Amplifiers and Amplification

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.

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.

Decibels

Decibels are a logarithmic scale of relative loudness.

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
Pain Threshold: 130 dB
Jet aircraft: 140-180 dB

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.

For example, an amplifier that has a distortion rating of 10% at full power output is unlistenable (perhaps even at lower output levels as well), however, an amplifier that outputs a .01% distortion level at full power output level would be imperceptible at all listening levels.

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. 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.

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 terms.

How Stated Power Ratings Can Be Deceiving

In addition to the continuous power rating, another factor affecting real amplifier power output, especially with surround sound receivers, is whether the manufacturer is basing their wattage output specification on one or two channels driven or with all channels driven. 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 with one or two channels driven, the real-world wattage output when all 5 or 7 channels are operating across all frequencies will be lower, possibly as much as 30 or 40% lower. However, it is important to note that the wattage output, as stated in the manufacturer's specifications, may not be available across all channels at the same time.

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.

Conclusion

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), and Dynamic Headroom.

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, 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 of some of the factors to take into consideration when buying a Receiver/Amplifier. Don't base your buying decisions from a single specification, such as a Watts-Per-Channel rating. There are many other factors to take into consideration, and never buy any receiver/amplifier without giving a good listen for yourself. Make sure the dealer allows a 30-day satisfaction period or other accommodation for return/exchange if you are not happy with the performance of the unit.