The Mixer: Functions & Connections Explained

Channels: More is Actually More

One of the first questions you need to ask yourself when thinking about buying a mixer is “How many channels do I need?” and the answer, of course, depends on the size of the band or ensemble that you’re planning to mix. The golden rule here is that it’s always better to have too many channels than not enough. For example, even a band with a standard setup can quickly eat up 16 channels, and you might also find yourself sound-checking a couple of bands ahead of the gig and want to be able to leave settings as they are. With a digital mixer this is largely solved by presets, but since bands can always come in with their own microphones or even more instruments, having the right number of physical channels is the most important feature to consider. Try not to get blinded by the numbers though. Some mixers only offer a limited number of preamps compared to the total number of inputs, leaving you with a bunch of leftover line-inputs that don’t have preamps. If you want to mix in maximum comfort, then a minimum of twenty-four channels is recommended.

Using the Equaliser

With most mixer control panels, the EQ knobs for each channel are lined up underneath the channel gain knob. Generally, equaliser controls are only used to cut or boost specific frequencies. DJ mixers or cheaper PA mixers often have just three EQ controls for the low, middle and high frequencies. More extensive mixers like the Behringer Eurodesk offer a parametric EQ section for the mid-range frequencies. A parametric equaliser is made up of two control knobs: one to select a specific frequency range (referred to as a ‘sweep’) and one to determine the cut or boost level applied to the selected frequency range. More expensive models are fully parametric and therefore, every frequency band includes a ‘sweep’. Take another step up and you’ll come across models like those included in the Soundcraft MH Series which include a third control knob for adjusting the ‘Q’. This means that rather than just being able to select the frequency range that’s cut or boosted, you can influence the extent to which the surrounding frequencies are adjusted. When boosting a frequency, a lower Q value is set to ensure a more natural sound. When cutting a frequency, it’s best to keep the margin as small as possible so that you’re removing as little of the unwanted frequencies as possible. Many mixers also offer an EQ section for the master signal. These are usually small faders for adjusting anywhere between three and thirty-three frequency bands, and are used to adjust the total sound which can be useful for boosting or tempering any troublesome frequencies or to counter the adverse effects of bad acoustics in the performance or recording space. This EQ section is usually pretty limited and is by no means a good replacement for two external thirty-three band equalizer since they offer far more in-depth tools for adjusting the left and right stereo signals, making it match up with the performance space.

The Sonic Colouring of Preamps

The first step in the signal path of a mixer is the point where the instrument is connected, so either a microphone or line input. A microphone has a lower output volume than a line level signal, which is another reason why microphone inputs are always fitted with gain-boosting preamps as standard, to strengthen the signal, and as we’ve already covered, the level of ‘boost’ is controlled using the gain/trim knob that’s usually found at the top of the channel strip. But preamps don’t simply raise the volume of the input signal, they actually make a significant difference to the quality of sound. This is determined by the character of a particular preamp, and how it colours the signal. Mixers made by brands like Mackie and Behringer are known for preamps that add relatively little colour to the sound, while Soundcraft and A&H preamps are known for adding warmth. When it comes to expensive and budget-priced models, the difference is often negligible, and the sound difference will be far more noticeable in the studio than it would be in a live setting. If you really want to profit from the depth and added dimension that a preamp can offer, then you’re better off looking at the more expensive end of the spectrum. There are actually many sound engineers that often use one or two external preamps to guarantee that they’ll get a specific sound out of the vocals, guitars, or drums.

Phantom Power

Unlike dynamic microphones, condenser microphones require 48 Volts of power in order to function. This is known as phantom power and can usually be fed to the microphone by your mixer by simply connecting it to the XLR port and activating the phantom power function using the on/off switch. Sometimes, this is a phantom power switch for the whole mixer, or you get a phantom power switch per microphone channel. In principle, phantom power doesn’t cause any harm to dynamic microphones, but just to be sure, it’s always recommended to switch the phantom power off when no condenser microphones are connected to your mixer. It’s also a good idea to switch off the phantom power before plugging in or unplugging anything, since it can cause a loud bang.

-10dBv & +4dBu

There are two standards for the level of a line signal: -10 dBv and +4 dBu.

• Professional studio equipment almost always operates using the higher level of +4dBu.
• Hi-fi equipment and instruments like keyboards use a lower level of -10dBv.

With many mixers, you’ll find a -10 dBv and +4 dBu selector switch, which can be found fitted to the inputs as well as the outputs. Say you connect a portable recorder (-10dB) to an output that’s been set to +4dBu – the signal is probably going to get too loud too easily and leave you with distorted sound. By switching the output to -10dBv, the problem is immediately solved.

Balanced & Unbalanced

The microphone inputs (XLR) are usually balanced to counter any microphone signal interference. In this dedicated blog, we dive deeper into the difference between balanced and unbalanced signals, but all you need to know here is that the better the mixer, the more likely it is that every input and output is balanced – including the line signals. In the case of balanced jack ports, a stereo jack cable needs to be used since, while a mono jack can be plugged in without any issues, you’’ll miss out on the benefits of having a balanced input or output. Normally, inputs and outputs are marked BAL or UNBAL so you know exactly what kind of port you’re using.

Microphones & Voltage

When a line output is taken from a stage piano and connected to a microphone preamp, a massive amount of gain is generated and you’ll get nothing but distorted noise. On top of that, line signals are usually unbalanced, meaning that when bridging longer distances with longer cables (from about five-metres up), the more sensitive the signal will get to noise and interference caused by electro-magnetic radiation. In this case, a DI box can also be used to weaken the high voltage of the line signal as well as balancing the signal and sending it as a microphone level to a mixer. Active DI boxes and condenser microphones have no voltage at all. These are fed phantom power (usually at 48 Volts) by the mixer via the connected XLR cable. Cheaper PA mixers often have a single phantom power switch rather than a switch per channel, which isn’t the best when, for example, you want to use a ribbon microphone or other equipment that’s more sensitive to electric current. In those cases, it’s better to seek out a mixer fitted with a phantom power switch per channel – and once you find one, always remember to only use the phantom power on/off switch when the channel is muted or make sure that the gain is all the way down, otherwise you’ll hear an electronic click or pop which, at a louder volume can damage both the ears of your audience and your speakers.

Low-Cut

Next to microphone inputs you can sometimes find what’s referred to as a low-cut button (or high-pass, or rumble). This button activates a filter that pushes back the lower frequencies. The filter frequency normally sits between 60Hz and 80Hz, so you can literally filter out the bassy sounds of things like footsteps and passing traffic, removing it before it reaches the preamp. Due to the nature of the low-cut filter, it’s not designed for the sound of a kick drum or bass guitar since it’ll remove all the ‘meat’ from the sound.

Insert (for Serial Effects)

The channels of a mixer could come equipped with inserts, so that the signal of a specific channel can be detoured through an effects unit before being sent back through the channel. To do this, a cable with one stereo jack at one end and two mono jacks at the other needs to be used. The stereo jack is plugged into the insert, while the two mono jacks are plugged into the input and output of the effects unit, creating a detour through the effect. This method should only be used when working with effects that lend themselves to insert effects (serial effects), since when sent through the insert, the whole signal is sent to the effects unit (a compressor, for example) and the treated signal then replaces the original signal.

Aux Send & Return (for Parallel Effect or Monitoring)

The aux send and aux return are used to add parallel effects to the sound. Via the aux send, the signal is tapped off, sent through an effects unit and then added back to the original signal via the aux return (mono or stereo). An effect like reverb is a typical parallel effect and is therefore always looped through the aux send and return. Each channel has a control knob for setting how much of the signal is fed through the aux send, which can be set to ‘0’ if a specific channel doesn’t need to be treated with the parallel effect. Some mixers also have a control knob for adjusting the overall volume of all of the aux sends, and for the aux return, there’ll always be a knob for controlling the level of the processed signal coming back through. The aux send can also be used for connecting up a monitor. In this case, every aux knob on every channel will be used to determine the level of each channel as it’s sent to the connected monitor. The aux return, meanwhile, can also be set up as an extra stereo or mono line input, but if you use it as a line input, you only have control over the volume level and will have no way to influence details like the EQ.

More About Auxes & Effects

The auxes, or ‘auxiliary sends’ always have a fixed place under the EQ section. With an auxiliary send, it’s possible to independently send each track to a separate output. This output can lead to an effects unit or a monitor, and the ‘return’ can either come in the form of a rotary knob or a separate fader that determines how much of the processed signal is mixed with the original signal. This way, all of the channels of the mix can (for example) be run through the same reverb effect. This isn’t just space and money saving, but ensures a more coherent mix. As such, it’s really important when picking out a mixer to take the rest of your setup into account, so that you’re certain that you have enough auxes to play with. When you’re working with a separate monitor desk, then you’ll have more than enough with just two sends, which can be used to add gear like an external reverb and delay. But there are many companies that offer analogue mixing desks with a digital effects processor, like Yamaha, Mackie and Behringer. By using built-in effects, you can use the front-of-house mixing desk to control the monitors, and the number of auxes needed then depends on the number of monitor mixes you need. An internal effects processor can be a useful solution when either space or money is limited, but the downside is that they can be far less convenient than external effects gear, especially since you rarely get the power to adjust any of the effect parameters. What you often get instead is an immense list of presets that are navigated by an unclear overview, which is not always the most intuitive way of working. The reverbs can often sound a little ‘cheap’ and electronic and the delays don’t usually come with an adjustable speed or tap tempo. If you’re setting up your mixer in the rehearsal room, some built-in effects will definitely do wonders for the vocals, but if you’re engineering for live bands, you’ll probably be better off using some external effects made by well known developers like TC Electronic or Lexicon.

Pre & Post Fader

An aux send can be tapped before or after the fader (so the channel volume control). When set to ‘pre’, it’s tapped before the fader, and when set to ‘post’, it’s tapped after the fader. When the signal is post-fader, then the processed signal (e.g. reverb) always maintains the same proportion as the original signal and therefore is affected by the fader setting, and usually, this is what you want. When the aux send is pre-fader, then the aux send signal remains constant, no matter the fader setting. The pre setting is great when you need to use the send as a monitor signal. Sometimes, you can set the send to pre or post fader with a button that’s usually placed just next to the aux send rotary knob.

Note: the aux send signal is (in principle) not always placed before the low-cut and equaliser functions of the channel, even though the channel is set to pre-fader.

Sub Groups

Sub groups (also referred to as groups or sub-busses) tend to be offered by larger mixers. Sub groups are extra outputs that allow you to send a selection of one or more channels out, which is useful for making custom monitor mixes for one or more of the musicians on stage. Using the controls included on each channel strip, you can determine which sub-group or groups the channel signal is sent to, while the signal is still sent to the main output.

Analogue or Digital

The time when analogue gear always sounded warmer than digital gear is long over, which is a real testament to how far digital mixers have come. Digital mixers are also able to offer a mass of benefits when compared to analogue mixers, the first being the size. Digital mixers can be much smaller and space-saving while still offering an array of essential effects – which in analogue terms would easily take up a couple of flight cases. The number of faders can be limited without sacrificing any inputs, and since digital mixers can be fitted with motorised faders, you can work with more banks. This means that as you flip from one bank to the next, the faders will automatically shift into the correct position. With just eight faders, you’re suddenly able to control up to forty-eight channels without having to lug around a two-metre-wide desk. With digital mixers, you can also create and save presets – presets that not only save effects parameters but the entire mixer configuration so it can simply be called up later. It goes without saying that, when it comes to mixing the live sound of two or three bands in one night, this feature is a life-saver. Multiple configurations can be saved, so if you use a mixer for rehearsals, you can strike the perfect balance for band practice and save it just in case a parameter gets accidentally tweaked. Many digital mixers also offer a recording function these days which is not just insanely useful when working on new material in the rehearsal room but for listening back over gigs. Good recordings can even be used as demos, and even album-quality material can be created with just your mixer. There are a number of options available when it comes to mixers that double as digital recorders. Most mixers will record the master mix to an internal memory, an SD card, or USB stick, and sometimes the file is immediately compressed. If you want to make the best quality multi-track recordings then you’ll want to look at mixers that double as audio interfaces, so you can connect it directly to a computer and record the final mix to a DAW.