Class D amplifiers that effectively suppress EMI provide high sound quality at low cost

Consumer audio engineers have been trying to replace the power-hungry analog Class A/B amplifiers with more power-efficient digital Class D amplifiers for many years. However, the high EMI emissions inherent in traditional Class D amplifier solutions can interfere with the operation of AM/FM radios and smartphones, and the high cost of adding filtering and shielding measures to meet EMI regulations has hindered the application of Class D amplifiers.

Silicon Laboratories, a high-performance analog and mixed-signal IC manufacturer, has released a 5W stereo Class-D amplifier that can effectively reduce electromagnetic interference (EMI). The amplifier uses unique technology to reduce the impact of EMI on sound quality, bringing high-fidelity sound quality to consumer audio electronics.

Rick Beale, director of audio processing amplifiers for broadcast systems at Silicon Labs, said that traditional Class D amplifiers generate a lot of EMI radiation in the 900MHz transmit and receive frequency band, which weakens the quality of over-the-air (OTA) transmission and reception signals of smartphones and their expansion devices. In addition, most expansion platforms use analog architectures and are very susceptible to noise generated by TDMA transmissions. The Si270x amplifier is a smartphone-friendly all-digital architecture that is not easily interfered by radiated noise. Silicon Labs' EMI reduction technology enhances OTA performance and compatibility. As the first member of Silicon Labs' Class D amplifier series, the new Si270x amplifier is widely used in price- and noise-sensitive consumer audio products, including smartphone expansion devices, desktop radios, TV sound bars and monitors, speakers, and battery-powered radios.

The Si270x amplifier uses multi-stage EMI reduction technology to suppress the interference of traditional Class D amplifiers from the source, making it easier to comply with EMI regulations and more suitable for AM/FM radios and smartphones. Compared with existing Class D amplifier solutions, the Si270x amplifier reduces radiated interference by 10 times within the EMI specification frequency band, 100 times in the FM radio band, and 1000 times in the AM band.

Rick Beale said that with its groundbreaking EMI reduction technology, Silicon Labs has solved the challenges faced by Class D amplifier technology. Si270x mainly achieves EMI suppression through four levels of technology: first, it uses extended PWM (pulse width modulation) to effectively suppress broadband switch radiation energy, reducing the radiation energy to less than 1/10 of the original, and effectively ensuring sound quality in the process; second, the modulated noiseless notch technology reduces 35dB of noise to ensure clear AM signal reception; third, the adjustable switch conversion makes the PWM conversion process smoother, making it easier to meet consistency requirements and FM reception; finally, Si270x provides on-chip feedback protection, which not only improves audio quality, but also suppresses derivative problems that may occur when reducing EMI effects and provides compensation for certain signal changes.

In terms of sound quality, its total harmonic distortion (THD) is -0.05%, and the signal-to-noise ratio (SNR) reaches 90dB ~ 95dB. The Si270x amplifier provides 7-band equalization adjustment functions for the main audio device and 2-band adjustment functions for the auxiliary audio device. The former is mainly for the output of the main audio stream, so it provides more adjustment flexibility. The auxiliary channel is generally connected to an auxiliary channel such as an external low-pass channel, and only some adjustment functions are required to meet the requirements. In order to avoid EMI caused by traditional Class D amplifiers, audio engineers often use low-cost but inefficient Class A/B amplifiers, which require larger and more expensive power supplies and heat sinks (some require 9V or 12V battery configurations). The Si270x amplifier allows developers to use smaller and lower-cost power supplies and fewer batteries in portable audio systems (such as speakers) without reducing performance. Compared with systems using Class A/B amplifiers, the Si270x can extend the playback time by 2.5 times, and only half the number of batteries (4 AA alkaline batteries) can provide up to 8.4 hours of playback time.

"In addition, the built-in audio processing DSP function of Si270x can also share some computing tasks with the main processor of the audio system," said Rick Beale. The integrated DSP audio processor uses Silicon Labs' unique dual-core 24-bit DSP core, which can balance the modulation of speakers, increase the volume with a compressor, and has built-in 3D sound effect support. Silicon Labs Si270x only needs low-cost filters to work with audio equipment using power-saving and EMI-suppressing Class D amplifier technology, which will completely change the existing audio amplifier market landscape.

Figure 1, Si270x block diagram