What to consider when selecting amplifiers for home theater.
Number of Channels
When you’re ready to upgrade a room with better sound, the first question you’ll need to answer is “how many channels” do you want? There are a number of common audio setups, including:
- Stereo. Two-channel audio includes a left speaker and a right speaker.
- Surround Sound. The most common configuration for surround sound is 5.1 channels, including five full-frequency surround channels: left, center, right, left-rear, and right-rear channels, and one low-frequency effects (LFE) channel, which is denoted .1, indicating that one subwoofer low-frequency speaker will be used.
- Spatial Audio. Basic spatial audio configurations, such as Dolby Atmos, are labeled as 5.1.4, indicating the same configuration described above, plus four “height” speakers placed on or near the ceiling, or aimed upwards to reflect off the ceiling.
Your amplifier must be able to reproduce sound at the volume levels you desire, with your chosen speakers, without distortion. While it is true that you should choose speakers with a power handling rating that matches or exceeds the maximum power per channel of your amplifier, it is harmful to operate your amplifier at output levels where the amplifier can no longer reproduce the sound accurately, leading to distortion. Distortion is much more difficult for speakers to reproduce than a clean sound signal. An over-driven amplifier is the most common reason for speakers burning up. So, it is just as important to select an amplifier with enough power, and low distortion at your desired power level, as it is to select speakers with an adequate power handling capability. Of course, larger rooms will tend to need higher power amplifiers and speakers than smaller rooms, as sound will travel further from each speaker to each person in a larger room, and sound amplitude diminishes the further the sound travels from the speaker.
Amplifier power ratings depend on the speaker impedance. A typical speaker impedance is 8 ohms. With a lower speaker impedance, such as 6 ohms, the speaker will draw more power from the amplifier at a given volume setting.
Amplifier manufacturers almost always publish their power output ratings along with a distortion specification. As you turn up the volume and demand more power from your amplifier, you can reach a point where distortion becomes a problem. An example AV receiver specification (with 11 channels) is shown below.
- Power Output (8 ohm, 20 Hz - 20 kHz, 0.05% 2ch Drive) 140 W
- Power Output (6 ohm, 1 kHz, 0.7% 2ch Drive) 175 W
Notice how the power rating of 140 watts per channel, with relatively low distortion of 0.05%, only applies when the amplifier is driving 2 channels. As this amplifier can power up to 11 channels, distortion would be quite a bit higher if the amplifier were asked to deliver 140 watts to all 11 channels simultaneously.
Also notice how the maximum power rating is higher for speakers with a lower 6 ohm impedance, but at this higher power level the distortion is much higher. At the rated power of 140 watts, the distortion is likely to be acceptably low, even with lower impedance speakers.
Good amplifiers deliver tight bass. Damping factor is a measurement that describes the amplifier’s ability to resist the electrical feedback that comes from low frequency speaker drivers (a.k.a. woofers). As a speaker powers a woofer, the speaker cone moves in and out. This speaker cone has mass, and inertia. The kinetic energy of the speaker cone moving may (momentarily) turn the speaker from a transducer that consumes electrical energy into a generator of electrical energy. Modern amplifiers incorporate feedback circuits that can measure and compensate for this effect, enabling the amplifier to tightly control the position of the low frequency speaker cone. Note that the length and gauge of the speaker wire also affects damping factor, so it is important to use the minimum necessary length of speaker wire, and a sufficiently heavy gauge.