Power

The Short Course
It’s important to know how much power you’ll need per channel, and it depends on several factors, including the size and acoustical characteristics of your room, how loud you like your music and movies, and how efficient your speakers are, as indicated by their sensitivity rating. (A 3-dB increase in sensitivity means that you need half as much power to produce a given sound level, while a 3-dB decrease in sensitivity means you need double the power to achieve that volume.) It takes less power to fill “live” rooms that have hard, reflective surfaces than “dead” ones that have lots of upholstered furniture and thick carpeting.

All things considered, you’ll probably need less power than you think: 40 or 50 watts per channel is plenty for most music listening, and unless you have a really big or really dead room or unusually inefficient speakers, 80 to 100 watts per channel will ensure action-movie climaxes don’t sound wimpy. Having more power on hand never hurts, but there’s no good reason to use a receiver with more than 100 watts per channel in a compact bedroom system, and very few people need more than that even in their main home theater systems.

In-Depth Guide to Power Specifications
On the surface, an amplifier or receiver power specification looks pretty straightforward — “100 watts per channel,” for example. Amplifiers are not light bulbs, however, and a complete power spec is quite a bit more detailed. The Federal Trade Commission (FTC) requires that a manufacturer’s primary power rating for a home audio amplifier (whether separate or built into a receiver) be its average continuous power into an 8-ohm impedance. It must also include the frequency range over which the rating is valid and the maximum distortion over that range at the specified output level. In other words, a typical power spec, in its full glory, might read something like this: “100 watts per channel from 20 Hz to 20 kHz into 8 ohms with less than 0.05% THD,” where THD means total harmonic distortion.

That spec packs in a lot of information. It tells you that each amplifier channel can deliver at least 100 watts at every frequency within the audio band, which probably means that the amp or receiver can deliver a bit more power at frequencies in the midrange, where most of the action is. It also tells you that the output waveform will be an almost perfect replica of the input — just larger. That’s the significance of the distortion figure, which indicates what proportion of the output consists of stuff that shouldn’t be there. With complex sounds, such as most music, the ear is surprisingly tolerant of distortion — several percent or more may go completely unnoticed. So if the maximum distortion is held to 0.1% or less, you can be sure that you’ll never hear it. (In normal use, any maximum distortion figure below about 1% is going to be pretty safe.)

Distortion is part of the power specification because when an amplifier begins to run out of steam, distortion starts rising. And when the amp is completely overloaded, it cuts off the peaks of waveforms — a condition known as “clipping” that produces a very ragged-sounding distortion. The distortion part of the power spec keeps the manufacturer honest. An amplifier or receiver might be able to put out more power at 10% distortion than at 0.5%, but the extra apparent oomph is just that — apparent. It’s too garbled to be useful.

Even within the realm of reason, there’s some maneuvering room. The same amplifier or receiver tagged with the typical power spec cited above might also be honestly rated at, say, 115 watts per channel from 40 Hz to 20 kHz into 8 ohms with less than 0.9% THD. The difference between the two specifications is inconsequential on all counts. Just recognize when comparing power ratings that this sort of leeway exists.

The last element of the standard power specification is the impedance of the load the amplifier’s output is connected to. Even before the FTC stepped in, the de facto standard for impedance had been 8 ohms for many years because most speakers have nominal (rated) impedances of 8 ohms or close to it.

That’s not the case for all speakers, however — some mainstream speakers have rated impedances as low as 4 ohms. Besides, the impedance of nearly all speakers varies with frequency, often substantially, which means that an impedance rating is really an approximation. An “8-ohm” speaker might have an impedance ranging between, say, 5 and 32 ohms over the audio frequency band. Depending on how an amplifier is designed, these changes in impedance could present significant challenges while playing music or movie soundtracks.

The reason is found in Ohm’s law, which states that for a given voltage in a circuit, the lower the impedance, the higher the current. Because power is voltage times current, an audio amplifier can usually deliver more watts as the load impedance is reduced — at least up to the point where it can’t pump out more current without shutting down or throttling back to protect its output transistors from overheating. An amp with inadequate output-current capability can choke on low-impedance speakers.

Most good amplifiers and receivers today, including many that come with cautions against using speakers with impedances rated less than 8 ohms, will handle 4-ohm speakers without terrible distress, and the best high-end amps hold up well down to 2 ohms or less. Still, if you’re using low-impedance speakers, it’s a good idea to make sure any amplifier or receiver you buy will be happy driving them. Look for a 4-ohm power rating in addition to the standard 8-ohm one. In particular, make sure that the rated power output into 4 ohms is at least as great as into 8 ohms — preferably, it should be at least 25% higher. If no 4-ohm spec is published, check with the manufacturer.

Finally, almost all amplifiers and receivers will deliver slightly more power in short bursts than they can continuously, which is great, since music often contains brief peaks (called “transients”) that are considerably louder than the average level. The dynamic-headroom specification is designed to account for this characteristic. It indicates, in decibels (dB), how far above its continuous rated power an amplifier can go for short intervals, with 1 or 2 dB being typical. Getting more than 3 dB of dynamic headroom from an amp usually requires either special circuitry or an artificially low continuous-power rating.

Back to Surround Sound Overview | Back to Resource Center Home

Print

Stumble It