A power amplifier is generally designed to do two basic things: increase the power of an audio signal without changing it and control the motion of a loudspeaker. Some of the specifications associated with power amps can be a bit cryptic, so let’s clarify the terminology.
Signal-to-Noise Ratio
Any electronic device generates noise, and hopefully that noise is far lower in level than the audio signal. The difference in dB between the signal and the background noise is signal-to-noise ratio. In the real world, you probably won’t have to worry about the S/N ratio of a power amplifier because (1) modern power amps are pretty darn quiet, and (2) the racket that the audience makes far outweighs noise generated by power amps and digital mixing consoles.
Frequency Response
Frequency response gives you an idea of the amp’s linearity. This spec (e.g. “20 Hz to 20,000 Hz”) should be accompanied by a tolerance such as “+1/-2 dB,” meaning that the response deviates from “flat” (0 dB) by plus-one or minus-two dB. A tight tolerance such as the one previously mentioned indicates that the amp is relatively accurate across the frequency range (you don’t want to see “+5/-8 dB”), but it doesn’t tell you how the amp behaves when connected to a real speaker. Since most modern amps have a relatively flat response, I wouldn’t bet the farm on a frequency response spec.
Damping Factor
A wise man once said, “objects in motion tend to stay in motion.” This applies to speaker cones. An amplifier moves a speaker cone to reproduce audio, but what happens when the audio signal stops? The speaker cone is supposed to stop moving. I like to think of damping as braking ability — like the brakes in a car, not like breaking glass. Damping factor is a measure of how well the amp controls the motion of the speaker cone. A damping factor higher than 30 or 40 is probably sufficient, and you’ll often see numbers of several hundred. Amplifiers with good damping reduce speaker hangover, providing tight bass — but will not cure your hangover.
Maximum Input Signal Level
This is the largest signal that may safely be sent to an amplifier’s inputs, usually expressed in dBu. Signals above this level drive the amp into “clipping,” or severe distortion. If the amp is smart and has onboard protection circuitry, it will shut down before it blows up. Hopefully, the maximum input is well above average operating level, providing headroom. As an example: If max input is +22 dBu and average operating level is +4 dBu, you have 18 dB of headroom.
Input Sensitivity
This number, usually expressed in volts, is the signal level required by the amp to produce full-rated power output. In the real world, you need to know that your mixing console can produce this amount of signal (or more) to sufficiently “drive” the amplifier. Typical input sensitivity numbers range from 1 to 2 volts.
Speaker or “Load” Impedance
The speaker impedance spec gives you an idea of how “comfortable” an amp is when connected to different speaker loads. In ye olde days, we had to match the output impedance of the amplifier to the input impedance of the speaker, but modern power amps are relatively tolerant of varying speaker loads. That being said, we still want to make sure that the manufacturer provides a power rating into the speaker impedance we wish to drive. For example, an amp rated for 100 watts into 8 Ohms (Ω) will match very happily and in fact deliver 100 watts into an 8Ω speaker. If we connect a 4Ω speaker to that amp, the amp may be forced to work too hard, possibly overheating or tripping a protection circuit. If we connect a 16Ω speaker to this amp, no one gets hurt, but the amp won’t deliver its full power rating. Nowadays, just about all power amps are rated for 4Ω and 8Ω loads. Things may get dicey when connecting a 2Ω load because some amps can’t handle loads that low. They’ll run hot and possibly shut down. Ideally, the amp is rated to deliver full output into a range of speaker loads from 2Ω to 8Ω.
THD, or Total Harmonic Distortion
You can think of THD as a comparison between the input signal to an amplifier and the output of that signal from the amplifier. Let’s suppose you connect a test tone generator to the amp’s input and play a 1 kHz sine wave (go back to Audio 101 and recall that a sine wave is a “pure” tone that has no harmonics). In a perfect world, the amplified output would be the same, pure 1 kHz sine wave — but with a bigger amplitude. Alas the world is not perfect, so what is sent to your speaker could be a 1 kHz sine wave with a smaller 2 kHz signal added in. This is an example of second harmonic distortion, because 2 kHz is the second harmonic of 1 kHz (1 kHz x 2 = 2 kHz). If we found there a bit of 3 kHz mixed in with that signal, we’d also have third harmonic distortion (1 kHz x 3 = 3 kHz). THD is a sum of all these unwanted, whole number multiples of the fundamental that creep into the output along with the original signal.
IMD or Intermodulation Distortion
See above for contrast. IMD is not harmonically related to the original (fundamental) signal. IMD results from combinations of complex audio signals that have varying frequencies — like music.
Crosstalk
Crosstalk is the amount of audio separation between channels. When crosstalk is poor, you’ll hear signal from one channel “leak” into another channel. Look for a crosstalk spec of at least 40 to 50 dB.
Output Power
Output indicates the number of watts the amp can deliver to a specific load, but we really want a power spec that includes the load impedance, a distortion value, and frequency range or bandwidth. For example, “100 watts per channel” is a very vague spec that raises a lot of questions. Is that peak power or continuous power? There’s a big difference. An amp that generates 100 watts continuously will more easily drive your speakers to louder levels than an amp that can produce a momentary peak at 100 watts.
How much distortion is being produced at that power rating? If it is a small amount of distortion (say below 0.1% THD) then you’re unlikely to hear it, but if that 100 watts comes at 10% THD, then much of the power is unusable for producing clean audio. Is that power rated into a 4Ω load, an 8Ω load or something else? Reputable manufacturers furnish power output into 8Ω and 4Ω loads, and sometimes into 2Ω loads, all of which clues you in to how the amp responds to difficult situations. The type of spec you’d like to see is “100 watts continuous per channel (both channels driven) into an 8Ω load at 0.07% THD, from 10 Hz to 22 kHz.” You may see another spec for the same amp stating “200 watts into a 4Ω load” with the same parameters — indicating that the amp should be able to safely handle a 4Ω load. This is useful information when considering whether or not you can connect two 8Ω speakers in parallel (resulting in a 4Ω load) for higher SPLs or increased coverage.
“Peak” power is the maximum short-term power an amp can produce. “Short term” means a fraction of a second. “Continuous” or “average” power is probably the most meaningful output power spec. It tells you how much sustained power the amp can generate, usually playing a sine wave into a specified load, at a specified frequency for a specific amount of distortion.
Notice I avoided “RMS” (Root Mean Square), which is sometimes confused with continuous power. That is not correct. A discussion of why is well beyond the scope of this article; for a detailed explanation, look it up in the Yamaha Sound Reinforcement Handbook.
AC Power Consumption
Do not confuse this spec with audio power output. This is the maximum amount of electricity required by the amp to generate the rated audio output. Sometimes it is expressed in watts, other times it is expressed in amperes. In the real world, you need to make sure that the electrical service used to feed the amplifier is capable of allowing the amp to run at full throttle. As an example, a power amp that draws 15 amps at maximum audio output, connected to a 10-amp electrical circuit will likely blow the circuit breaker when you hit the gas pedal. The unfortunate thing is that the amp will probably start up and “idle” without blowing the circuit breaker, which can occur when the show gets loud.
These are not all of the specifications used to describe power amps, but they are probably the most important. In a future column, we’ll take a look at how power translates to SPL.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
