Shape matters

Our Hi Fi systems are hungry for clean power: the better the power the better the sound. I don't suppose the crowd reading this is unaware of the importance of power to the sound quality of our high-end systems. But a good question is why and perhaps even a better question is what? What does "clean power" mean? Clean power means perfect DC. DC without any AC on it and DC that is perfectly steady. Remembering that we must convert the AC from the wall to DC, getting rid of all the AC and keeping the DC steady and free of AC would seem like a challenge - and it is, in fact, a big challenge. So let's start understanding the process we go through of converting AC to DC and getting that clean power. It all starts with the AC and ends up as music. The shape (quality) of the AC has a big impact on the music because it has a major impact on the DC - and as we'll learn, turning DC into music is the easy part. AC means alternating between + and - and the purest, gentlest way to alternate between these two is called a sine wave. Yesterday we covered why sine waves matter- their gently curving shape is what our Hi Fi system's power supplies need to make the clean power - while the square shape we showed gives really terrible power. A sine wave is one of the few perfect shapes in nature - moving back and forth without any unnecessary motion. As engineers we rate the quality of a sine wave by its perfection of movement; the measurement system we use quantifies the lack of unnecessary motion called harmonics (THD). Harmonics are motions moving faster than the one we are interested in. So purity of movement, back and forth between + and -, are what we want to produce pure DC. How do we create this perfect motion called a sine wave? Did someone say "we need a perfect shape for AC"? No. The perfect shape of AC comes from the very process of generating it in the first place. Here's a picture of that process: See the magnets on each end? And notice the spinning coil of wire in the middle? As the coil spins closer to the magnet the voltage rises up in the coil gently with a single purpose and when the coil is closest to the magnet (called the peak) you get the most voltage. Then the process goes in reverse gently moving away from the magnet. Sound familiar? This single purpose gentle movement closer and further from the magnet, in a circular motion, results in a sine wave. It's that simple. Sine waves happen because of the way we generate the power - not by design. If this system were a phono cartridge and instead of a spinning coil, the coil moved back and forth at the end of an attached needle, we would call it a moving coil cartridge.