The weight of a traditional amplifier, like the one shown here, comes largely from its massive transformer—the circular object on the left. The amp’s discrete components on the circuit boards make it easy to repair by a qualified tech. In this type of amp, excess current is turned into heat and then dissipated by heat sinks (right) or a fan.
Class A and B vs. Class D
Let’s look at linear amps a little closer. In the sine wave in Fig. 2 we can see that the wave has a central point, which is known as the X axis. Graphically speaking, this corresponds to the position of the string at rest. The wave goes positive first, then negative, then returns to rest before beginning its next cycle. To amplify this we need a central point about which to cycle the electrical equivalent of this wave. In a pure class-A tube or solid-state amplifier, this central point is half of the available volts. The tubes or transistors then turn more on or off to produce the amplified wave, which means the tubes or transistors process all of the signal, all of the time—there are no breaks, joints, or delays in the signal. It remains as sonically pure as possible.
In a class-B tube or solid-state amp, the center point is at zero volts, and two supplies are used—one positive and one negative. In such a case, two sets of tubes or transistors are used, with one set handling the positive half, and the other the negative half. This setup makes class-B amps far more efficient, with greater output potential. However, as the wave crosses the zero point and the positive half turns off and the negative half turns on, there has to be a slight overlap between the two sides to avoid breaks in the output (sound). This is where the aforementioned bias setting comes into play, ensuring that both sets are turned on slightly all the time. In a class-AB amp, the bias is set higher than a class-B amp, making the overlap between the two halves larger.
In recent years, a third amplifier type—class D—has become increasingly popular, thanks to its small size, light weight, extreme efficiency, and affordability. In class-D amps, the transistors turn full on or full off very fast—up to 500,000 times per second—positive on then off, negative on then off, and so on. Without getting too deep into electrical stuff, let’s just say that class-D amps are more efficient because the sound from the instrument is used to control the length of time transistors in the power section are on or off. Because of this greater efficiency, class-D amps generate far less heat than class-A and class-B amps. But they are not without their problems. The output must be filtered to get rid of the switching noises, and extensive protection mechanisms are needed to prevent damage to the power supply, speakers, and other parts due to overheating or power overload. If a class-D amp does get overloaded or overheats, power will switch off for 5–10 seconds and then reset.
How to Make Sense of Power Ratings
Okay, folks, we’re getting closer and closer to the crux of our power conversation. Remember when we talked about continuous power output? For a very long time that was the most widely accepted basis for amplifier power ratings, because it has a direct correlation to real-world needs. However, over the last few years myriad other power-rating formulas have been invented or brought back from obscurity. To be blunt, the only reason I can see for some of this is to fool consumers who can’t make sense of all the technical terminology and to make products sound more powerful than they actually are. These other rating types include burst power, peak music power output (PMPO), and fast pulse power before clipping (FFP).
To measure an amplifier’s power, we feed a sine wave (Fig. 2) into the front of the amp, connect a load of the correct impedance to the output, and turn it up. Maximum output is reached when the top or bottom of the waveform starts to flatten out, a phenomenon known as clipping. The output is then measured and noted.
Now let’s look at how output ratings are listed on some products currently for sale on the market. (We aren’t disclosing makes or models, because the point here is to make sense of all those numbers, terms, and acronyms.) If an amp is listed as having “continuous power output of 500 watts RMS into 4 ohms at 1% THD,” it means that, at the point of clipping, the amp can produce 500 watts of electrical power on a sustained, continuous basis.
Those two words—“sustained” and “continuous”—are key because they indicate a stamina that is crucial to certain musical situations. That’s a good thing, and here’s why: The waveform we’ve been looking at so far depict steady-state waves, but when you’re playing bass it’s not just one steady note with no dynamics. There’s a larger peak at the onset of the note when the string is plucked, followed by the decay as the string ceases vibrating. Fig. 5 looks more like the waveforms in composite music reproduction, where there are peaks due to loud sounds (such as drum beats) and lower sounds (like vocals and backing instruments).
In 1959 the British tube manufacturer Mullard noted in its Circuits for Audio Amplifiers guide that the average continuous output power required for general music reproduction (e.g., a recording of a band) was approximately an eighth of continuous power at full volume if the transients (large peaks in the waveform caused by sudden, loud signals) were to remain unclipped. This is a power rating system that has reared its ugly head again recently, especially with regard to lightweight class-D amplifiers. Most class-D bass amps today use small power modules designed for general music reproduction—not low-frequency-heavy bass—and they are rated using this 1/8th-power system rather than the more robust continuous average power rating.
Here’s part of the power specifications for a bass amp using a typical class-D power module:
• 300W at 1% THD+N, 4Ohm
• Full power bandwidth (20Hz – 20 kHz)
• 113dBA dynamic range (300W, 4Ohm)
• THD+N = 0.005% (1W, 4Ohm)
An amp with these specs will be marketed as a 300-watt amplifier. But what the specs don’t tell you is that this class-D amp module can only sustain 300 watts of output for a maximum of 27–60 seconds! This is more or less the norm for all class-D modules, and it’s perfectly acceptable for, say, playing MP3s of your favorite band. But if we go back to Mullard’s average continuous formula and calculate backward, we find that only 1/8th of those 300 watts—about 38 watts—is continuously available. This rather optimistic math is used as justification for selling what’s really a 60-watt amp as a 300-watt.