CRITICAL FIGURES

Marketing is extremely important in driving amplifier sales. That’s why the performance figures are critical for the success or failure of a given amplifier product. Marketers are always looking for ways to differentiate their products, and in the process, sometimes they live “close to the edge” on the truthful data front.

If there’s a trick out there to rate a 500-watt amplifier at 1000 watts under certain conditions, the marketing crew is all over it. They know that the buyer looks at certain numbers before anything else, and power output usually tops the list.

Other performance figures can also be manipulated to increase the power rating. The most common trick: Have you ever seen a DVD receiver-plus-loudspeakers (also known as “home theater in a box”) package that claims something like “350 watts of total power”? This power spec is written on the box in a larger font that anything else.

But in the fine print, you’ll also notice the caveat: “at 10 percent distortion.” If you’ve ever heard what 10 percent distortion sounds like, the term “garbage” comes to mind. So how do they get away with this? Because they’re not lying, but rather, are using specmanship to state a higher power output figure.

This tactic applies to pro audio as well, but in a less obvious fashion. Rating distortion at 10 percent is a throwback to the days of car audio power boosters, which ran bridged amps from the 14 volts of power readily available in cars. In order to get a 40-watt output rating, power output was rated at ultra-heavy clipping.

Some chipmakers in the Class-D arena also use tricks of this type in their data sheets. In fact, certain ones even rate power output as a “saturated square wave.” This means that the output is clipping (driven past its output voltage range) nearly 100 percent of the time!

There are several standards for rating power that help keep manufacturers in line when releasing specs. These rules come from the Federal Trade Commission (FTC), Electronic Industries Alliance (EIA) and others. The problem is that different equipment manufacturers use different standards for rating power output. Most pro audio manufacturers state the standards used, so keep that in mind when you compare specs – always try to understand the context of the specs, and compare apples to apples.

Another sleight of hand found in the home theater genre is measuring amp channels in pairs rather than all five (or more) being driven simultaneously. It can be argued that this is actually a more practical approach due to the nature of movie sound, but it also doesn’t mean certain advantages aren’t being taken and touted.

MULTIPLE VERSIONS

There are many permutations of Class- D amps. To list a few options, there are open loop (no feedback) and closed loop versions, analog input and digital input versions, also fixed switching frequency and variable switching frequency versions. One that is very common to pro audio is the analog input, closed loop, fixed switching frequency type. Measurement tricks for this type of Class-D amp are much like those used to measure conventional “analog amps.”

In the consumer world, digital input Class-D is has gained ground; especially what is called “true digital,” which equates to digital modulation, open loop. Naming this variation “true digital” is misleading, albeit commonplace. (All Class-D is analog at some point.)

Switching amps generate high-frequency output voltage “ripple.” Alternating drive current through the output filter causes this “side effect.” The details are quite involved, so an explanation of the nuances here would be impractical. In any case, the output ripple can cause measurement equipment to produce false readings, usually by way of exaggerated distortion or noise readings.

To compensate, some chipmakers and measurement equipment manufacturers have devised switching amp measurement filters, which can be used to fictitiously improve performance figures, sort of like reverse discrimination. Be wary of amps measured with manufacturer-specific filters.

Some Class-D designs turn off the outputs when the input level is below a certain threshold, much like gating is used to reduce background noise picked up by a microphone. This can create exaggeratedly high SNR figures, and is more common in open loop digital input amplifiers than for analog input amps.

The primary motivation for non-conventional amplifiers is efficiency, but how is efficiency measured? Unfortunately, it’s typically measured as a single data point at maximum power. Some Class-D manufacturers measure efficiency at higher power level than that for which the amplifier is rated.

For example, an amp might be rated to supply output of 100 watts at 1 percent THD+N, but efficiency might be rated at 150 watts output, where THD+N is off the charts. This is done to eek out the closest-to-100 percent number possible. As a rule, efficiency should be stated at listening level (1/8th power) and at full power (1 percent THD+N).

Another useful figure is idle power dissipation, which indicates how much power is wasted when the amp is “quiet.” Class-D amps that turn off the output stage during the absence of audio give an exaggeratedly low idle power dissipation figure. Turning the output stage on and off can cause audible effects such as clicking or popping. Even in designs with decent noise performance, turning the idle noise on and off is audible.