The first thing most folks look at when they see a shiny new receiver sitting on display is the power rating. The rating is measured in watts per channel.
The rating is usually measured as RMS, or root mean square, which means that it's a measure of sustained power, as compared to peak, which measures the highest possible instantaneous peak.
The problem is that you can't take these ratings at face value because manufacturers play a lot of interesting tricks when they give these watt ratings. The result is that a receiver can be more or less powerful than another receiver with the same rating. To really get a feel for a receiver's power, examine the following four measurements closely:
- Distortion: Power is measured at a certain number of watts at a certain level of distortion (noise created by the amplifier). You want low distortion (of course). The tricky part comes in when you examine how the amplifier's power output is measured - specifically at what distortion level it's measured. An amp that's measured, for example, at 100 watts per channel at 0.02 percent THD (total harmonic distortion, the standard distortion measurement) is quieter and probably more powerful than one that's measured at 100 watts per channel at 0.2 percent THD. You can really do a direct comparison only if both are measured at the same THD percentage. Another way of looking at this issue is this fact: If manufacturers measure power at a higher distortion rating, they can squeeze more "on paper" power out of the receiver.
- Impedance: Almost all amplifiers are rated at 8 ohms impedance (a measure of electrical resistance), so you can compare ratings this way; but a few are instead measured at 6 or even 4 ohms. These lower resistances can give an artificially high power rating - be suspicious.
Not all amplifiers built into receivers (or even separate power amplifiers) can power 4-ohm speakers without overheating, popping a circuit breaker, or just plain breaking down. Check to see whether a receiver can support these lower impedances if you choose speakers that require it. Lower impedances are rare unless you're buying some really high-end gear - most home theater speaker systems are rated at 8 ohms of impedance (though the actual number varies as the speakers reproduce different frequencies).
- Frequency range: Lower frequencies (the bass frequencies) require more amplifier power than higher frequencies. Because of this, some receiver manufacturers test their systems not at the full 20 to 20,000 Hertz (Hz) range (which is called full range), but with a limited range (such as 40 to 20,000 Hz). This limited-range testing can create an artificially high power rating. Receivers that are measured at full range are often called full bandwidth rated.
- Number of channels driven: Home theater receivers should be capable of driving at least five speakers (some have amplifiers for six, seven, or more speakers for extra surround channels). Power ratings should state how many speakers were actually being driven during the system test. Preferably, all channels are driven simultaneously at the stated power. Some systems give power ratings in stereo mode (with only the front left and right speakers driven), which means that the power with all speakers being driven is less than the stated amount.
