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Spike it!

— By Paul McGowan —
So why is a Class D switching amplifier not a great choice for building an AC regenerator? It sure seems like it would be perfect: it's efficient, powerful, low distortion and, best of all, physically small. And, as I mentioned in yesterday's post, if you buy a commercial off-the-shelf UPS sine wave supply, that's how it's built. So, clearly, it works. It's just not something you want in your high-end audio or home theater setup. The biggest issue Class D amplifiers have, when used to deliver AC power, is peak current delivery. They are just not very good at it. If all we were attempting to do with our Class D power amplifier is drive a pair of loudspeakers, or perhaps even a big electric heater or light bulb, a Class D amp is a great solution. It can deliver loads of current (watts) with very little loss or heat. It's really pretty perfect. Unfortunately, most of our equipment does not look like a loudspeaker or electric heater to an AC regenerator or power conditioner. This is because most equipment is very demanding when it comes to current. That demand comes in short little bursts requiring upwards of 100 amps! How's that compare to a loudspeaker? A big pair of inefficient power-hungry speakers may require upwards of 2 to 5 amps; a far cry from 70 to 100. This unusual demand our equipment places on AC power is called Power Factor. PF is a technical term engineers use to describe the relationship between the voltage and current coming out of our home's wall socket (or regenerator). An incandescent light bulb, an electric heater or, for that matter, most loudspeakers have a PF of 1. What's that mean? Simply stated, the voltage and the current track each other perfectly. Here's a picture of what a PF of 1 looks like: PF 1 When we next attempt to drive a power amplifier, preamplifier, DAC, projector etc. that use a conventional power supply something different happens. The PF goes down to less than 1 and the result looks like this: PF .7 Note the sine wave is the voltage, and then the little spiky things the current. This is where a Class D amplifier gets in trouble. When a Class D amplifier is asked to deliver a large amount of current quickly, it collapses and causes a deformation of the sine wave shape. This produces distortion plus a lack of needed energy at a critical moment in delivering power to our equipment. Bottom line? Class D performance under the conditions presented by conventional power supplies is far worse than simply plugging into the AC wall socket and should be avoided. That's reason number one why we would never consider using Class D in an AC regenerator, and neither should you. And there's more... tomorrow.

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