Understanding Amplifier Power Output Specifications

Denon AVR-X4300H Home Theater Receiver - Inside View
Denon AVR-X4300H Home Theater Receiver - Inside View. Image provided by D&M Holdings

Don't Base an Amplifier's Quality Just on Its Wattage Output

POWER! It seems that power is the name of the game these days, in politics, business, relationships, and in home theater systems. You check out those ADs in the newspaper for a receiver or amplifier for your audio system, the one feature that sticks out is the Watts-Per-Channel rating. One receiver has 50 Watts-Per-Channel (WPC), another one has 75, and still another has 100.

The more watts the better right? Not Necessarily.

Most people think that more watts means more volume. An amplifier with 100 WPC is twice as loud as 50 WPC right? Not exactly.

Let's explore some answers to the above questions.

How Stated Power Ratings Can Be Deceiving

When it comes to real amplifier power output, especially with surround sound receivers, a lot depends how honestly the manufacturer determines the power output rating it chooses to promote.

For example, when you see Ads or product announcements where the manufacturer states power ratings, you can't take that number at face value, you need to look more closely on what the manufacturer is basing their statements on.

For example, in home theater receivers that have a 5.1 or 7.1 channel configuration, is the stated wattage output specification determined when the amplifier is driving just one or two channels at a time, or it is the specification determined of the amplifier when all channels are driven simultaneously?

In addition, was the measurement made using a 1KHz test tone, or with 20Hz to 20KHz test tones?

In other words, 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 5 or 7 channels are operating at the same time across all frequencies will be lower, possibly as much as 30 or 40% lower.

A better indicator is to base the measurement when two channels are driven, and, instead of just using a 1kHz tone, use 20Hz to 20kHz tones, which represents the widest frequency range sensitivity that human may have. However, that still doesn't fully take into account the amplifier's power output capability when all channels are driven.

On the other hand, not all channels actually require the same power at the same time as variations in audio content affect the requirements for each channel at any given time. In other words, a movie soundtrack will have sections where only the front channels may be required to output of power, but the surround channels may be just output lower volume ambient sounds. By the same token, the surround channels may be called upon to output a lot of power for explosions or crashes, but the front channels may by de-empahized at the same time time.

Based on those conditions, look for a power specification rating phrased in the following manner, that is more practical to real-world conditions: 80 watts-per-channel, measured from 20Hz to 20kHz, 2-channels driven, 8 ohms, .09% THD.

What all that terminology means is that the amplifier (or home theater receiver) has the ability to output 80-watts per channel (which is more than enough for average size living room), using test tones over the entire range of human hearing, when two channels are operating with standard 8-ohm speakers.

Also included is the notation that the resulting distortion (referred to a THD or total harmonic distortion) is only .09% - which represents a very clean sound output (more on THD later in this article).

Continuous Power

An additional factor is this equation is the ability of a receiver or amplifier to output its full power continuously. In other words, just because your receiver/amplifier may be listed as being able to output 100WPC, doesn't mean it can do so for any significant length of time. Always make sure that, when you check for Specifications, that the WPC output is measured in RMS or FTC terms, and not terms such as Peak Power or Maximum Power.

Decibels and Amplifier Power Output

To further illuminate the topic of amplifier power output, here is a brief overview of how Decibels factor in to amplifier power capability. 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 a minimum perceptible change in volume, 3 db is a moderate change in volume, and about 10 db is an approximate perceived doubling of volume.

To give you an idea of how this relates to real-world situations the following examples are listed:

0 db is the threshold of hearing
Whisper: 15-25 dB
Background noise: about 35 dB
Normal home or office background: 40-60 dB
Normal speaking voice: 65-70 dB
Orchestral climax: 105 dB
Live Rock music: 120 dB+
Pain Threshold: 130 dB
Jet aircraft: 140-180 dB

In order for one amplifier to reproduce sound 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 watts per channel needs to be 1,000 watts per channel to be twice as loud. In other words, the relationship between volume and wattage output is logarithmic rather than linear.

Distortion

In addition, the quality of the amplifier is as (or more) important than just the wattage output. An amplifier that exhibits excessive noise or distortion at loud volume levels can be unlistenable. You are better off with an amplifier of about 50 WPC with a low distortion level that a much more powerful amplifier with high distortion levels.

However, when comparing distortion ratings between amplifiers or home theater receivers - things can get "cloudy" - as you might notice, on its spec sheet, that amplifier or receiver A might has a stated distortion rating of .01% at 100 watts of output, while amplifier or receiver B might have a listed distortion rating of 1% at 150 watts of output.

The rush to conclusion might to be assume that amplifier/receiver A might be the better receiver - but you have to take into consideration that the distortion ratings of the two receivers were not stated for the same power output. It may be that both receivers might have the same (or close) distortion ratings when both are running at 100 watts output, or when receiver A was driven to output 150 watts, it might have the same (or worse) distortion rating as Receiver B.

On the other hand, if an amplifier has a distortion rating of 1% at 100 watts and another another has a distortion rating of only .01% at 100 watts, then it is more obvious that the amplifier or receiver with the .01% distortion rating is the better receiver, at least with regards to that specification.

As a final example, if you run across an amplifier or receiver that has a stated distortion rating of 10% at 100 watts, it would be unlistenable at that power output level - it is possible that it might be listenable, with less distortion, at a lower power output level. However, if you run into any amplifier or receiver that lists a 10% distortion level (or any distortion level higher than 1%) for its stated power output - I would probably steer clear - or, at the very least, try to get some additional clarification from the manufacturer before buying.

Distortion specifications are expressed by the term THD (Total Harmonic Distortion).

Signal-To-Noise Ratio (S/N)

Also, another factor in amplifier quality is Signal-To-Noise Ratio (S/N), which is 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. A S/N ratio of 70db is much more desirable that a S/N ratio of 50db.

Dynamic Headroom

Last (for the purposes of this discussion), but not least (by any means), is the ability of your receiver/amplifier to output power at a significantly higher level for short periods to accommodate musical peaks or extreme sound effects in films. This specification is very important in home theater applications, where extreme changes in volume and loudness occur during the course of a film. This specification is expressed as Dynamic Headroom.

Once again, Dynamic Headroom is measured in Decibels. If a receiver/amplifier has the ability to double is power output for a brief period to accommodate the conditions described above, it would have a Dynamic Headroom of 3db.

The Take-Aways

So, when shopping for an receiver/amplifier, be wary of wattage output specifications and also take stock of other factors such as Total Harmonic Distortion (THD), Signal-To-Noise Ratio (S/N), Dynamic Headroom, and also the sensitivity of the speakers you are using (more on this at the end of this article).

In conclusion, your amplifier or receiver, although the centerpiece your audio or home theater system, other components such as Loudspeakers, Input devices (CD, Turntable, Cassette, DVD, Blu-ray etc...) are also links in the chain. However, you can have the best components available, but if your receiver or amplifier isn't up to the task, your listening experience will definitely suffer.

This has been a very brief overview on how to interpret amplifier power specifications, as well as other factors that need to taken in context.

The important take-away is when shopping for a Home Theater Receiver/Amplifier, don't base your buying decisions from a single specification, such as a Watts-Per-Channel rating.

Although each specification contributes to the ultimate performance capability of the receiver or amplifier, it is also important to emphasize that a single spec, taken out of context with other factors, does not give you an accurate picture of the receiver/amplifiers true capabilities.

For an additional perspective on this subject, check out four articles from our site: What is the Relationship between Loudness and Amplifier Power?, Amplifier Power and Speaker Efficiency, How Much Power Do My Speakers Need?, and Speaker Sensitivity: An In-Depth Explanation

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