Things that make you go, hmmm…
“Why do big power amps have level controls on them? It seems everyone just turns them all the way up and sets their levels elsewhere (crossover, EQ, etc.).”
Some of it is just for convenience, but they really can (and often should) be used for level setting. We’ve talked a lot about gain structure in past issues of inSync (check the archives) and no doubt will cover it more in the future. The level controls of the power amps should be set so you maintain consistent gain staging throughout your system, while allowing ample headroom. If you need to turn them up all the way to achieve that (as is often the case) then you certainly should. It’s just not a good practice to turn them all the way up and then operate the previous gain stage – whether it be an EQ, crossover, or your mixer – at only a small fraction of its nominal output level. This seriously degrades your signal to noise level and also sets up a potentially dangerous situation.
I‘ll tell you a quick story to illustrate. About 15 years ago I was brought in to mix a concert of a major up and coming band at a large arena in the midwest. The PA consisted of 20 full range boxes per side, and 10 subs per side. All were custom made by the sound company, who shall remain nameless. Their normal practice was to operate all the amps all the way up and make adjustments at the crossovers. This can be a nice convenience when the crossovers are out at the house mix position, as was the case. But when I noticed how far askew the relative levels were on the crossovers (bass WAY up, mids and highs WAY down) I recommended (well, it was a little more than a recommendation actually) we make some adjustments on the power amps to even things out better. We had about half of the adjustments done when a forklift driver dropped a huge riser on the snake, splitting it in two. Now, on a good day you can cut an audio snake in two (not that I really go around trying this) and nothing serious will happen other than needing a new snake. However, in this case a huge spike was sent to the power amps, many of which were still at full volume. Guess what? Aside from a very loud noise for a second until circuit breakers tripped, most of the speakers connected to the unadjusted power amps were destroyed, many of them blowing literally into pieces. In fact, little pieces of titanium were seen falling out of the horns onto the arena floor. Some of the speakers connected to the amps that had already been turned down failed as well, but most of them survived and we were at least able to put on a show.
Obviously this story is a bit extreme (though true) and there were other problems with the system, but the point is you really shouldn’t run the levels on your power amps any higher than you need to. This is especially true when powering high frequency drivers with large power amps. They don’t normally need as much power to keep up with the low end so turn them down. How high do you run the amps?
It’s not that hard to figure this out. Get a signal into your mixer and set it up so you have a nominal level at its output. Not all the way up, just a good nominal level; right up to the red on its meters. Then set up the other downstream devices so they pass this through at their nominal levels. If you’re using an electronic crossover set it up so it’s at near unity gain across all of its outputs. If you’re not sure where that is just turn it up most (not all) of the way. You need to keep an eye and ear out for distortion, but the idea is to drive the components on the front end pretty hard while leaving plenty of headroom. Now adjust your power amp levels so your PA is operating as loud as you can possibly imagine it needing to go for the venue you are in. If some of the amps start to clip then you know you are out of headroom at that point. You either need to reconfigure the system or leave the levels set there. Actually I would back it off a bit. You don’t want your amps to clip before your mixer even goes into the red. Just use some common sense and judgement here and you’ll be okay. Once all this is done you will have optimized your signal to noise ratio and set up a gain structure to best enable you to get a quality sound out of your system. You can apply these same techniques to your studio monitoring setup as well.