Audiovisual receivers buying guide

AV receivers sell the idea of turning your living room into a cinema. How much should you spend?
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04.Power specs

There's a lot of emphasis on power in the marketing material for AV receivers, because selling them on their technical specifications for sound quality is difficult. The amount of power you need is determined by your listening distance, desired volume and speaker requirements.

The power rating of the amplifier tells you the maximum power available for the loudspeakers, and the power to each speaker is referred to as a 'channel'. The technical term for it is the Root Mean Square (RMS) power measured in Watts (W).

For typical domestic listening, power ratings between 50 W and 100 W RMS per channel should be sufficient. You don't have to use all that power, you just need to have sufficient in reserve to avoid distortion in the loud sections. This is sometimes called 'head room' or 'dynamic power'.

To make an undistorted 50 W RMS system sound twice as loud you don't need 100 W, you need more than 500 W - and you also need speakers that can handle the power without distortion. Most domestic speakers require only about 50 W or less and even more expensive models may be rated at only 120 W per channel at 8 ohm.

What to look for in the specs

A good receiver spec might look something like:

  • Minimum per-channel power 100 W RMS
  • 8 ohm, 20 Hz-20 kHz
  • 0.04% Total Harmonic Distortion (THD)

It may appear daunting, but it's just a shorthand way of describing the amplifier's ability to produce a given amount of power without introducing sound distortion.

8 ohm

An amplifier is like a miniature power station supplying electrical power to the loudspeaker. Ohms are a measure of the load a speaker puts on the amplifier. Domestic equipment commonly quotes a load of 8 ohms. Some manufacturers also quote power available for four, six or 16 ohms. The higher the number the more power is needed to drive the speaker.

If an amp has a power output of 55 W at 6 ohm, when recalculated at 8 ohms it's reduced to 41.25 W.

20 Hz-20 kHz

Humans can hear a wide range of frequencies - from very low, below 20 Hz, to very high, around 20 kHz. The amplifier needs to state the range of frequencies for which full power is available. This is usually described as the power bandwidth.

Unlike the mains power supply that maintains a constant voltage, the amplifier output voltage continuously changes in response to audio signals. As the volume and frequency changes, so does the amount of power needed to drive the speaker.

0.04% Total Harmonic Distortion (THD)

This is a measure of the change introduced by the system to the original input. THD figures below 0.1% for amplifiers are achievable and are generally inaudible for all but the most trained ears. We can hear low levels of distortion in audio signals and therefore can hear when the amplifier starts to overload.

So it's important to know the maximum power output at a given and low (ideally inaudible) level of distortion.

The major contributor to distortion when playing loud sounds is usually the speakers, not the amplifier.

For a more detailed (and technical) explanation of how amplifiers work we suggest you read Making sense of amplifier and receiver specifications by Graeme Huon.


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