Wattage, Ohms and What to Consider When Buying an Amplifier

by Bert Markgraf

Amplifiers take weak signals from electronic components such as microphones, guitars, DVD players and mobile music players and change them into powerful signals that can drive large speakers to create robust music for a home theater or large audiences. To avoid speaker damage and poor sound quality, amplifiers must match the speakers they are driving. Amplifiers and speakers both have power and resistance specifications, expressed as watts and ohms. These ratings are a key factor in matching an amplifier to the speakers of your sound system.

Amplifier Characteristics

The watt rating of an amplifier determines the loudness of the sound coming out of your speakers. Ohms describes how much resistance the speakers connected to the amplifier can have. The amplifier characteristics usually specify the RMS (Root Mean Square) watts per channel and the ohms per channel. Sometimes there are several values, such as 40 watts into 8 ohms and higher watt values into lower ohms. This is the power you can expect with speaker configurations that correspond to the specified resistance. To get the rated amplifier power, you must connect speakers with the specified ohms.

Speaker Ratings

Amplifier watts indicate how much power the amplifier can deliver; speaker watts tell you how much power the speaker can handle when its resistance matches the amplifier ohms. If you connect an amplifier that is more powerful, it may blow the speaker by damaging the mechanism that produces the sound. If your amplifier has less power than the speaker can handle, the speaker doesn't get the power it needs and may produce inferior sound.

Combining Speakers

Sometimes you want to connect two identical speakers per channel. You usually do this by connecting them in parallel, running two wires from each amplifier terminal to each of the speaker terminals. In this case, the speaker power is twice as much as an individual speaker and the resistance is half as much. If you have two speakers rated 40 watts at 8 ohms, the two speakers in parallel will have 80 watts at 4 ohms. Connecting speakers in parallel is often useful to make a speaker configuration match an amplifier.

Choosing the Amplifier

Typical amplifiers for home theater range up to 100 watts per channel, with several channels for the various speaker positions. For stereo applications like mobile devices or computer amplifiers, 40 watts per channel is often enough. Both these amplifiers usually specify 8-ohm speakers. Car amplifiers often have several hundred watts total power and feature a separate channel for a subwoofer that amplifies your base, taking up a lot of the extra power. PA systems can have several thousand watts, depending on how large an area you need to cover. These systems often specify lower speaker resistances of 4 or 2 ohms.

Photo Credits

  • Jupiterimages/Photos.com/Getty Images