HT Measurements Explained: Absolute Power - Power Amplifiers Page 2

Continuous power, however, is important when you are trying to fill a large room or are looking to re-create theaterlike sound levels. For continuous-power measurements, the test equipment constantly compares the input with the output. Distortion is the difference between the two signals and can sound like static or a breakup of the signal. The test results are represented by the semi-U-shaped graph (Figure 1). Signal distortion usually decreases gradually as the output level increases, until the signal reaches the limits of the amplifier. At this point, the distortion increases dramatically. While we specify the power output at the initial onset of distortion, the more-important measurements are at the point where the distortion makes up 0.1 percent of the signal and at the point where the distortion makes up 1 percent of the signal.

Figure 1: Our measurement chart shows the amplifier's power output versus distortion. We note the power before distortion and at 0.1 percent (A) and 1 percent (B) distortion.

The load on the amplifier, initially, is an 8-ohm resistor, as most speakers represent an 8-ohm average load to the amplifier. Ohms are a measure of impedance, which is an electrical measurement of how much resistance to current flow the speakers offer to the signal. However, a "nominal" 8-ohm load means that, at some frequencies, the impedance to the signal may be higher and, at other frequencies, it's lower. As the impedance drops, the amplifier can increase its output but must draw more current to do so. We test amplifiers, when they demonstrate an ability to handle it, with lower-impedance 4-ohm loads, as well. A theoretically perfect amplifier will double its power every time the impedance drops in half, but in reality few are capable of this.

In addition to being affected by impedance, an amplifier's output is limited by the current capabilities of its power supply. In other words, the amplifier may be able to deliver its rated power as long as only one channel is being driven, but it may not be able to keep up if more than one channel (and particularly if all five or six channels) is reproducing difficult signals simultaneously. We try to measure one channel of a dedicated amplifier with both one and then all channels playing the same signal. Some larger amplifiers draw more current with all channels driven than our 15-amp voltage regulator can handle and are subsequently tested with only one or two channels running. Likewise, receivers are tested with two channels driven because they don't always offer an easy means to drive all five channels with the same signal.

Our second test measures the amplifier's frequency response at the output. If you read our description of frequency response as it relates to loudspeakers in the October 2000 issue's Boot Camp, you know that a "flat" frequency response is one where the device reproduces each sound, from low bass to high treble, with the same volume level, or amplitude, as the input signal. The resulting frequency-versus-amplitude graph appears as a flat horizontal line. We measure an amplifier's frequency response just as we do a speaker's. However, instead of printing the graph, we describe the results in terms of how far the line deviates from a reference point. A description that reads "flat, within +/-0.2 decibels from 20 hertz to 20 kilohertz" means that some frequencies are 0.2 dB louder than the reference point and some frequencies are 0.2 dB quieter than the reference point. A difference of 0.2 dB is audible to trained listeners if it occurs over a wide enough band. The less deviation across a wider frequency range, the better.

C. Powered (or active) speakers have an amplifier, with its exposed heatsink, built-in.

Finally, we measure an amplifier's Total Harmonic Distortion plus Noise (THD+N). This test looks at the harmonics created when the equipment tries to reproduce the signal. For example, we input a 1-kHz sine wave and measure the output's harmonics, or multiples of that signal, which would be at 2 kHz, 3 kHz, 4 kHz, etc., as well as the amount of noise produced. Harmonics tend to change the tonal characteristics of the sound. In musical instruments, this is good; in amplifiers, it is not. In the case of an amplifier, the lower the measurement, the better. The test equipment performs this test at selected frequencies from 20 Hz to 20 kHz and simultaneously filters out the test signal itself at every given frequency, allowing a measurement of just the resulting distortion and noise.

There's more to an amplifier, and life, than just power. As you can see from the various tests we perform, understanding how an amplifier reacts to various loads, what variance there is in the tonal balance, and how much noise or distortion it adds to the signal can greatly affect an amp's performance. This may not tell you what an amplifier sounds like, but it can certainly help you narrow down your choices to a select few that will fit in your system. Happy hunting.