The technological advancements in live sound amplifiers over the last couple of decades are truly impressive.
Modern amps often have built-in DSP (signal processing with which to "tune" your loudspeakers); network connectivity to control and monitor system health; and more efficient power delivery that allows for extremely powerful, yet lightweight designs. These advancements, however, often make it even more confusing to make an informed choice.
Regardless of whether you do the limiting in the amps or in standalone DSP, do it somewhere!
The goals for a system designer remain the same: maximize the potential of your sound system (PA); somehow protect the PA from damage; and keep electrical power consumption, heat dissipation, and weight (particularly for touring) as low as possible.
With all of the options available, how do you find the best fit for your PA? I hope to bring a little clarity to what I believe are the key considerations.
Many loudspeaker manufacturers make it a fairly easy decision, thankfully, assuming your budget allows you to follow their recommendations (many of the available amplifiers today are remarkably expensive).
Whenever possible, though, following the manufacturer's recommendation will likely yield the best results and minimize the likelihood of damage to your loudspeakers. And because many of the amps available today have DSP, loudspeaker manufacturers will often also provide tuning presets that you can load into the amplifiers, to get the best sonic performance from your system.
For example, the JBL VTX rig at Lakewood Church could be powered by any suitably powerful modern amplifier. However, the manufacturer recommends Crown iTech HD amps, and provides tuning presets for them. This made the choice easy for us, as the pairing of these loudspeakers and amplifiers just works, and it sounds great with little effort. Then there is the additional comfort factor of knowing it will be unlikely that we will "blow up" the loudspeakers, as JBL has helped to ensure the amplifier settings will prevent the loudspeakers from ever being overloaded.
Many other loudspeaker manufacturers do the same thing, and some take it a step further. d&b audiotechnik, for example, supplies their own amplifiers, making the process even simpler. And then there are companies like Meyer Sound, who (with a few exceptions) make exclusively self-powered loudspeakers (with amplifiers built into the loudspeaker cabinet).
Ideally, then, you would purchase amplifiers that the loudspeaker company endorses and use the presets already configured within the amplifier. However, sometimes you need to figure it out on your own, and that's what we'll talk about now.
The program content we feed to loudspeakers has both a peak (instantaneous) power level and an RMS (root means square, or average) power level. This is because normal program content (e.g., music and speech) have dynamic range there is a difference between the average energy in the signal (which is approximately perceived as loudness) and the temporary peaks (transients) that naturally occur, but which don't have much effect on our perception of loudness. These two different power levels happen to correlate to the two main ways loudspeakers sustain damage.
Loudspeakers have two failure modes. Thermal failure happens when the RMS (average) energy is too high, for too long, and the internal components quite literally melt. Mechanical failure happens when the peak (instantaneous) energy is too high, and that causes the components to attempt to move too far and end up breaking as a result.
The conventional wisdom about matching amplifiers to loudspeakers says that you should have an amplifier capable of delivering twice as much power as the loudspeaker can handle. If you are trying to get the most SPL out of your system, you need to push it to its limits.
If you have an amplifier that isn't quite as powerful as it should be for that condition, you will clip (distort) the amplifier outputs whenever you attempt to drive the system to its maximum SPL. Whenever you clip a signal, you automatically get a byproduct in the form of unwanted harmonic content. This extra content raises the amount of RMS energy feeding the high-frequency drivers of your PA, and can cause thermal failure. This is where the notion comes from that undersizing your amplifiers is dangerous to your PA.
The reality is that you can damage a PA with almost any amplifier, if you're careless enough. The solution is to use limiters to protect the loudspeakers from both thermal and mechanical failure. In order to prevent thermal failure, you need an RMS limiter that will stop the average power from exceeding the "continuous" power rating (the thermal limit) of the loudspeaker. In order to prevent mechanical failure, you also need a peak limiter that will stop the instantaneous peaks from exceeding the "peak" power rating of the loudspeaker.
Knowing this, then, the ideal amplifier choice is often one that has a little more power than you need (so there is always a little unused "headroom" to ensure that you won't clip), and which provides both RMS and peak limiter options as part of its built-in DSP.
You can, of course, provide these limiter functions in a stand-alone loudspeaker processor (DSP), such as a Lake, BSS, Biamp, Ashly, or XTA product (to name a few). In this case, it is often advisable to run the amplifier levels "wide open" and manage gain in the DSP, so that nobody can come turn up the amplifiers later and cause problems.
Even with the excellent limiter options available in standalone DSP, though, I still like the idea of the limiters being in the last possible place in the signal chain (i.e., in the amplifiers). However, if you need a large quantity of amplifiers for your system, it can certainly be more cost-effective to let standalone DSP do the job and purchase amplifiers with no internal DSP.
Let's come back to the idea of an "undersized" amplifier. Remember that it's when the amplifier clips that causes problems, so the solution is simple: set limiters so that doesn't happen. As long as you don't clip the amplifier, there is absolutely nothing wrong with using an amp that is "too small." You won't get the maximum SPL potential out of the PA, but you may not need it, anyway. Many loudspeaker systems are capable of getting much louder than needed, so a little wasted potential may not be a big deal in the long run. Plus, loudspeakers tend to behave in a nonlinear fashion as you approach their limits anyway.
The takeaway should be that the amplifier size you choose is a function of balancing the maximum SPL you need out of the PA, with cost. The protection, whether the amplifier is oversized or undersized, comes from having both RMS and peak limiters to protect the loudspeakers (and prevent amplifier clipping in the case of an "undersized" amp).
Regardless of whether you do the limiting in the amps or in standalone DSP, do it somewhere!
I mentioned in the beginning that many modern amplifiers (usually those that have built-in DSP) can be networked for remote monitoring. I am a big fan of this function. Not only do you have the ability to monitor levels and limiter activity, you can also mute the system this way.
When doing a "burp" test (short bursts of pink noise done one amplifier channel at a time to test the loudspeakers), it is incredibly handy to be able to do this via a computer in the room with you.
The other major function that many modern amplifiers have is an ongoing load test, which continually monitors the connected loudspeakers. By comparing the reported load (e.g., 1.7 ohms) to the expected load (perhaps 4 ohms for a pair of 8 ohm speakers in parallel), you can see that something is wrong. Perhaps the loudspeaker cable is developing a short, or a voice coil is melting). This load test doesn't usually conclusively prove what is wrong, but it will often give you an idea of what to investigate. And with the complexity of modern PA's, any "health" data we can get is helpful.
In a nutshell, keep the following points in mind:
Whenever possible, consult the loudspeaker manufacturer and use their recommended amplifier pairing suggestions and presets.
Use both RMS and peak limiting to fully protect your loudspeakers.
Don't be afraid to use a smaller amp than your loudspeaker can handle, if you don't need the SPL, and you use limiting to prevent the amplifier from clipping.
Otherwise, pick an amplifier that delivers around two to four times the continuous power rating of the loudspeaker.
Do your loudspeaker tuning either in a standalone DSP or in the amplifier's DSP (if available), but please don't do it in your console. Console-based PA tuning is susceptible to accidental fiddling by others and means that swapping out your console will also result in you losing your system tuning.
Generally, don't be tempted to load an amplifier channel lower than 4 ohms, even for amplifiers that can handle it. It typically requires larger loudspeaker cable, both because of the higher current involved and to overcome cable loss, and it can also make some amplifiers nonlinear.
And finally, if you need help making the choice, or determining limiter settings, do not be afraid to reach out to a preferred integrator, a consultant, or (maybe) the wonderful online community. If you pick the latter, though, just be prepared to judiciously exercise your comprehension skills!