Paul's Posts — 21 October 2012
By Paul McGowan
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.
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Soundminded
Today power quality matters more than ever. Our society has become completely reliant on it. It’s not just business that depends on it. When you go into a bank, go to a checkout counter at a retail store, your entire stay in a hospital, everything you rely on your computer for, you depend on reliable power of acceptable quality. What is acceptable for some equipment can be unacceptable for other equipment. It depends on the equipment itself and what it’s being used for.
Yes shape does matter. Engineers with the help of mathematicians have categorized less than ideal power waveforms into nine different categories of defects. Different types of power quality equipment address different distubances and to differnet degrees. For example, how many of these disturbances does a power strip with a surge protector and a small filter address. Only one or two probably. The MOV inside only protects your equipment from being damaged by an overvoltage spike. A few small capacitors and inductors may surpress some induced common mode and transverse mode noise. But they will do nothing for severe harmonic distortion. What about an expensive power cord? That won’t address any of them.
What about the kind of UPS you buy for your computer at a retail store? It’s called a line interactive UPS. It will address up to eight of them. The one it cannot effectively address are frequency variations beyond a certain point. It will address a power outage however. But its harmonic distortion can be horrendous, up to 40%. That won’t matter to your computer but it may be entirely unacceptable to your high end audio equipment. What about a much more expensive double static conversion UPS? that will address all 9 but the waveform may have up to 5% harmonic distortion. Does that matter for audio equipment? I don’t know. It probably depends on the equipment’s power supply design and what the harmonic components are. What about a power regenerator? It will produce no more than 0.5% distortion, one tenth of a double static converson UPS. But it has no batteries, no power storage elements (as far as I can tell) and so does not address a power outage. What about a rotary UPS? Well that’s probably about as good as you will get. Pillar makes a very good one. Be prepared to spend megabucks for one…if they even make one small enough for a home audio system. Usually these are intended for industrial use.
Do audio systems need UPSs and backup generators? That depends. For home listening probably not. It’s inconvenient to occasionally be without your favorite music but that’s about the limit of it. However, if you run a recording studio, have many musicians, technical people, administrative support on site for a recording session and power is unavailable, all the money spent for that session may be lost. And if it’s at a leased space you’re recording at like a hall that money is lost too and the whole thing has to be rescheduled.
One problem for new audio systems is the vulnerability of computer hard drives to sudden outages. That subjects them to data corruption, even damage. I’d been told this since forever by computer experts but like the shoemaker whose children go with holes in their shoes mine was damaged though just such neglect. A UPS for a computer hard drive is very advisable. If you use a laptop, you effectively already have one built in.