Improving your PA system Part 2
So you’ve read Part 1 and decided that your existing speakers or speaker/amp combination are good enough – or you’ve made plans to buy a nice pair of active speakers. You’ve got a decent enough mixing desk and microphones and now you’re thinking – what next?
Go and buy an active subwoofer. Definitely, absolutely, no question. Buy it from the same range as your active mid/high speakers. “But”, I hear you say, “we don’t need any more bass, we’re not a dance act, it sounds just fine as it is”. Well, OK, if it really does sound fine then don’t spend any more money, go invest in something else. But before you go, think about this: the frequency range of a CD, and the human ear, is about 20 to 20,000 Hz. The first 350 of those 20,000 hertz accounts for HALF the power used in an average system. Those Yamaha speakers I was raving about in Part 1 struggle to reproduce frequencies below about 60Hz, which includes the whole bottom octave of the piano and the lower two strings of the bass guitar.
Adding a sub to your system does two things: most obviously it adds in that bottom octave, giving a lot of depth and visceral feel to the music without necessarily increasing the perceived volume; secondly the sub frees up the top speakers from having to try to reproduce those deep bass notes. Speaker coils can actually heat up from the power needed for the bass, leading to a harsher tone, and the amplifier’s power is being used up trying to push small speakers to produce tones they’re not really designed for. Taking that high energy load off your top cabinets gives them more room to ‘breathe’ and produce those silky smooth vocals you’ve been craving. Sometimes it’s convenient to have a small lightweight system, but if your budget and muscles can handle it, a sub is definitely the next step for a high quality portable PA.
Don’t be passive
If you have a traditional amplifier-speaker combination, you may be tempted to add a passive subwoofer that goes in between your amplifier and existing speakers. That will work, but I don’t recommend it. Remember that half of the power lies under 350Hz – your amp is going to have to pump out all the power needed to reproduce that added bass, maxing out and heading into harsh, distorted territory quickly. A bigger amp would help, but prices go up steeply for higher power models. You’ll miss out on one of the big advantages of using a separate amplifier for the bass: by separating out the different frequencies, your system can sound twice as loud with the same amplifier power. Bear with me on this one… two 200 watt amplifiers (one each for the bass and treble) will sound like 800 watt, not 400 watt. It sounds crazy, and is beyond the scope of this article to explain, but there’s a great article at ESP that goes into all the details. To stick with separate amplifiers and passive speakers you’re really going to need a second amplifier, external active crossover unit, and probably a rack to wheel it all around in. And you’re going to have to set it all up correctly. Is an active speaker system starting to sound attractive yet?
There’s two ways to wire up active subs and tops, depending on how the manufacturer designed the system. Older units tend to include the whole crossover (signal splitter) in the sub speaker, so you have to go from your mixer into the sub then back out into the tops. Newer systems (including the QSC K and Yamaha DXR series) have processing inside each speaker, with a little switch on the tops to tell them that a sub is being used. The advantage is that you can wire the speakers up any way that’s convenient so long as they each get a signal from the mixer. I should also mention that you don’t have to buy a pair – bass frequencies are generally non-directional and don’t add to the stereo image, so a single subwoofer will do fine. It’s traditional to buy a size up from your tops (i.e. 12 inch sub for 10 inch tops, 15 for 12, 18 for 15) but not absolutely essential if budget is tight.
Changing the subject
So, my advice for improving your PA system boils down to buying better speakers? Pretty much, yes. Ok, so I did cheat by stating at the beginning that I’m assuming that your mixer and mics are OK, which kind of begs the question. But I said that because I’ve used a lot of small PA systems where money has been spent on outboard gear like graphic equalisers and compressors to try to improve the sound, when it’s almost invariably the speakers that let the side down. A decent pair of professional active speakers, preferably coupled with a matching sub, will be tuned to near-perfection by the manufacturer, include smart digital signal processing, and won’t let you overdrive them. You will need far less heroics at the mixing desk to get a good sound and you won’t break your back hauling racks of heavy amplifiers around.
Of course, there’s always improvements to be made with good quality wiring, better microphones, and mixing desks. I’ve got articles in the works to cover all of these, so watch this space.