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Understanding receiver and amplifier power

02.14.2008 — It’s no secret that it takes a lot of knowledge to truly understand the world of home audio, which is littered with confusing numbers and terms.

Wattage, current, distortion, frequencies, and impedance are just a few of the specifications anyone looking for an amplifier of home theater receiver is going to come across.

With the thousands of choices in each category, it takes a little bit of background to use these specifications to make a great choice.

In this primer from TechLore, we take a look at the power specifications of amplifiers and receivers.

In Steps the Watt
Evaluating and stating the wattage of an amplifier has become the single most important number to the amplifier shopper.

Unfortunately, wattage is a highly misunderstood specification, which dupes people into buying products with a level of performance much lower than what is anticipated.

One-hundred watts seems to be the magic number for most, thinking that as long as you have 100 of them, and no less, they’ll be fine. However, even passenger side mirrors on your car say that things aren’t always as they appear.

To be fair, wattage is an important number that can give a clear understanding of an amplifiers performance. A Watt, named after the British scientist James Watt, is a unit of power.

Since power is expressed as a unit of energy divided by a unit of time, a watt is commonly defined as a unit of power equal to one joule per second. Simply stated, a watt is the amplifiers ability to do work, like make the drivers in a loudspeaker move.

Wattage in an amplifier is like horsepower in a car. The more horsepower a car has, the faster the car goes. But unfortunately, this isn’t always true.
Take into consideration that a 150 horsepower engine might be able to rocket a 1200-pound car, but it’s not going to budge a 5-ton semi-truck. The weight of the car is similar to the weight (efficiency) of the speaker connected to an amplifier.

Standardizing the Specs
In the competitive atmosphere that has taken over the entire electronics industry, manufacturers and marketers struggle to make their products appeal to the masses.

Massaging the specifications of their products is a sure fire way to attract buyers in a sea filled with similar products.

There is, to this day, no truly defined way to measure the power output of an amplifier. As you will learn below, the numbers do not mean much until they can be understood in context, and broken down in a manner that allows fair comparisons between products.

When amplifiers went from tube amplifiers (that use vacuum tubes) to solid-state amplifiers, power ratings for amplifiers got extremely confusing. Finally, the FTC stepped in ruled that all power claims had to include the conditions in which they were measured.

Unfortunately, the FTC did not rule that amplifiers had to be tested on the same playing field. This means that in order to make fair comparisons, one would need to know how to interpret what the conditions of the measurement means.
Specifically, the FTC ruled that:

  • Power measurements must be stated in RMS (sustained) watts.
  • The bandwidth (band of frequencies) used in the measurement must be stated.
  • The impedance (load) had to be stated.
  • The number of amplifier channels played into that load had to be stated.
  • Total harmonic distortion figures had to be stated.


Specifications Explained
More often than not, looking at amplifier specifications can reveal a great deal about the amplifiers real world performance.

Below is an explanation of each specification, and how to interpret them.

RMS Power
There are two ways of expressing power, peak and RMS. Peak power is the amplifier’s ability to provide an instantaneous burst of power.
However, peak power measurements are somewhat fictitious since they cannot be sustained for long. RMS (Root Mean Square) power ratings are a mathematically correct way to express an amplifier’s usable average power, which is a representation of real-world performance.

Bandwidth refers to the frequency range used to produce the claimed power specification.

Most amplifiers have a harder time producing the frequency extremes, such as low bass frequencies or high treble, than it does with midrange frequencies. Humans with perfect hearing can discern a frequency range from 20Hz to 20kHz (20,000Hz).

Since music and movie soundtracks involve a very wide range of frequencies, it is good to know what range the amplifier was tested under. This ensures that the amplifier can hold its own producing the dynamic effects of music and movies.
Many manufacturers often try to massage the specifications of amplifiers to give them a higher claimed wattage. When you see a power rating that says “x watts @ 1 kHz”, it means that they only tested one specific frequency to get its power rating.
Not only does this not give you any indication of whether or not the amplifier can hold its own during frequency extremes, but it is unfair to buyers since the measured wattage would be considerably less if all frequencies had been used.

Solid-state amplifiers usually produce lower wattage into higher impedances (electrical resistance) than lower impedances.

This, however, is not always true for tube amplifiers.

According to Ohm’s Law, a perfect amplifier would produce twice the wattage into a 4-ohm load than it does into an 8-ohm load.

But since lower loads can drain amplifiers quickly, the power supplies tend to produce less operating voltage. So in the real world, lowering the impedance of the speaker will not necessarily yield twice the wattage from the receiver.

When evaluating power specifications, the output wattage will typically be larger when a lower impedance is used for testing.

Number of Channels Driven
Long ago, when amplifiers had only two channels, the term “all channels driven” meant that the test included both amplifiers being tested simultaneously.

Today, products contain 5, 7, or even 10 amplifiers in one chassis, and this term can be a bit deceiving. It would make sense that “all channels driven” would mean that every channel in the product were all tested simultaneously, but unfortunately, this is not always true.

If, for example, a product contains 7 amplifiers, yet they only used two of them at the same time while testing, the claimed wattage on the receiver would be significantly higher than if all 7 amplifiers had been used.

It is important to read the fine print to find out exactly how many amplifiers had been used for the test.

Total Harmonic Distortion (THD)
Distortion measurements do not necessarily speak volumes.

At some degree or another, all amplifiers introduce distortion, which is when an amplifier changes the input signal. In most cases, the distortion increases as the power demanded grows.

Distortion measurements can give an indication of when an amplifier is operating in a non-linear fashion.

Manufacturers can also use this specification to their advantage. As stated above, claimed wattage is the effect of the test variables.

So, if the manufacturer chooses to turn up an amplifier until it reaches 10% THD, they could report a higher wattage (even though it would probably sound terrible). High wattage at very low distortion is desirable.

Comparing Specifications - An Example
Here are some real world power specifications to compare. While reading them, try to figure out which one is probably the better amplifier.

For this example, assume that both amps in question contain five channels.

Amp #1 - 75W per channel, 5Hz - 80kHz, @ 8 Ohms, 5 channels driven with no more than .08% THD.

Amp #2 - 100W per channel, 80Hz - 20kHz @ 6 Ohms, 5 channels driven with no more than 1.0% THD.

So the question on everyone’s mind is, “Which one is the better amplifier?” At first glance it may appear that #2 is the better amp, considering it boasts 25 more watts than amp #1.

However, the surprising answer is that amp #1 is probably the better amplifier. Here’s why:

As you can see, the stated frequency response on amp #1 is considerably wider than amp #2. This means that amp #1 was producing a much wider range of extreme frequencies than the second.

In fact, amp #2 cut out low bass frequencies entirely, indicating that it may not be able to hold up under extreme bass passages.

It also shows that amp #1 was tested using a higher impedance than #2, and as you learned above, higher impedances produce lower wattages.

Amp #2 used a 6 ohm load, which would give it a higher claimed wattage than if it had used an 8 ohm load.

Amp #1 has less distortion than amp #2. While distortion is not always a clear indicator of an amps sound quality, it does show that #1 is altering the input signal much less than amp #2.

Considering that amp #1 was tested harder than #2, it’s impressive that it maintained low distortion under a rigorous test.

This article was written by TechLore’s Matt Whitlock.


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