Digital Switching Amplifiers (commonly known as Class-D) have been around for years. Nevertheless, it is nearly impossible to engineer a conventional Class-D amplifier that handles the full requirement, 20-20,000Hz, for full-bandwidth music reproduction. A Class-D amplifier works by utilizing a high-frequency sawtooth waveform to modulate the music signal (to learn more about how Class-D amplifier works, click here).

The constant presence of the sawtooth waveform, which is very high in frequency spectrum and its inevitable frequency jittering, can mask or corrupt low-level music signal. The output filter designed to filter out noise and overtones caused by the sawtooth waveform adds a 180 degree phase shift to Class-D output stage, causing possible instability and adding distortion due to its own inherent non-linearities.

Additionally, the output filter presents frequency-variant output impedance that can interact with a speaker's complex impedance. Variants of Class-D amplifiers with the addition of Digital Signal Processor claim to improve music reproductions. However, because of their lack of close-loop design, especially from the speaker's terminals, spurious interaction between the speaker's complex impedance and back-EMF with the amplifier's resonant output filter can result in harsh sound reproduction. The fundamental flaws of conventional Class-D amplifiers remain unresolved.