Which Amplifier Class Is Best? Part 2
Following on from the previous part, get to know the remaining 4 amplifier classes: Class D, T, G, and H, along with a comparison of their pros and cons.
Last time we covered 3 amplifier classes: Class A, Class B, and Class AB. Today we will talk about the remaining 4 classes, along with a table comparing the pros and cons of each class.
Amplifier Class D
Class D power amplifiers are completely different from Class A, Class B, or AB. In the classes above, the output amplification stage amplifies or delivers power according to the strength of the input signal, whereas Class D converts the input signal into a pulse-width waveform known as Pulse Width Modulation (PWM), in the form of a square wave, while ordinary audio signals are sine waves. This converted signal is used to drive the operation of the output amplification stage, switching it on and off according to the width of the pulses fed in to trigger the output stage, similar to how digital signals work, that is, defined as opening or closing the circuit. This leads some people to mistakenly think that Class D means digital, but in reality it may be similar only in terms of on-off operating behavior, not in terms of how it amplifies the audio signal. The limitation of Class D is that it is usually restricted to fairly low frequencies, because the output amplification stage must filter out the PWM square wave to return it to a sine-wave frequency signal. In general, a typical Class D amplifier filters at 500 Hz, so the usable frequency is no higher than about 250 Hz or a little more. Above this frequency the sound becomes blurred or choppy, and trying to amplify across the full frequency range often causes Radio Frequency Interference (RFI). Other classes are not yet widely known, and given the way it works, this type of power amplifier is well suited to driving subwoofers, emphasizing hard-hitting, powerful bass, but is not popular for driving mid-high speakers.
Amplifier Class T
Class T power amplifiers were developed to reduce the weakness of Class D, which lacks stability at high frequencies, by using digital capabilities to help. By increasing the switching operating frequency, it can switch at higher frequencies of around 85 KHz, and then a low-pass filter circuit at around 40 KHz is used, resulting in a Class D-style amplifier that operates effectively up to frequencies higher than 20 KHz. This lets the circuit work across the full audio channel (20-20,000 Hz) and enables Class T to drive both subwoofers and mid-high speakers.
Amplifier Class G
Class G power amplifiers use two or more power supply rails and work by having the amplification stage switch to a higher supply voltage when the input signal grows stronger, giving higher efficiency and reducing voltage loss in the transistors. However, they have issues during the transition between power supply rails as they switch for each signal strength, and because of the complex circuit design, this class of power amplifier may not be very popular today.
Amplifier Class H
Class H power amplifiers are similar to Class G, except that at the point where the signal clips there is no change to the input signal frequency. In terms of the circuit it is like a Class D power amplifier, but the output stage is arranged like a Class AB circuit, so this type of power amplifier can drive both mid-high speakers and subwoofers (low-frequency speakers).
As mentioned, each class of power amplifier has a different way of working within its circuit. Some classes may be suited to mid-high speakers, and some to subwoofers. So when selecting a power amplifier, we should choose one appropriate to the intended use of each class, which will give us high-quality sound that is good, just as we want.