Class D AC

In yesterday's post Built ins I asked the question: if an AC regenerator is good at improving performance of audio equipment, why not build one into each and every product you make? Why go to all the trouble of building a separate AC Regenerator, like that found in a Power Plant? The first thing we should ask is just what is an AC regenerator? Briefly, an AC regenerator takes the AC from the wall power outlet, converts it to DC (battery voltage), and then regenerates a new AC output from which you power your equipment. Unlike a power conditioner, which removes high frequency noise from the power line with a low pass filter, the AC regeneration concept can be viewed more like that of rebuilding the power rather than cleaning it. There's a simple animated 2.5 minute video explanation you can watch here to better understand it. It's a little bit of a goofy, salesy video, but if you're new to the concept it'll help. To make an AC regenerator you need a power amplifier connected to a sine wave generator. So the first and most obvious answer to the question of why not build it in is simple: real estate. A power amplifier, power supply and sine wave generator are larger than most of the audio products we manufacture, thus doubling or tripling their physical size. But, wait a minute. Power amplifiers don't have to be huge. There are many great sounding power amplifiers on the market using Class D technology that should do the job. Why not build one of those into the product? That's a great question and one we are frequently asked. It is possible to build a regenerator from class D technology. In fact, a sine wave UPS (Uninterruptable Power Supply) device you might have connected to your computer, to protect it from a power outage, likely uses Class D switching technology for the amplifier. It works. Unfortunately it has problems, many times worse than that coming out of the wall outlet. Class D uses a technology known as PWM (Pulse Width Modulation), consisting of a series of long and short pulses to recreate the sine wave or music it is amplifying. The transitions between pulses must be removed before they are presented to the loudspeaker or powered unit, because they are noise; lots of noise. To do this designers add a filter to the Class D amplifier output, and this creates three main problems: increased noise, higher output impedance, and an inability to handle large current demands. We'll go through these problems tomorrow.