Adding a Subwoofer to your PA System

Adding a subwoofer to your PA system involves several steps:

Firstly, you need to identify the type of subwoofer that best suits your needs, so either active or passive, and the size and power required to match your PA speakers. For general PA usage, this will normally be a choice between a 15-inch or 18-inch unit, depending on the results you hope to achieve.

Active subwoofers come with built-in amplifiers, making them easier to integrate, while passive subwoofers require an external amplifier, so additional cost and extra cables are involved. In most cases, a mobile user such as a live band or DJ will opt for an active subwoofer system, as they are just a simpler and more modern option and you are assured that their built-in amplifier is perfectly matched to the woofer capabilities. They are also far less effort to hook up, requiring just a line-level input.

Though there are several methods of set-up possible, for a standard two-speaker PA rig you will most likely be connecting a sub to work in one of two ways, which is either full range, or as the low-end in a mid-top system. This is a personal choice, though is often dictated by the type of connections available on the subwoofer you are using.

Full range means your top speakers are playing a full audio signal as usual, with the subwoofer being provided the same signal and using its controls to cut off the unwanted mid and high frequencies. Some prefer this method, especially if they only have a single subwoofer, as they like their main speakers banging away at full tilt. Which is fine, if that's your preference.

The more technical option for many is the mid-top method though, as it takes the stress of producing heavier bass notes from your main speakers completely, allowing them to run more efficiently at louder volume. The subwoofer produces all the bass from the signal to your chosen cutoff frequency and passes the remaining midrange and treble only to the main cabs. This method requires the subwoofer to have what's known as a ‘high-pass’ crossover output, or the use of an external crossover system.

Choosing between an active and passive PA subwoofer depends on various factors including ease of use, setup flexibility, budget, and if it will be for installation or mobile use.

• Active PA Subwoofers: Active subwoofers come with built-in power amplifiers, simplifying the setup process greatly as you won’t need an external amplifier or speaker cables to feed it. They usually feature integrated crossover controls, allowing for precise frequency cutoff adjustment, and may include phase correction switching to help with integration and placement. Some may also include the aforementioned 'high pass' filter option to their output.

  Depending on their design and budget level, they will offer XLR, 6.35mm Jack, or RCA inputs and outputs to suit easy connection to your PA speakers and mixer. Some models include extra sockets for daisy chaining of additional subs and even options like DSP sound shaping.

  The convenience of active PA subwoofers makes them a popular choice for those who prefer a straightforward setup. They are fitted with impedance-matched power amplifiers and will include protection circuits to prevent damage from overloads or short circuits, adding an extra layer of reliability.

• Passive PA Subwoofers: Passive subwoofers, on the other hand, require an external amplifier and a crossover system, as the cabs themselves are just a woofer in a box. This adds complexity to the setup but can offer greater flexibility in terms of system configuration.

  Many people just prefer keeping electronics out of their speaker cabs, as failure of an amplifier on a passive rig can be quickly dealt with, whereas an active speaker is out of commission until its issue is fixed. Passive subwoofers are usually lighter and more portable since they don’t have built-in amps, which can be advantageous for mobile setups.

  When deciding, consider your specific needs. If you’re looking for a plug-and-play solution with minimal fuss, an active subwoofer is likely the best choice. However, if you have the technical know-how and want the flexibility to customize your system, especially if it's a venue installation, a passive subwoofer might be more suitable. Additionally, consider the size and scale of your events; larger venues might benefit more from the tailored power solutions that passive subwoofers can offer.

  One caveat to the passive route which doesn't get mentioned is the quality and type of amplifier found in modern active subs. Most are now supplied with highly efficient and lightweight Class D units, and it is often considerably more expensive to purchase a passive subwoofer and separate power amplifier of the same specification.

Connecting active PA speakers to an active subwoofer is relatively straightforward, thanks to the built-in amplification and crossover features. However, the connection methods differ depending on the inputs and outputs that particular subwoofer has available, and of course, if you are using a single sub or a pair.

As previously mentioned, for a mid-top setup your subwoofer must have a high pass crossover filter that can be routed to its output, which many lower-cost subs do not include. It can still be done though, but would require the addition of an external crossover or speaker management unit.

The other issue you may find is that some subwoofers have left and right inputs and outputs which make wiring from a stereo mixer to one sub and two top speakers simple, but many only have one input and output, complicating things a little.

Line Connection Methods

We will focus on stereo wiring here as it's the one that people ask about the most, but the methods are pretty much identical for mono. It’s important to understand that stereo is an effect that's only present on recorded music. A live band for instance is all mono, so there is no need to worry about left and right in the same way, as both top speakers will be outputting an identical mono signal.

• Single Sub with 1 input: An active subwoofer can be fed a line-level signal from a mixer (the left channel) which then links up from the subs output to the left PA speaker. The right PA speaker is fed directly from the right output of the desk. This is perfectly fine as bass below 200Hz is recorded as mono, and is output on the left channel as standard.

  The sub will receive the full audio signal, and then you set its frequency cutoff control so that it only produces low bass, but it will pass the full left signal up to the PA cab. These connections can be via RCA or XLR signal cables depending on your equipment, though always use XLR if it's available for its added noise rejection.

• Single Sub with 2 inputs: If the subwoofer has left and right inputs and outputs you can simply take a stereo feed from the mixer's main output to them, then link out from the sub to each of the main speakers. It can also be wired the same way as the single input option if you prefer to do it that way.

• Dual Subs Using a pair of subs actually makes things much simpler in most people's minds, regardless of the number of inputs they have. You can take separate cables from your mixers left and right outputs (XLR preferably) to each sub, then link from each sub to each top speaker.

  Any of these connections will have the top speakers receiving a full signal, with the sub ignoring any frequencies above its set crossover/cutoff level.

• Mid-Top Setup: The wiring for a mid-top setup will be similar. The exception is that you will usually have two subwoofers, or if it's a single sub it must have left and right inputs and outputs, and either way the subs must have a high pass filter to the outputs. To do this with any other type of sub would require an external crossover unit to split the filtered signal feed correctly to the top speakers.

  It's a more complicated setup than many care for, but it gives superb results, especially when using smaller PA speakers as they no longer have to reproduce any low bass signals, allowing them to focus just on midrange and treble for a really dynamic and detailed presentation.

• Crossover Settings: Active subwoofers have an adjustable crossover (also called a frequency cutoff), which allows you to manually set the point at which the sub will work. A good starting point is around 80Hz which covers sub-bass (20-60Hz) while dipping into the standard bass frequency range, but this may vary based on your main speakers’ capabilities and the overall effect you prefer, which is why it's adjustable!

Connecting passive PA speakers to a passive subwoofer is more complex as it involves separate power amplifiers and will usually require a crossover system or speaker management unit. It also differs because you won't have cables going between the subs and top speakers as you do with active units. The speakers and subs in a passive rig all get wired back to the amplifier(s).

• Signal Connection: This will depend on the amplifier's features. If the amp powering the subwoofer has built-in crossover adjustments available then you can just connect your mixer’s output directly to it using either an RCA or XLR cable.

  If it doesn't have these adjustments on board, and most don't, you will also need a crossover or speaker management unit which allows you to take the output signal from your mixer and distribute it out to several power amplifiers with specific frequencies boosted or cut. For a low-cost crossover, this will be a selection of buttons and pots giving you adjustment to different frequencies, while a management system will likely be digital with a display screen and menu system. Both essentially do the same job though, and are required in a passive PA speaker system to ensure the speakers all work as they should.

  As a brief explanation, if you don't use any type of crossover and just feed a passive sub with a full signal, what happens is you are sending an electrical signal full of the low frequencies the sub is designed for, but also all the mid and high frequencies too. It can't convert these high frequencies into movement, but it will try, and they will quickly destroy the woofer's voice coil by making it overheat.

  It's not so essential for top speakers as they can handle a full signal, with their built-in passive crossover distributing the frequencies to the appropriate drivers, but it’s just good practice to be able to cut certain frequencies even in a full range setup, as it enhances the sound and helps protect the speakers. A crossover system is however a necessity if you wish to run your rig in a mid-top configuration.

• Speaker Connection: Most power amplifiers have two channels and two volume controls (it's two mono amps in one box), and you will only be using one side to run a single subwoofer. Some amplifiers include a bridge mode which allows both power channels to be combined for more power into one output. (not all amps are suitable for bridging so don't ever just wire across two channels, despite what the internet experts tell you).

  From the amplifier’s speaker output, connect the speaker cables to your passive subwoofer. Ensure you use cables rated for the power output of the amplifier, and use terminated cables such as Speakon or 6.35mm Jack if the sockets are available, as they provide a superior connection to bare wire.

  It’s worth noting that large subwoofers are incredibly power-hungry, and should ideally have their own amplifier, as trying to run an entire rig from one amp will produce provide extremely poor results. If it’s a powerful enough amp, you can run a pair of subs from it (one per side), but your main PA speakers should always have their own amplifier, running either in mono or stereo depending on your preference via your crossover system.