Primary Consideration – Power Output
Power output is one of the most important considerations in choosing a receiver. Power is expressed in watts per channel and the decision about how much power you need should be based on your selection of loudspeakers, the size and acoustic characteristics of your listening room, and how loud you like to listen. It is always best to match the power requirements of the speakers with the output power of the receiver. Some speakers require more or less power, expressed as loudspeaker sensitivity (in decibels, dB), which is a measure of how much sound output is produced with a specified amount of amplifier power. Power output and speaker volume is not a linear relationship. For example, a receiver with 100 watts per channel will not play twice as loud as a receiver with 50 watts per channel using the same speakers – the difference in maximum loudness would be barely discernable, only 3 decibels (dB). Rather, more amplifier power will allow the system to handle musical peaks without straining.
Generally, a speaker with lower sensitivity (88dB-93dB) will require more amplifier power than a speaker with a higher sensitivity (94dB to 100dB or more) to play at the same volume level. Most receivers are rated with a minimum power output of 75 watts per channel and higher. When comparing power output, it is important to know how the power is measured. The most accurate measure of power is RMS (Root Mean Square, a mathematical formula), as opposed to peak output power. Some manufacturers inflate specifications by measuring power at a single frequency, say 1kHz, instead of the entire range, 20Hz-20kHz. When comparing receiver power outputs, always make sure they are measured the same way.
5.1, 6.1, 7.1 Channels – How Many Do You Need?
A/V receivers are distinguished from two-channel or stereo receivers by having five or more amplifier channels to power speakers that can reproduce movie theater sound or multi-channel music in your home. Most DVD discs and other multi-channel sources are encoded in Dolby Digital 5.1 and/or DTS 5.1 channel sound for playback on home theater systems. A basic system consists of 5.1 channels of sound. The five channels are left and right, like a stereo system, a center channel for movie dialog or music vocals and on-screen sound, and left and right surround channels, for special effects and surround sound. An additional subwoofer channel (the .1 LFE channel, Low Frequency Effects) adds very low bass for music sources and special effects on DVD movie sound tracks. The composite of the five main channels plus a subwoofer channel produces a “soundfield” that envelops the listener. A 5.1 channel system is capable of playing programs encoded in Dolby Digital and DTS.
6.1 channel a/v receivers have an additional rear-center channel output, and are becoming increasingly popular, even at lower prices. Some DVDs are encoded with 6.1 channel Dolby Digital EX and DTS-ES, and can be played back on this type of system. If properly installed, 6.1-channel sound can create a more enveloping surround sound effect.
7.1 channel receivers have three front channels, two surround and two surround-back channels, plus a subwoofer channel. The additional rear channels produce a soundfield with more precise placement of surround effects. Some 7.1 channel receivers offer THX™ soundfield enhancement, which is a system developed by Lucas Film™ designed to present film sound or multi-channel music with the most authentic quality. THX processing is offered as Select or the more advanced Ultra 2 format, which is optimized for movies and multi-channel music. Many other manufacturers have proprietary soundfield programs, called DSP, which also provide enhancements for music and movie sound. Sony, for example, has Digital Cinema Sound™ and Yamaha has Cinema DSP™. Many 7.1 channel receivers also permit the two surround-back channels to be reassigned to a second zone for a multi-room system, leaving the main system with 5.1 channels.
