Tech Tip:Using a Powered Subwoofer to Augment Your Existing PA
Expand the power of your current setup without breaking the bank
More and more people are recognizing the benefits of a powered subwoofer, and not just DJs or techno musicians who need to pound out an earth-shaking beat. A subwoofer handles low-end frequencies very well and can offload some of those responsibilities from your PA’s midrange-oriented cabinets. You can buy a three-speaker system that includes a subwoofer (or will accommodate one when the time comes to upgrade), but anyone can use a subwoofer in any system at any time. The trick is in the mixer and the feature called an aux send or aux bus.
Using your mixer’s aux bus, you can selectively route low-end signals to a dedicated subwoofer, leaving your top boxes, or midrange cabinets, to handle everything else—guitars, vocals, keyboards, and the drum kit minus the kick. For bands that have been getting along with their current PA and are starting to play larger venues, or just realizing they need to kick up the output a notch, investing in a subwoofer will often supply the additional power you need at an economical price. And you don’t have to scrap your current system!
Take the sub way
Here are some of the advantages of bringing a subwoofer into your system.
Fig. 2: Use the Aux Send level control (on the channel strip) and the Aux Send output to feed the subwoofer.
- Maximize your total power by directing midrange and treble frequencies to the top boxes and low end to the subwoofers. This means that your system will be louder at the same power output, or that your maximum output power will be the same as a larger system’s.
- You can be more directional with your midrange cabs. This is because sub- woofers are non-directional; you can place a sub almost anywhere near the stage (within reason) and achieve a similar effect as if it were directly under (or connected to) the midrange boxes.
- You can keep stage rumble out of open microphones. This is especially important in outdoor gigs with makeshift stages that often ring and rumble when people walk across them or when vehicles pass by in close proximity. For rumble reduction, you need a high-pass (low-cut) filter that can be found in most crossovers and if you do a lot of outdoor venues and festivals, they’re well worth the money. And even when you can’t always hear it, without a high-pass filter your midrange components are expending energy inefficiently in attempting to transduce those extraneous low-frequency energy impulses in your system.
- The two different signal paths can be processed separately. You can obviously apply individual effects and EQ to each channel, but sometimes it’s helpful to process the entire bus, such as when using feedback eliminators, crossovers, or delay compensation. In these cases, a separate subwoofer path allows you to modify just the mid/high signal, just the low-end signal, or both. Want more bass? Don’t EQ, just turn up the aux send control!
Adding an aux sub to your system is easy. First, decide on what instruments will go to the subwoofer. That’s usually the kick drum and bass (electric or acoustic), though you could additionally add a floor tom and split keyboard part. Figure 1 shows a drum kit whose kick mic is set up to go to Channel 1 in the mixer.
Once the mics and inputs are taken care of, you have to set up your mixer so that the drum is sent to the subwoofer. This is done using an aux send. The aux send volume control is on the channel strip. The output of the signal is usually a back-panel jack to the right side as you face the mixer. Run a cable of the appropriate length from the aux send output jack to the subwoofer’s input (see Fig. 2). If you have other instruments, run those at the appropriate level using the channel’s aux send level too. It will be grouped with the kick and output to the subwoofer as well (also shown in Fig. 2). You can now think of the aux send control as a low-end EQ. Turning up the level increases bass response.
Deciding whether a dedicated sub will suit you is a simple test to make, as it’s pretty easy to borrow a subwoofer, and you should hear the results almost immediately. Routing gets slightly more complicated if you’re going to hook up a feedback eliminator or a crossover. But the first step in maximizing your available power is to get the subwoofer into the system and feed it the signals it’s best suited to reproducing. That leaves the midrange boxes free to do their thing, achieving the same volume level with less power, or using the same power to boost the output a little. And with a small PA, every little bit of clean, efficient boost helps.