Lesson 3: Mixers

The Mixer is the central controller for the whole sound system. Every sound source goes into it, and from it is driven all the output devices. Digital mixers are fast becoming the way of the industry because of their high quality noise and dynamic range figures, their flexibility in the number of processes that can happen to each chanel, and also their ability to be programmed and automated, but to help understand the individual functions it pays to look at the signal flow of an analogue mixer.

Mixer design step 1

The most basic of all mixers would be to simply join the output wires from the different input sources, but this presents problems in 3 areas. If you refer to the table of sources from the previous lesson you see differing voltages and impedances, and the characteristic of balanced or unbalanced. If these are not treated correctly the resultant output can be lacking in either volume, tone, or go to the other extreme and cause overload by being too high.

Mixer design step 2

The simplest solution is to put a switchable input stage that is flexible enough to handle 3 different voltage levels, 2 different impedances, and balanced or unbalanced. Our mixer now has all inputs set up to match their various sound sources, so theoretically all the channels should be at the same volume.

Mixer design step 3 Unfortunately, you soon find out that not all singers sing at the same volume, not all guitarists play at the same volume, not all drummers... you get the idea, and so we need individual volume controls for each channel. These are commonly known as "faders". For added control we could add a master volume control. We now have a very operative mono mixer, and I have used several like that through the years.

Mixer design step 4

Next thing we might notice about the output sound is that some sources might sound a bit "tinny", while others sound a bit "boomy". It would be good to add some tone controls to each channel.

These could be as simple as a single treble control, or as complex as a multistage "parametric equaliser", which will be talked about in the lesson about Effects Processors.

Mixer design step 5

OK, so now the sound out the front is sounding good, but the band is complaining that they can't hear themselves up on the stage, and the mix they want on the stage is different to what the audience wants to hear. The solution is simple. We create a second mixer using the same set of inputs, put another volume control on each channel going out to the 2nd mix "bus", and another output master volume, and we have the foldback send. The placement of the junction in the circuit is important. If it comes after the fader it will be affected every time we adjust the sound out the front, so it needs to be placed BEFORE the fader. This gives rise to the name "pre-fade send", and its not uncommon to see several prefade sends, allowing several different mixes for different foldback locations on the stage. This point is also useful for doing recording mixes.

Mixer design step 6

Something is still missing. The sound sounds "2 dimentional". All the sounds are jumping out of the speakers right into your face. There is no depth to the sound. This is the job of the reverb unit. This is explained more in the effects chapter, but for now its important to note that, if we put the effects sends before the faders (like the foldback send), and the effects returns straight into the main bus, when we turn the chanel fader down the sound can still get out to the house through the effects bus. To get around this problem we put the effects send junction AFTER the fader. This gives rise to the name "post-fade send", and its not uncommon to see several postfade sends, allowing several different types of effects for different voices/instruments on the stage.

Mixer design step 7

Our mixer is starting to look quite impressive but its still only mono, which is fine for live mixing, but not for recording. The solution is to add another bus and a "Pan" (panorama) control on each chanel to choose if we want that sound to come out of the left bus, the right bus, or any combination in between.

Finally we have a fully functional mixer. Anything we add after this are options (eg, phantom power, meters, pre/post fade monitoring). Its nice to have lots of options on our mixer, but it really comes back to what situation we want to use the mixer in. A multitrack recording studio will have many output busses to choose from instead of just left and right, whereas a live performance mixer might have many prefade sends to give many foldback mixes around the stage.

We're now ready to move on to Amplifiers. Click here to go to the lesson about types, classes, and configurations of Amplifiers, or Click here to return to the Stage & Studio Equipment index page.