How to Compare Amplifier Power Ratings by Patrick Quilter

The pastor has just given you the OK to purchase a much-needed power amplifier. So you start your hunt, collecting stacks of product specification sheets to compare the specifications. After all, you can count on the numbers, right?

Wrong. Unfortunately, the reality is that product specifications aren’t as straightforward as they seem, and can easily be misinterpreted. But you won’t get stuck comparing apples to oranges if you know how to decipher the numbers to get the story behind the specs.

Although there are numerous specifications used to measure amplifier performance including noise, harmonic distortion, etc., this article will focus on comparing amplifier power ratings, one of the most important specs when looking for the right amplifier to match your speakers. Most amplifiers from reputable companies will have good performance within their power ratings.

Variables

Measuring amplifier power depends on several variables. The first variable is the level of distortion present in the amplifier output when making the measurement. If you drive an amplifier so that it occasionally clips (let’s say with 1% THD), you will measure more power than if you reduce the output so you only have 0.1% THD. This is one way manufacturers can show higher power numbers.

The second variable is the frequency range across which the power is measured. If you measure power at a single frequency, typically mid-band at 1 kHz, you will see higher numbers than if you measure over the entire audio spectrum (20 Hz- 20kHz). This is because power tends to roll off at lower frequencies, and distortion tends to rise at high frequencies.

A Tale of

Two Standards

Thankfully, there are two common standards that make it easier to compare amplifier output ratings: FTC and EIA. The FTC standard, established by the Federal Trade Commission, requires a manufacturer’s stated power rating must be met, with both channels driven, over the advertised frequency range – usually 20 Hz to 20 kHz – at no more than the rated total harmonic distortion (or THD). See Example 1.

The EIA rating, established by The Electronic Industries Association, reflects the power output for a single channel driven at mid-band – typically 1 kHz – with 1% THD clipping. This standard (shown in Example 2) inflates the amplifier’s power points to 10 to 20% higher than the FTC ratings.

Of the two, the FTC rating tells you much more about the product than the EIA rating. The FTC rating gives you the average power output for both channels over a wide frequency range and lower distortion level. This is a much more conservative – and realistic – measure of an amplifier’s average output power. But in order to claim more power, some manufacturers might list only the EIA numbers; others will disclose both FTC and EIA output ratings enabling you to easily compare manufacturer’s specs.

Two-ohm Catch

Most manufacturers, however, don’t publish a 2-ohm FTC spec even if the amplifier can easily handle normal program material into a 2-ohm load. An FTC rating requires that the amp undergo a "warm-up" test, meaning the amp’s output ratings are measured after 60 minutes at 1/3 power and five minutes at full-rated power. Since 2-ohm operation is right on the edge of where an amp’s current-limiting protection circuits will kick in, the amp will automatically go into protection mode before the five-minute threshold. Fortunately, music waveforms are less demanding than constant-level sine waves or pink noise used for testing. Because of this, most manufacturers only list the EIA power rating for 2-ohm operation.

More Than

Just Power

Now that you know how to read the numbers, should you always buy the amplifier that has an extra 10% more power? Not necessarily. To hear a difference in loudness, you typically need to double the power going to your speakers. In other words, you will hear the difference between 400 watts to 800 watts, a 3 dB increase, but you won’t hear a difference between 400 watts and 440 watts (+0.4 dB). Bottom line: A minimal difference in watts isn’t audible, so don’t trade off other important amplifier features, such as reliability and sonic clarity, for a little more power.

Read the

Fine Print

Just like all product specifications, read advertised power points carefully. The numbers can be misleading unless you know what you’re comparing. Whether you use FTC or EIA ratings, the important thing is that you compare measurements using the same standards for frequency range and distortion.

Understanding what ratings standards are behind power point claims is a fundamental first step in choosing the right amplifiers for your speakers. And that can mean the difference between a downed sound system on Sunday and one that’s perfectly matched so that the show – or service – goes on.

Example 1

A "150W amp" as measured by the FTC Standard Specification

Continuous Average Output on Both Channels Driven

8 ohms, 20 Hz to 20 kHz, 0.1% THD = 150 watts

(The unpublished EIA rating might be as high as 175-185W at 1kHz, 1% clipping)

Example 2

A "170W amp" as measured by the EIA Standard Specification

8 ohms, 1 kHz, 1 % THD = 170 watts

(The unpublished FTC rating might be as low as 135W, 20Hz to 20kHz, 0.1% THD).

Patrick H. Quilter is founder and chief technical officer of QSC Audio Products, Inc. www.qscaudio.com or info@qscaudio.com