Stated Power Ratings Can Be Deceiving
When it comes to real amplifier power output, especially with surround sound receivers, you can’t take a manufacturer’s amplifier power rating statements at face value. You need to look closer at what they base their statements on. For example, for home theater receivers with a 5.1 or 7.1 channel configuration, is the stated wattage output specification determined when the amplifier is driving one or two channels at a time? Or is the specification determined when all channels are driven simultaneously? In addition, was the measurement made using a 1 kHz test tone or with 20 Hz to 20 kHz test tones?
Zeroing in on Stated Power Ratings
When you see an amplifier wattage rating of 100 watts-per-channel at 1 kHz (which is considered the standard mid-frequency reference) with one channel driven, the real-world wattage output when all five or seven channels operate at the same time across all frequencies is lower, possibly as much as 30 to 40 percent lower. It’s better to base the measurement with two channels driven, and, instead of using a 1 kHz tone, using 20 Hz to 20 kHz tones. These represent the widest frequency range that a human can possibly hear. However, that doesn’t take into account the amplifier’s power output capability when all channels are driven. In a home theater receiver, not all channels require the same power at the same time. Variations in audio content affect the requirements for each channel at any given time. For example, a movie soundtrack has sections where only the front channels may be required to output significant power, while the surround channels may output less power for ambient sounds. On the other hand, the surround channels may need to output a lot of power for explosions or crashes, but the front channels may be de-emphasized at the same time. Based on those conditions, a power specification rating phrased in context is more practical to real-world conditions. One example would be 80 watts-per-channel (WPC), measured from 20 Hz to 20 kHz, two-channels driven, 8 ohms, .09 percent THD. What all that jargon means is that the amplifier, stereo, or home theater receiver can output 80 WPC using test tones over the entire range of human hearing when two channels are operating with standard 8-ohm speakers. This is more than enough for an average size living room. Also included in this example is the notation that the resulting distortion (referred to as THD or Total Harmonic Distortion) is only .09 percent. This represents a very clean sound output.
Continuous Power
Another factor to consider is whether a receiver or amplifier can output its full power continuously. Just because a receiver or amplifier is listed as being able to output 100 WPC doesn’t mean it can do so for any significant length of time. When checking amplifier specifications, see if the WPC output is measured in RMS or FTC terms and not in Peak or Maximum Power.
Decibels
Sound levels are measured in Decibels (dB). Our ears detect differences in volume level in a non-linear fashion. Ears become less sensitive to sound as it increases. Decibels are a logarithmic scale of relative loudness. A difference of approximately 1 dB is the minimum perceptible change in volume, 3 dB is a moderate change in volume, and about 10 dB is an approximate perceived doubling of volume. Here is how this relates to the real world:
0 dB: The threshold of human hearing15 to 25 dB: Whisper35 dB: Background noise40 to 60 dB: Normal home or office background65 to 70 dB: Normal speaking voice105 dB: Orchestral climax120 dB+: Live rock music130 dB: Pain threshold140 to 180 dB: Jet aircraft
For one amplifier to produce sound that’s twice as loud as another in decibels, you need 10 times more wattage output. An amplifier rated at 100 WPC is capable of twice the volume level of a 10 WPC amp. An amplifier rated at 100 WPC needs to be 1,000 WPC to be twice as loud. This follows the logarithmic scale mentioned above.
Distortion
The quality of the amplifier isn’t only reflected in wattage output and how loud it gets. An amplifier that exhibits excessive noise or distortion at loud volume levels can be unlistenable. You are better off using an amplifier of about 50 WPC with a low distortion level than a more powerful amplifier with high distortion levels. However, when comparing distortion ratings between amplifiers or home theater receivers, things can get cloudy. On its spec sheet, amplifier or receiver A might have a stated distortion rating of .01 percent at 100 watts of output, while amplifier or receiver B might have a listed distortion rating of 1 percent at 150 watts of output. You might assume that amplifier or receiver A might be the better receiver, but you have to consider that the distortion ratings of the two receivers aren’t stated for the same power output. It may be that both receivers have the same (or comparable) distortion ratings when both run at 100 watts output, or when A is driven to an output of 150 watts, it might have the same (or worse) distortion rating as B. On the other hand, if an amplifier has a distortion rating of 1 percent at 100 watts and another has a distortion rating of only .01 percent at 100 watts, the amplifier or receiver with the .01 percent distortion rating is the better unit with regards to that specification. As a final example, an amplifier or receiver with a stated distortion rating of 10 percent at 100 watts would be unlistenable at that power output level. It may be more listenable with less distortion at a lower power output level, but if you run into an amplifier or receiver that lists a 10 percent distortion level (or any distortion level higher than 1 percent) for its stated power output, get some clarification from the manufacturer before buying.
Signal-to-Noise Ratio (S/N)
Another factor that affects amplifier quality is Signal-to-Noise Ratio (S/N). This is a ratio of sound to background noise. The larger the ratio, the more the desirable sounds (music, voice, effects) are separated from acoustical effects and background noise. In amplifier specifications, S/N ratios are expressed in decibels. An S/N ratio of 70 dB is more desirable than an S/N ratio of 50 dB.
Dynamic Headroom
Last (for the purposes of this discussion) is the ability of a receiver or amplifier to output power at a higher level for short periods to accommodate musical peaks or extreme sound effects in films. This is important in home theater applications, where extreme changes in volume and loudness occur during a film. This specification is expressed as Dynamic Headroom. Dynamic Headroom is measured in decibels. If a receiver or amplifier can double its power output for a brief period to accommodate the conditions described above, it would have a Dynamic Headroom of 3 dB.
The Bottom Line
When shopping for a receiver or amplifier, be wary of wattage output specifications. Also, take stock of other factors such as Total Harmonic Distortion (THD), Signal-to-Noise Ratio (S/N), and Dynamic Headroom. Plus, pay attention to the efficiency and sensitivity of the speakers you use. An amplifier or receiver may be the centerpiece of your audio or home theater system. Other components such as CD players, turntables, and Blu-ray Disc players may also be links in the chain. You may have the best components available, but your listening experience will suffer if your receiver or amplifier isn’t up to the task. Although each specification contributes to the ultimate performance capability of the receiver or amplifier, a single spec, taken out of context with other factors, doesn’t give an accurate picture of how your home theater system will perform. It’s important to understand the terminology thrown at you by the ad or salesperson, but don’t let the numbers overwhelm you. Base your buying decision on what you hear with your ears and in your room.
