The basic job of an audio amplifier is to increase the power of an audio signal enough to let the speaker recreate sound. How loud those speakers get depends on two factors:
- The power of the amplifier: How many watts of head-banging power the amplifier pumps out.
- The sensitivity of the speakers: How many decibels of loudness the speaker produces when a given number of watts comes from the amplifier.
Amplifiers are one of the largest issues you confront when putting together a multi-room home-entertainment system, so be sure to put adequate time into researching your options. Don't make the mistake of assuming that you can just run speaker wire all over the house and then use your present amplifier. The salesperson at your local stereo store should be able to help you figure out all your amplifier needs. To get the best help from the stereo-store techie, draw a picture of the interior of your house and note where you want each device and speaker to go, along with accurate distances and likely conduit paths. This way, the salesperson can help you calculate your needs based on the equipment that store carries.
Impedance matters
When making the match between an amplifier and speakers, you need to take into account the speakers' impedance. Impedance is the force that the current coming from your amplifier pushes against. All you need to know about impedance is that if you don't have enough, you may damage your amplifier! That's because lower-impedance speakers may cause an amplifier to overheat, which is never a good thing for a piece of electrical equipment.
The impedance rating of your speakers and your amplifier must match. Most speakers are rated at 8 ohms impedance, but some are rated at 4 ohms. Lower-impedance loudspeakers are more difficult for amplifiers to power. In fact, many inexpensive amplifiers are incapable of powering 4-ohm speakers and may overheat or just plain not work if you try to combine them.
Wiring more than one speaker to an individual amplifier channel greatly decreases the overall impedance that the speakers present to the amplifier. So even if you have an amp rated for 8-ohm impedance and speakers that are rated at 8 ohms as well, if you wire two speakers to the same output (in different rooms, for example), you effectively halve the impedance - to 4 ohms. Adding more speakers lowers the impedance even more. Pretty soon you're going to damage your amplifier.
Matching impedance
Adding more than two sets of speakers to an amplifier may cause it to malfunction, because you lower its impedance. If you're thinking about installing a multizone system, you can skip this part - multizone systems are designed with individual amplifiers for each set of speakers.
You can avoid this impedance problem in one of the following ways:
- Use separate amplifiers for each extra pair of speakers: If you use a separate amplifier for each set of speakers, you won't need to worry about impedance matching.
- Use an impedance-matching device: This device lets you connect multiple speakers to an individual amplifier channel without causing impedance problems, but it does decrease the amount of power that each speaker receives (and therefore the speaker's maximum loudness).
You may be tempted to hook several speakers up to one central unit, but overall, the more amplifier power you have, the better. If you can afford additional amplifiers - they're relatively inexpensive - go ahead and get them.
If you decide to go with a single amplifier for your home-audio network, you need to get some sort of impedance-matching device into your system. You have a couple of choices:
- A central impedance-matching transformer system: This device is a small box that accepts a single stereo pair of speaker-wire connections and has several sets of speaker outlets. The internal transformer matches the impedance, so even if you have several pairs of speakers simultaneously connected to the amplifier, the amplifier functions correctly. Many of these transformer boxes also double as speaker selector switches - on/off switches for your speakers - so you can control which speakers are playing music and which are blessedly silent.
- An impedance-matching transformer built into an in-wall volume control: These volume controls, which you place in each room with speakers, perform the same function as a central matching transformer, plus they let you adjust the volume without using a remote control. Some of these devices even have built-in on/off switches, so you can disable the speakers in the room - a nice feature to have in a single-zone system when you don't want to be disturbed.
If your audio network will be limited - feeding only two rooms, for example - you can probably get away with not using an impedance-matching system. Just be sure to choose an amplifier or receiver that has two sets of speaker outlets, and make sure that it can handle the impedance load that your speakers put upon it. After you get beyond two sets of speakers, though, you should consider installing an impedance-matching system. Skipping this step is not worth the risk of damaging expensive equipment.
