are always going to be grossly inaccurate. If you understand enough about electricity and trigonometry to be able to calculate the cosine of the phase angle between the voltage and current to find the power factor, you are better equipped than most with a digit voltmeter and current clamp.

Everyone knows that when you overdrive an amplifier, it will distort. This distortion is characterize as a fuzzy sound in your speakers. The distortion is caused by the amplifier reaching its electrical limits and trying to exceed them. This situation is known as clipping because the voltage rails in the amplifier cannot be exceeded without clipping off the tops and bottoms of the waveform. What many people don't understand is that while the amplifier is being driven into clipping, the output power of the amplifier continues to increase and the waveform begins to resemble a square wave. Here in lies one of the most common reasons for burned loudspeaker voice coils.

You see, if you take a sinewave and a square wave of exactly the same voltage, the square wave actually produces double the power. Yes, that 25 W per channel amplifier is actually capable of producing 50 W of power if overdriven badly enough. Ever wonder how you could burn up a speakers voice coil rated at 50 W with a 40 W amplifier? Now you know."

If i messed a few words up my bad it was alot of chit to wright.