Aux-Fed Subs

This can be a slightly controversial subject with opinion split between those who love it and those who really don’t care for it much at all. Running a system in an aux-fed sub configuration can help to clean up and enhance the clarity of a live mix. There are significant advantages, but a little thought about routing and alignment is necessary to implement them properly.

The Concept
When establishing a mix, an engineer would likely use high-pass filters on the majority of inputs, many having no useful information below 100Hz or so. This tends to be primarily important on vocal microphones in order to cut rumble, stage noise, plosives and proximity effect. Since the HPF on most consoles is 12dB per octave, to set it with a corner frequency of 100Hz would result in a 15dB cut at 50Hz. This is significant attenuation, but summing a number of these sources together, particularly microphones in the same area picking up almost the same spill will result in this unwanted low-frequency energy becoming considerably more pronounced and audible.


The primary idea behind aux-fed subs is as a way of removing this unwanted low frequency energy from the subwoofers completely, applying signals only with relevant information in order to get a cleaner sub-bass output with minimal power wastage and maximum headroom resulting in a punchier mix. A by-product is that more direct control of the sub output level is attributed to the mix engineer. This may or may not be a good thing.

To realise this, it is necessary to alter the system control configuration and to set up an extra output from the mix console. Traditionally, an auxiliary is used for this, hence the term ‘aux-fed subs’, but as I will go on to discuss, any output can be used that can generate a unique mix derived directly from the input channels i.e. auxiliary, group or centre bus.

It depends how the system is designed as to how this will be configured, but the stereo output which would usually be routed to the mid/high cabinets and subwoofers is routed only to the mid/highs, with the extra console output feeding the subs seperately. Any console inputs with relevant sub-bass information are routed to the sub output. Of course, all speaker drive lines must be processed exactly the same as they would using a traditional stereo system, which may mean extra processing is required if using outboard system control.

Variable or Fixed Level?
Engineers may like to mix with aux-fed subs because it gives them greater control over the subwoofer output on a channel by channel basis. Personally, I don’t like to work this way, as allowing such variable control is detrimental to the system alignment.

If I were to mix a show using a standard stereo output configuration and I wanted to add bass to the bass guitar, I would use the low frequency control on the channel EQ, selecting either a shelving or bandpass filter with control over frequency and bandwidth. I use the same technique with aux-fed subs, although other people may prefer to turn up the sub feed. The difference is that increasing the level of the subwoofer results in a broad uncontrollable boost the same bandwidth as the sub bandpass, extending down to the lowest range of the subwoofer with a sharp roll-off at the low-pass filter cut-off frequency, usually 24dB per octave. Assuming the system has been aligned with a smooth frequency response through the crossover frequency, this would result in a stepped bass guitar response with the lowest notes being at a certain level and dropping drastically at the crossover frequency, probably as fast as one note to the next.

A sub feed does not necessarily have to be driven from an auxiliary; alternatively a fixed level bus such as an audio group or centre bus which track the input channel fader but do not have individual level controls of their own can be used. I believe this is a much better way of configuring a sub bus, as it doesn’t allow indiscriminate adjustment of sub-bass levels on a channel by channel basis, simply enabling low frequency information to be removed from sources where it is not required, the very reason for implementing a separate sub bus.

I believe on an individual channel that the sub feed should be on or off and when on, it should be equal volume to that if the console was driving just a stereo output. Of course, if you have to use an auxiliary for this purpose, simply setting it to either off or 0dB will achieve the same thing.

Alignment Issues
When aligning a sound reinforcement system, the sub must be phase-aligned (or time-aligned) to the next pass-band through the crossover region. The crossover frequency is discovered not by reading off the frequency programmed into the spectral divider, but by measuring the system and determining where (spectrally) the output from both pass-bands is at the same level. This means that before time-alignment, each band must be level matched. It also means that changing the level of a band will render the alignment inaccurate; the crossover frequency will change and the system will no longer be phase-coherent through the crossover region.

FOH Mix Control

This should explain why variable sub level is undesirable from the mix console, as varying the level of the sub master will change the crossover frequency, hence the alignment.

When aligning an aux-fed sub system, the reference signal should be played out from the mix bus and sub bus at the same level. If gain or attenuation of the subs is required, it should be performed in the system controller. At this point, the subs can be phase-aligned to the mid/highs, after which the attenuation levels on the console should maintain their relativity i.e. if the alignment was performed with the mix bus fader at 0dB and the sub bus output fader also at 0dB, this relationship must be maintained. If the volume of the entire mix needs to be reduced, each bus should be attenuated equally. If the sub output relative to the mid/highs is altered, the alignment must be verified.

General Issues
The sub feed should always be post-fader; reducing the level of the source in the mix bus should also reduce the level in the sub bus. If it is run off a pre-fade send, you could end up in a situation where the input fader is pulled all the way down, but the source is still routed to the sub at full volume. Some consoles also configure the pre-fade sends to be pre-EQ which is useful for running monitors from FOH but not for subs. Any EQ applied on the channel should be applied to both buses simultaneously.

The pick-off point is also significant when using audio groups. If an input is routed to the sub and an audio group simultaneously, the sub feed will be post input fader but it will still be pre group output fader. This means that if you pull down the level of the audio group master, the sub feed is unaffected. It also means that if you compress the audio group, the sub feed will be unprocessed resulting in a varying sub level relative to the mid/high level depending on the dynamics of the source. Of course, VCAs are not affected by this problem; as a VCA controls the level of the input fader, any post-fader sends track both input fader and VCA changes.

When mixing on an aux-fed sub system, you should always be mindful of the crossover frequency between the main system and subs. This will have a bearing on what sources are required in the subs; the higher the crossover frequency, the more sources will need to be applied. Just like when dialling in high-pass filters, this is a job for the ears! In my experience, aux-fed subs don’t work with a crossover frequency above 100Hz.

Personally, when I mix pop music on an aux-fed sub system, I start with the Kick Drum, Floor Tom and Bass Guitar only routed to the subs. Keyboards may or may not need to be routed to the subs depending on how much LF information they have, but if it’s piano/strings/lead, I tend to leave them out. Playback should always be routed to the subs.

Aux-fed subs are my preferred way of mixing, especially after spending a year mixing FOH for a reggae artist where bottom end is so important. I find that they offer increased flexibility when mixing with no side-effects, provided they’re utilised properly and the system is aligned well. I prefer to run them off the centre bus for two reasons; it is simple to route an audio group to the centre bus if necessary and the centre master tends to be right next to the stereo master which makes adjusting the master volume and keeping relative levels simple. Even better, on some consoles you can program the centre output to track the stereo output.

My preference as a system tech is to set up the system with aux-fed subs so that they are available if a band engineer wants to mix that way, and if they don’t, just route every channel to the sub at 0dB; it is just like mixing with just a stereo bus provided the output controls are relative.

Tags: , , , ,

5 Responses to "Aux-Fed Subs"

  • Rick Perry says:
  • iminthemix says:
    • Steve says: