How Much Power?

The primary function of a receiver is to power speakers, and the receiver’s power (its ability to play loud) is measured in Watts RMS, per channel. Although Watts RMS is a straight-forward electrical calculation, the way the measurement is taken and stated is not always the same between competing receivers. It's not even the same from receiver to receiver within the same brand; so read the power rating carefully because there are plenty of ways to mislead the uneducated.

On some inexpensive receivers, the Watts RMS measurement is only for one frequency, typically 1 KHz, which is only part of the frequencies humans can hear. I don’t know about you, but I haven’t come across any sound tracks of music made up of only one note. What you are looking for is a Watts rating for the entire spectrum of human hearing (20Hz - 20 KHz) into a speaker with 8-ohm of resistance. 8-ohm is the typical resistance of the majority of speakers.

The other measurement that is also stated with the Watts rating is the amount of distortion that was present for the rating, usually stated like THD < 0.1% ( THD stands for Total Harmonic Distortion). High quality receivers will have a THD of less than 0.1%. Distortion of the intended sound is not what you are after, so the lower the number the more accurate the reproduction of sound.

Now that you have a good grasp of what the watts measurement is all about let’s tackle power. How much power you need depends on: your preference for volume; the size and characteristics of the room; and most of all the sensitivity of your speakers.

The first thing to understand about power is there isn’t much difference between a receiver rated at 40 or 60 watts. There is a logarithmic relationship between power and audible loudness. That is, to double the loudness of a receiver that is rated at 10 watts per channel RMS, you would need a receiver with 100 watts per channel. Unless you will be using your new receiver in an unusually large room, or live at the end of an airport runway, you really don’t need a lot of power.

In fact, some of the very best amplifiers are rated at about 50 watts per channel and if you were to listen to the 1812 Overture on one of these systems, you’ll be sprinting to turn the volume down when the cannons fire. The reason for this is they have lots of dynamic power or “headroom” which is the ability to give you a significant increase in power output for short periods of time.

“Headroom” is far more important than continuous power for home theatres which tend to have extreme shifts in volume from dialogue to transitional music or special effects. If you saw “Top Gun” in the movie theatre, or walked into any AV store in the late 80’s, you will be familiar with how extreme the shifts in volume can be.

If you are reviewing the spec sheet of a receiver to compare headroom, compare the receiver’s watts rating with an 8 ohm load, against its rating with a 4 ohm load. Better quality receivers will approach double its output from 8 ohms to 4 ohms. In a perfect world the receiver would be double the watt rating, but we don’t live there.

The final thing you want to consider is how many channels the receiver can power, and that all channels are powered equally. If you are shooting to have a completely immersive experience with video and complementary sound, then you want all channels to be powered equally, so that the jets in Top Gun sound like jets whether they approach you from the front, back or sides.