Class-D amplifiers are a type of audio amplifier that use a different type of transistor to amplify the signal. They are often used in portable audio devices, such as radios and mp3 players, because they are more efficient than class-A amplifiers.
An Introduction to Amplification
At the most basic level, a traditional amplifier is a signal booster. Give it an input signal, and it uses electricity and various gain stages to increase the amplitude until you end up with a much higher signal than the one that came in. The end result: a louder signal than the input signal.
There are multiple types of amplifiers like class A, class B, and class AB. Classes A and B have their own benefits and drawbacks, while class AB amplifiers combine elements of each to maximize the benefits and minimize the disadvantages of each. For years, the vast majority of consumer electronics like A/V receivers and home theater systems have used class AB amplifiers.
Why are we spending so much time talking about other amplifiers if this article is about class-D amplifiers? Because they work very differently.
How Class D Amplifiers Work
A class-D amplifier is also known as a switching amplifier. Instead of boosting the input signal via linear gain stages, a class-D amplifier uses a concept known as pulse width modulation.
This is a complicated subject, but this converts the input signal into pulses. The output stage then switches back and forth at a high frequency, corresponding to the pulses that the signal was converted into. This amplified signal is then processed and run through a low-pass filter to return it to the original waveform and remove high-frequency noise.
This conversion to and from another type of signal is somewhat similar to how digital-to-analog and back conversion works, but this is far less complex. That said, the process (likely combined with the “D” in class-d) leads to people sometimes mistakenly referring to class-D amplifiers as “digital” amplifiers.
What Makes Class D Amplifiers Special?
Because the pulse width modulation lets amplification happen at much higher frequencies than normal, class-D amplifiers require much smaller power transformers than class AB amplifiers. That means you can pack much more amplification into a smaller space.
These amplifiers are highly efficient, producing more volume using less power. Class-D amplifiers frequently have maximum possible efficiency values of 90 percent or more, while class AB amplifiers rarely go higher than 60 percent efficiency.
Of course, class-D amplifiers aren’t perfect. Because of the low-pass filter, which is meant to filter out unpleasant noise, the high end can suffer, so these aren’t great for audiophile use. Some class-D amplifiers can also exhibit distortion, and some people, audiophiles in particular, just aren’t fans of the sound of these amplifiers in general.
Even with the downsides, that combination of high efficiency and small size makes class-D amplifiers perfect for certain applications.
Common Uses of Class D Amplifiers
As you might imagine, class-D amplifiers are a great option for any audio project where size and power efficiency—think battery life—are important. Because of this, you’ll often see class-D amplifiers employed inside Bluetooth speakers.
Bluetooth headphones are also a good candidate for class-D amplifiers, though because the volume requirements are fairly low, other types of amplifiers work for this particular case as well. You’ll also find class D amplifiers in many other products, including portable headphone amplifiers and even stereo amplifiers.
One major area where class-D amplifiers are frequently used is subwoofers. Looking at the pros and cons of class-D amplifiers, it almost seems as if they were created for subwoofers. This is because the audio issues essentially disappear when the amp is only used for bass frequencies, and the small size and efficiency let you pack massive power into a subwoofer.
Take a look at our list of the best subwoofers you can buy, and if it’s powered, there is a good chance that subwoofer is using a class-D amplifier.
