Here’s the “hold the baloney” sandwich on Class D Amplifiers. An expert from Lenard Audio.Class D amplifiers are described as PWD Pulse Width Modulation amplifiers. Class D is similar to a Switch Mode Power supply. Switch mode power supplies and Class D amplifiers are low mass, generate very little heat and approx 90% efficient. The majority of small amplifiers less that 20 Watts in computers and domestic application including TVs etc are now Class D amplifiers. Above 20 Watt the complexity and problems of Class D increase exponentially. Similar to switch mode power supplies, Class D amplifiers are virtually un-serviceable and life after 7 years considered a blessing.
Switching on and off a light bulb, electric motor, electric heater etc at high speed to control power is not rocket science to understand. With a little imagination a speaker could be switched on and off across 2 batteries and by varying the on time compared to the off time (providing the switching speed is above hearing range) the mass of the cone will average the energy to represent an audio signal.
Pre 1970 Valve technology reigned supreme. Outside of experimental computers the first transistors were used in small cheap radios which is how the Japanese electronics industry and Sony began. Transistors do and not have a linear function similar to valves, but are ideally suited to switching on and off. Class B analogue transistor amplifiers above 10 Watts were originally unreliable. It was assumed that transistors would not be suited for making high powered class AB amplifiers.
In 1964 Clive Sinclair an electronic inventive genius along with fellow engineer Gordon Edge developed the first PWM amplifier. The X-10 was marketed as 10 Watts but only produced 2 to 3 Watts. The Z-12 arrived shortly later which reliably produced 12 Watts, and the race was on. Almost every young electronic audio enthusiast started experimenting with PWM amplifiers. It was believed that PWM amplifiers capable of 100s of Watts would be available at low cost before a man would set foot on the moon.
www.nvg.org/ Planet Sinclair
wikipedia.org/ Clive Sinclair
Similar to Dr Frankenstein everyone was originally unaware of the monstrous lurking problems that were about to be unleashed. Any radio or TV in close vicinity to a PWM amplifier was blocked out by the Rf noise interference generated by the high frequency switching of the transistors. Test equipment capable of analysing switching frequency rise times that approached the limits of physics was not readily available. Government communication authorities quickly outlawed the use of any device that interfered with Rf transmissions. Another 30 years would pass before the science for controlling PWM Rf noise radiation was better understood. Very few high power PWM amplifiers today are CE Communication emissioncertified.
Understanding how a PWM amplifier works requires a lateral shift in thinking, but once the basic principle is seen it appears almost obvious. PWM does not require separate switching for each half of the wave form as in the first pic. A fixed high frequency square wave 100kHz to 1MHz is width modulated by the audio sine wave. At the output is a filter stopping everything above 20kHz from getting to the speaker.
The above pic is expanded to show only 16 pulses across an audio sine wave. When the square pulse widths are equal the averaged energy output is zero. As the pulse widths become wider than equal the averaged energy shifts toward the +Ve half of the audio sine wave. As the pulse widths become less than equal the averaged energy shifts toward -Ve half of the audio sine wave. But how is this achieved?
(1) To make the transition from a audio sine wave to a fixed 100kHz to 1MHz modulated square wave that simulates the energy of the sine wave, requires an intermediate step. First, a fixed 100kHz to 1MHz triangle wave is generated. The triangular wave must be approx x 100 the highest audio frequency. As the sine wave moves across the high frequency triangle wave a comparator recognises which part of the sine wave is above or below the points crossed on the triangle wave.
(2) As the triangle wave is crossed by the sine wave the MOS-FETs are turned on or off generating a high energy square wave of varying width. The MOS-FETs are not capable of turning on and off instantly. In the real world the speed of light is a theoretical concept. The Slew of the MOS-FETs governs how quickly the change from off to on and off again can happen.
(3) This transition from off to on represents the leading or trailing edge of an extreme high frequency which is magnitudes higher than the 100KHz to 1MHz fixed switching frequency. As the corners of the leading and trailing edges of the square wave make the transition from vertical to horizontal Rf ringing noise is generated. This Rf ringing also creates secondary problems inside the MOS-FETs that result in excessive heating and potential breakdowns.
The lower part of the above pic shows a more realistic view of the hi frequency square wave in comparison the comparatively low frequency audio sine wave. It clearly shows the noise of the switching frequency imposed on the audio sine wave that has to be filtered out. The high frequency modulating frequency has to be filtered out leaving the resultant energy of the amplified audio. The LPF Low pass filter only allows everything below 20kHz to get to the speaker.
Class D amplifiers are similar to the switch mode power supplies in computers and therefore non user serviceable. Failures are simply dealt with by warranty replacement or purchasing a new amplifier. The technology of professional high power Class D amplifiers is so complex and fragile that reliable operation of after 7 years should not be expected.
A deeper technical understanding of PWM amplifiers and their limitations is on this PDF published by component manufacturer IR (International Rectifier)
Application Note AN-1071 by Jun Honda and Jonathan Adams
sound.whsites.net PWM An excellent technical review of PWM problems and solutions.
wikipedia.org/ Switching amplifier
www.digiamps.com PWM Amplifier Modules
