Audio Basics: A Is For Ampere Page 2
But there's current and there's current. An electron only weighs 1/1837 as much as an atom of hydrogen, so it is not exactly power city. Dumping 10 of them every second into a wire is a futile pastime, but give us a few billion of them every second and they can move woofer cones. The density of the electrons flowing through a wire determines the amount of current (measured in amperes, symbol "A," or decimal fractions thereof) being passed, and this is what enables a given voltage to do work. In fact, voltage times current equals power (measured in watts, or W). So there!
Power is the ability to get something done in a certain period of time. Ask any successful politician, he'll tell you. A single Joe Blow with a mission is the equivalent of a few electrons. If he pushes long enough for his belief, people may start to listen to him after five, maybe ten years. But a congressman with the same belief (and millions of electrons' worth of PACs behind him) may get it codified into national law within a few months, because he is more powerful than Joe Blow. Political power is the ability to get things done faster. Electrical power is exactly the same thing. Think of it this way: Voltage is the idea, current is the clout, and power is the result of both of them acting together. Energy is expended whenever power is delivered; ie, work is done, and the energy required to do work is measured in joules. The relationship between energy and work/power is simple: 1 watt equals 1 joule per second. If your amplifier power supply stored 250 joules---typical of a very, very expensive solid-state amp or a moderately priced tube design---this supply could theoretically deliver 250W into a load for one second or 1W for 250s.
This is DC current flow. Most of us realize, though, that an audio signal is AC, which would imply that the electrons flow in one direction one moment and in the other the next. So let's consider AC, Alternating Current, for a moment.
The simplest soundwave---one having a fundamental frequency but nothing else---is a sinewave. A sine is a trigonometric function, but it's easier to visualize as the movement of a bicycle tire valve, as in fig.1. The graph plots the height of the valve in inches, relative to the hub of the wheel, on the left-hand vertical scale, vs uniform periods of elapsed time in seconds plotted on two horizontal scales. The lines extending to the right and then downward plot the changing height of the tire valve as the wheel turns. As you can see, the plot points---joined here by a line---trace the shape that has become the symbol of audio.
Fig.1 How the height of a bicycle tire valve traces a sinewave. The top group shows the positions of one spoke and the valve, vs elapsed time; the left group shows the valve's height vs elapsed time
Note that the horizontal center line of our sinewave plot, corresponding to the hub of the wheel, is marked 0, while the area above it is marked "+" and that below it is marked "-." If this were a soundwave, 0 would be the room's atmospheric (barometric) pressure, the pluses would be air-compression phases, and the minuses would be rarefaction phases. If this were an audio signal in a circuit, 0 would be no current flow through the circuit, + would be flow in one direction, and - would be flow in the other. That's the fundamental description of alternating current. It's also the pattern of every cyclical phenomenon known to man, like averaged seasonal temperatures, the motion of the tines of a tuning fork, the day/night cycle, and your spouse's mood swings.
Sonically, the pure sinewave doesn't exist in the real world, but is always accompanied by one or more harmonics---other tones which are multiples of the lowest one (the fundamental)---and it is their relative intensities which give different sounds their sonic "signatures" which can tell us what produced them and how wet the performer's lips were at the time. The audible spectrum for most young people ranges from 20 to 20,000 Hertz (cycles per second), and the high-frequency extension shrinks with advancing age until, at 60, it is typically limited to 10kHz or below. (Aural decrepitude was somewhat of a blessing during the heyday of analog discs, because it was a more effective filter of mistracking and surface noise than any electronic noise-reduction system of the time. With digital audio, it is much less of a blessing.) Don't be worried by the "k" in front of the "Hz": to avoid writing strings of zeros for high frequencies, this shorthand simply means multiply the basic unit by 1000. 10kHz is identical to 10,000Hz.
That, then, is what we're working with in audio: direct current, and alternating current spanning a fairly wide range of frequencies. What, then, does sound reproduction require that we do with them? Well, we need to be able to control the magnitudes of voltages and currents, we often need to separate AC from DC currents, and---because audio signals start out being very weak and must end up being quite strong---we need to amplify them. Finally, we have to convert them into soundwaves. We'll consider these one at a time rather than all at once, so as not to confuse the less brilliant minds reading this.