Power amplifiers have been changing at a fairly rapid pace. Switching power supplies have reduced weight and offered added protection from line voltage variations. Class-D topology has cut the size of power supplies and heatsinks. Amplifiers are also shedding their conventional rack-mount chassis and taking up residence in loudspeaker cabinets.
In light of these developments, it’s time to take a new look at the amplifier-loudspeaker relationship. We begin with the usual questions: How many watts will that loudspeaker handle, and how many watts will that amplifier supply? If the numbers are reasonably close, then they should work together just fine, right?
Maybe so in the “consumer” audio world, but on the professional side, factors such as optimized sound quality across very large spaces, as well as reliability and return on investment, are much more paramount.
Professional power amplifiers are conventionally rated in wattage output with added terminology along the lines of “continuous sine wave” and/or “RMS” (root mean square and commonly thought of as “average”) at a certain “THD+N” (total harmonic distortion plus noise) into a certain “load impedance.”
The quote marks around the terms are used because the meaning of each can vary when applied to sound reinforcement. Voice and music are not continuous, and therefore, they are not sine waves but instead have a wide variation in levels, typically 10 dB to 20 dB from moment to moment. Peak-to-RMS on a sine wave is typically 3 dB. Meanwhile, a higher THD+N allows a greater power rating, while lower THD+N reduces that power rating.
The human ear discriminates differently. It will tolerate, and perhaps even find pleasing, a surprising amount of second harmonic. At the same time, a tiny fraction of seventh harmonic will run the ear (and the person attached to it) out of a room.
Further, the load typically used to test amplifiers is just a fixed resistor with little reactance at all. Anyone who has seen a loudspeaker impedance curve knows that there is very little of it that looks like a resistor. To make things even more interesting, different amplifier topologies react differently with the same signals and loads.
The most obvious example of how old thinking has led to excess and delusion is in the horsepower race. “My loudspeaker can handle 10 gigawatts and can take all that your 100-terawatt amplifier can deliver! You know – headroom!”
These words are usually uttered by the same people whose Hummer routinely cuts off your Accord on the interstate. However, if the intent is to at least occasionally deliver understandable lyrics, along with music that has instrumental separation and clarity, there needs to be more consideration of the total picture.