Comm Techniques Yield All-Digital High-Power Audio Amp

Virtually every electronic product today is digital, including products in the once all-analog domain of communications. Audio power amplification is about the last thing to be digitized. And thanks to communications techniques, D2Audio has created all-digital high-power audio amplification.

Class D switching amplifiers have been around for a while, especially in IC form, producing power up to several watts or more. Some higher-power units exist. Despite their main goal of very high efficiency, they lacked the fidelity to match traditional but less efficient class AB linear power amplifiers. D2Audio integrated DSP and feedback to achieve the very high power as well as world-class total harmonic distortion (THD) in a class D amplifier.

The primary reason for using switching amplifiers is to achieve high efficiency. Common-class AB power amplifiers are very linear with low THD. But they're also power hogs with efficiencies no better than about 50%. As a result, half of all the total dc input power goes to heat. That's why big heatsinks are always necessary in the higher-power units over about 50 W.

Class D switching amplifiers use a pulse-width-modulation (PWM) scheme to convert the varying analog audio input into a binary signal whose duty cycle varies with the input. The PWM signal then switches power MOSFETs off and on at a higher frequency. That high-voltage PWM signal is applied to a low-pass filter that removes the high-frequency signal, leaving an amplified audio signal across the speaker.

With such an arrangement, efficiencies over 90% are common. Also, the need for big heatsinks or other cooling methods virtually disappears. The main problem with this approach, though, is the switching noise in the output and higher THD than common linears. The noise and THD levels are adequate for many lower-powered audio applications that don't require stereo-grade audio.

D2Audio developed a switching amplifier that uses feedback to correct the noise and distortion. The PWM output from the power MOSFETs as well as the speaker current are monitored and converted to digital signals that are applied to a patented DSP chip. The chip adjusts the PWM signal to correct for the distortion and noise.

What results from this adaptive digital correction system is a THD+N as low as 0.05% in some models, which is even lower than that obtainable in traditional linears. Such feedback correction techniques have been used for years in RF power amplifiers. Now, the audio world can enjoy the benefits of high power, very high efficiency, and ultra-low THD+N.

The company offers a line of intelligent digital amplifier modules containing its patented DSP PWM controller chip. These modules are designed for OEM applications in stereo receivers, multichannel surroundsound systems, autosound, public address systems, and anywhere else high-power audio from about 100 to 2000 W is needed.

The modules accept analog inputs that are converted to PWM at 384 kHz. They also accept serial digital audio at 44.1, 48, 96, and 192 kHz from CDs, DVDs, and other digital sources. The DSP controller chip produces the appropriate PWM output for the MOSFET switches. (This patented chip is not sold separately.) As an aside, D2Audio is the only switching amplifier company that's THX certified, the standard for theater-like audio quality.

Many different models and configurations are available (see photo). The company also provides a graphical development system called Audio Canvas, which OEMs can use to customize the DSP controller chip to their application.