This article was first published several years ago in Live Sound International magazine, yet it still provides timeless reference information in clearing up misunderstood electro-physics of power amplification technology.
Q.} What does a power amp do, anyway?
A.) In a nutshell, a power amp takes your audio signal and provides both voltage gain and current gain to drive loudspeakers. The voltage gain is necessary because line-level audio runs around a volt, peaking at maybe 8 to 12V, and loudspeakers need more voltage than that to produce usable sound levels.
Voltage gain is determined by the amp’s circuitry and the position of its attenuators or gain controls; it is typically expressed as a mathematical multiplier, such as “40x,” or in decibels, such as “32 dB” (which is equivalent to 40x, by the way).
In an amp with a gain of 40x, O.25V in will produce 10V out, 1V in will produce 40V out, and so on, up until the output reaches the amp’s clipping point. A considerate amp manufacturer will tell you either on the front or rear panels or somewhere in the user’s manual what the amp’s full gain actually is.
The current gain is necessary because compared to the amp’s input, loudspeakers are very-low-impedance devices. You can think of impedance as the ratio of the voltage across a device to the amount of current it allows through. Therefore, the lower its impedance, the more current a device will allow to flow when you put a certain voltage across it.
An amp input might typically have an impedance of 10, 15, or 20 kilohms, which means that for a line-level signal, the input will draw minuscule amounts of current. That’s why crossovers, mixers, and other devices we use to drive amp inputs don’t have to put out much power.
But loudspeakers have to do lots of work, pushing and pulling air to create sound. They draw current because current, or electron flow, is useful. Electrons flowing through the speaker’s voice coil make the magnetic fields that react with the loudspeaker’s magnet to push the cone or diaphragm in and out.
Basically, current allows the loudspeakers to do serious work. Thus an amp has to have plenty of current capacity, although how much it puts out depends entirely on the output signal voltage and the load impedance.
With no load connected, meaning, a load impedance of Inflnity, the amp will produce voltage but no current. With an 8-ohm load, the amp will produce practically the same voltage and will put out current. With a 4-ohm load, the amp will again produce the same voltage and put out twice as much current as at 8 ohm, and so on (assuming you’re always running below the clipping point).
That’s what an amp does. How it does it will be explained in other answers. So read on.
Q.) If you take a 500W amp and turn it down 3 dB, then it’s a 250W amp, right?
A.) No, it’s still a 500W amp. It just takes 3 dB more input signal to reach that 500W full power level. For example, if the amp’s input sensitivity (the voltage at which it reaches full rated power for a given load) is 1V, then turning the gain controls down 3 dB means it will instead take a signal level of l.4V to reach full power.
