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  • November 02, 2023 4 min read

    In the ever-evolving landscape of technology, we’ve witnessed a series of breakthroughs, from the wheel's invention to today's smartphones. As these innovations change our daily lives, some become essential, while others fade into obscurity. In the realm of sound and audio equipment, a similar evolution has taken place, particularly in amplifier technology. Class D amplification is emerging as a front-runner. 

    For those unfamiliar with the term, an amplifier amplifies signals. It is typically used for driving headphones or speakers in the audio domain. But not all amps are the same. Amplifiers come in various designs, each with distinct characteristics. These designs are grouped into "classes" based on circuit configuration and operational methods. The classes, denoted by letters from A to H, represent the underlying characteristics of amplifiers.

    The origin of Class D amplification

    Class D amplification, often referred to as "digital" or "switching" amplification (though it isn't truly digital in nature), can be traced back to the invention of British scientist Alec Reeves in the 1950s. At that time, amplifiers were already named in the sequence of A (Class A), B (Class B), and C (Class C). Thus, it was naturally named Class D. However, the naming sequence at the time did not equate D with digital, so it's not advisable to directly associate Class D amplifiers with digital amplifiers, as this can cause confusion. 

    The correct definition of Class D amplifiers should be PWM (Pulse Width Modulation) amplifiers. It modulates the audio signal, converting it into waveforms of the same voltage amplitude but with different pulse widths, and then pairing it with a low-pass filter to restore the original signal waveform. (Another encoding technology is PDM Pulse Density Modulation, which is a digital format. Digital amplifiers should be related to PDM encoding.)

    Unlike traditional amplifiers that use the transistor's linear conversion region, the PWM technology of Class D amplification uses the transistor's fully-on and fully-off states. In other words, it continuously switches the transistor on and off, similar to the digital 0s and 1s. 

    But what makes Class D special? 

    The most important signature of Class D is energy efficiency. Class D can achieve the highest efficiency while maintaining the lowest distortion among all amplifier classes. Efficiencies of 90% or higher are possible with Class D designs, whereas Class AB amplifiers typically have efficiencies between 50% to 70%. 

    Secondly, Class D has the advantages of size and weight. They can be extremely lightweight and slim, making them perfect for compact setups. 

    Thirdly, Class D amplifiers operate at cooler temperatures. They do not heat up like other amplifier designs. The cool operation ensures longevity and safety.

    Nowadays, many electronic devices with audio output are battery-powered and portable. Energy efficiency, heat dissipation, and space-saving characteristics of the amplifiers are keys. Thus Class D shines in the modern world.

    Why is Class D controversial in Hi-End Audio?

    How to define an "ideal amplifier" depends on its practical application. In the high-end home audio domain, a good amp should be the one that ensures that speakers deliver wonderful sound. Sound quality is the main criterion here and factors like efficiency, power consumption, size, heat, etc., are less important and are not key attributes to judge the amplifier.

    In the early stage of Class D amplification, the distortion was not ideal. From 1960 to 2000, Class D amplifiers, besides having advantages in heat and efficiency, struggled to compete with traditional amplifiers in sound quality. Mature technologies like Class A and AB or Dynamic Class A are the main players in the HiFi audio world. The Class D amps produced in the early days received negative reputations in the audiophile community and many have formed a stereotype perception toward Class D.

    However, the world keeps changing. Over the past two decades, due to increased demands of mobile devices and technological improvements in semiconductor industries, Class D amplification has blossomed under the efforts of engineers. On one hand, it increases efficiency; on the other, it reduces distortion, making today's Class D amplifiers comparable and possibly even superior to traditional power amplifiers in sound quality. 

    Choose well-engineered Class D

    In the modern days, if Class D amplifiers sound bad, it can largely be attributed to EMI electromagnetic interferences, modulation technology, switching crystal timing control and state switching speed, power source anti-turnover capability, and output filter component requirements. 

    Knowing where the problems are, many countermeasures can be adopted to overcome them. Take the K221 Class-D Amp from DA&T Audio as an example, the design uses spread-spectrum in modulation technology, optimizes triangle wave to reduce the intrinsic non-linear distortion, uses PWM power management supplies, adds a high-power demand anti-coupling circuit between the power amplifier and the power supply to improve performances. To further reduce distortions, it increases the open-loop gain and properly uses negative feedback, chooses custom-made output filter components with low non-linear distortion and low magnetic saturation, matches the output timing of the transistors, and uses multi-layer thickened PCBs. K221 also uses ground-isolation technology to massively reduce the EMI interferences. 

    As a result, the K221 has a distortion as low as 0.007% and provides up to 500 watts of power output per channel, while providing a sound that is unhurried and natural. When playing classical music, the instruments are distinct with precise positioning. Good bass control and articulation, making it very comfortable to listen to, yet they are very affordably priced.  It's worth a pilgrimage for audio enthusiasts who look for high-value amplifiers. 

    Technology is all about progression and adaptation. If you love listening to music, podcasts, or watching movies with crystal clear sound, a nicely engineered Class D amplifier is worth investing in. Just as we've moved past using CDs, and cassette tapes in favor of digital streaming music, Class D amplifiers are setting the new standard for audio quality. The high efficiency, inexpensive high output, and low distortions are undoubtedly likable characteristics, thus Class D amps are widely used in many professional scenarios, e.g., professional speaker arrays and subwoofer arrays in IMAX and Dolby Atmos movie theaters, mastering studio monitors in recording studios, PA equipment in live concerts. Whether you're setting up a new home theater or upgrading your sound system, it might be time to tune into the future with Class D.

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