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Understanding Amplifier Power Ratings

Watts appear to be an easily understood figure that conveys a reasonable picture of how powerful an amplifier is at a glance. Or so it would be if the "kick it up a bit" marketing boys had not gotten to it. 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 in favor of, " that old marketplace magic." This basically means "caveat emptor" rules. We will do our best here to give you some basis for judgment, however.

There are a number of terms that apply to descriptions of a unit's power output as stated in watts.

A watt itself is a unit of energy that can be correctly described as the power used when energy is expended at the rate of one joule per second. A Joule is a quantity of work or heat that is generated equivalent to the force of one Newton moving an object at the point of application, a distance of one meter.

Now this wonderfully concise physical description can be as well applied to the actions of amplifiers upon their loads (speakers) as it can to any other phenomenon. When electrical power generated by the amplifier is applied to a load to do work, (move a cone back and forth) the resulting energy expenditure can be uniformly measured in a standard way, and applied to all other similar situations.

There is only one comparative power designation that is truly helpful, and that is: RMS

Which stands for Root Mean Square, the only reasonably accurate, consistent, comparative measurement of power exchange that should be used for both amplifiers and speakers. RMS is an average of the power transfer between a generator and normative load that is obtained by measuring the AC power with the formula P=EI cosine Ø where cos. Ø is the power factor.

Generally( but not always), RMS is half or less of the value usually attributed to alternative methods such as:

Peak power
Max power
IPP - Instantaneous Peak Power
PMPO - Peak Music Power Output

and other measurement methods, that while having a nebulous mathematical consistency, are still little more than schemes to create higher numbers to increase sales. Some manufacturers of low price components have recently taken to promoting power figures of as much as 10 times the RMS specification. Be wary ...very wary!

Many manufacturers provide specifications without mentioning what kind of watts are being measured. Again, this almost always means that those are not RMS watts, and the actual power is half or less (often, much less) than the number indicated.

Where are watts found?
It is important to remember that amplifiers create an amount of electrical potential that does not become watts untill the energy is converted to another form. This happens in the speaker, where the voice coil consumes watts of electrical energy as it converts it to sound amplitude as measured in decibels.

Multiple power points
Another favorite trick is to just throw out a number for woofers with two voice coils or amplifiers with 2 or 4 channels. This invariably turns out to be the maximum watts for all functions. So:"300 MAX watts Four Channel Amplifier" really means 150 watts RMS divided into four channels or 37.5 watts RMS per channel. Often it can mean less. Decidedly less impressive, eh? Or, for a woofer, "400 watt dual voice coil woofer" really means 200 watts RMS divided by two coils or 100 watts RMS per coil. This could yield disappointing results. Unfortunately, these descriptions are not always consistent. If they were, it would be easier for consumers to pin them down.

But that's not all! There are plenty more complications where these came from. For instance in amplifiers, there are amps with not only multiple channels, but the capability of being bridged. This means that any two ( but only two) channels can be bridged to form an amplifier of approximately double the capacity of a single channel. It does this by taking a mono input signal and dividing it between the positive peaks of the signal and the negative valleys, and amplifying each separately. These separated polarities are then reunited at the amplifier terminals to form a unified signal again for the woofer.

Thus, two 75 watt RMS amplifiers can become a single 150 watt RMS amp. In a four channel bridgeable amplifier, four can be come two, each with twice the power of a single channel.

But look out here! Most amplifiers are of the class AB variety. this means they can be bridged to a 4 ohm load only, yet some brands advertise the total of the 2 ohm load each channel can carry separately. This is very misleading and should not be confused with the amplifiers described in the next paragraph.

Multiple power ratings
Increasing numbers of amplifiers, particularly monoblock, high current or digital models, have the capacity to handle low impedance loads, sometimes all the way down to one ohm. As the impedance figure goes down, the wattage figure goes up. This reflects the fact that efficiency increases as the load becomes heavier (ohms figure gets smaller) Consequently, the greatest efficiency can be realized with amplifiers capable of delivering stable operation into very low impedance woofer circuits.

While 4 ohm remains the industry standard for many car amps and single coil woofers, capacities are tending downward, mostly because of the increase in the watt numbers attendant to lower impedance carrying ability. At a minimum, better quality units will list "2 ohm Stable" as a feature. Since most speaker loads are highly variable in operation, and will have occasions when operational impedance of a 4 ohm system can go below 2 ohms, it is good to have this capacity so that distortion is reduced at high amplitudes, especially.

Dual voice coil woofers can have coils that are 1, 2, 3, 4, 6, and 8 ohms. This means that such woofers can be configured to operate either singly or separately at the most efficient level. These woofers can be connected in a variety of ways to the multiple impedance amplifiers. Dual voice coils can be connected either in parallel or serial circuits. A woofer with two 4-ohm coils connected serially would produce an 8-ohm total, or in parallel (with both + terminals and both - terminals tied to one set of amplifier terminals) a 2-ohm load. Of course, a single voice coil can be used also, albeit at half the rated power. Check our Impedance Calcultor to get it right.

With two woofers connected in a serial circuit, two 2-ohm woofers can present a 4 ohm load, or two 1 ohm woofers can present a 2 ohm load. The best common configuration is to use two dual voice coil woofers with 4 ohm coils. All four coils in parallel will make a highly efficient 1 ohm load, with power handling equal to the total RMS of both woofers.

But in all cases, the idea is to match both the impedance and the output power capacity of both the amp and woofer units. A good relationship is for the Amplifier to supply between 10 to 40 percent more power to the woofer than the woofer's nominal RMS watt handling rating. Thus, a 200 watt woofer should optimally have a 225 to 280 watt amplifier channel connected to it.

But remember that all watt indications above are …. (what kind?) ….RMS, that's right.

Unfortunately, there are other ways in which watt measurements are taken that lend themselves to the kind of exaggeration and hyperbole that creates so much confusion for shoppers and owners of such equipment. Let us examine a few ways in which test conditions make it possible for manufacturers to make doubtful claims:

The power supply
All amplifiers will be able to supply larger waveforms when higher than usual voltages are applied to them. An amplifier that produces 200 watts at a normal 12 -13.5 volts may produce up to 300 watts when the power is boosted to 15 volts. Of course such an arrangement does not obtain in too many cars, and it is stressful on the amplifier, but that does not stop some sellers from using it.

Number of operating channels
When power is measured on a channel with only that channel operating, the resulting watts will be substantially higher than if all channels are operating and loading the unit's power supply uniformly - the condition in which most people operate their system.

Unspecified load factors
Some makers will use lower resistance than customary for obtaining power measurements, that is, a very low resistor value from half an ohm to 2 ohms and this is presented as the un-attributed, presumably standard impedance value. The value for the advertising cheat of not saying anything, is that the reader tends to presume everything.

Biased testing factors
Differences in ambient temperature, the amount of total harmonic distortion (THD) produced, and tested frequency bandwidth can also impact the number of RMS watts that can be produced. For instance a 250 watt amplifier operated to extreme capability will produce 500 watts RMS, but only with 18 percent distortion. While people are more tolerant of high distortion than is generally realized, this amount would be painfully unacceptable to most.


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