Audio Mixer – The Knowledge

You Need

The mixing of a number of audio signals is such a common thing to do that one would expect the Net to be riddled with articles on how and why signals are mixed. There are plenty of circuits that show how it can be done, but very little that explains the benefits or drawbacks of any particular scheme.

In the early days, there was little or no requirement for mixing. In most cases, the band or (small) orchestra used one microphone, and the amplified output went straight to air for broadcasts or direct to the cutting lathe for recordings. This was before tape or wire recording was used. Because there was so little need for mixing, very simple schemes could be used. Peoples’ expectations were low too – at the time it was sufficiently amazing that recordings or “wireless” were even possible, so no-one was listening for any of the issues discussed below.

Even though there were issues, there were also ways to ensure that they did not impinge in any way on the listeners’ enjoyment of the programme material. If audio circuits had to be switched, master level controls would be reduced momentarily to minimise switching noises for example. As audio broadcasts and recordings became more complex, simple manual techniques were no longer suitable because of the number of channels.

Many of the earliest mixers may have had perhaps 4 channels at most. Even such a small mixer started to become problematical though. As channels were switched in or out there would be level changes on the remaining channels. Likewise, even adjusting a level control (fader) could cause the overall programme level from other channels to change.

To understand the reasons, we need to look at the circuits that were used (and still are in many applications).

Basic Principles

For the novice, it may seem a little silly that we need to use a whole bunch of circuitry to mix signals. Surely if we just connect the outputs of the various sources together they will mix just fine, no? No!

In fact, many people have done just this and managed to get away with it, but it’s purely good luck rather than good management. Consider that most modern equipment uses opamps or other “solid state” output circuits, and these generally have a very low output impedance. 100 Ohms is typical, but some are a little more, others less.

A 1V signal fed from a 100 ohm output (A) into another 100 ohm output (B) will do two things …

Form a voltage divider, so instead of 1V we only get 0.5V
Present a total load of 200 Ohms to the driving equipment, causing a current of 5mA to flow between A and B. Few opamps can drive this much current without excessive distortion, and there is no longer any useful headroom.

At a peak voltage of 5V (a perfectly normal transient for example), the driving equipment will be expected to provide 25mA. This exceeds the ability of most opamps, so the signal will distort. Needless to say, equipment “B” sees the same problem. Worst case is when “A” has a positive-going transient and “B” has a negative-going transient. The maximum expected current flow can be very high, and opamps will distort badly.

So What Is and What does an Audio Mixer?

An audio mixer is an electronic device that channels incoming audio signals while maintaining control over such effects as volume level, tonality, placement, and other dynamics for music production. In professional sound mixing an audio mixer is sometimes called a soundboard (sound board), mixing console, or mixer.

Traditional audio mixers are physical pieces of equipment with inputs for instruments; digital devices such as drum machines, auxiliary line-ins, and microphones. Mixing technology is also available via software, but requires an advanced sound card that features instrument inputs. Alternately, one can transfer pre-recorded tracks to a computer for use with audio software.

Modern digital audio mixers are made for both professional and nonprofessional use, covering a wide range of quality and price. Studios commonly use a dedicated audio mixer, while in the nonprofessional market, an audio mixer is often coupled with a digital recorder. The least expensive, non professional models feature 4-track digital recording with built-in mixer. Additional channels add to price, all else being equal, with high-end models featuring 24 channels.

A good audio mixer also features panoramic potentiometers otherwise known as “pan pots.” This control places an audio track to the left, center, or right within the mix to create a full stereo image. Traditionally, vocals are centered, with lead and rhythm guitars taking up opposite ends of the mix, and drums filling the background. This builds an acoustical environment, as if the band is surrounding the listener. Keyboards, percussion and other instruments are also carefully placed within the image. In some instances a drum roll or lead riff might “slide” or “roll” (pan) from one stereo channel to the other for effect, creating a sensation of movement.

Sound, audio or recording technicians or engineers, use recording and sound editing equipment to mix and record audio. Mixing engineers must typically have experience working with audio electronics and computers, and should have a strong communication skills. If you feel you have what it takes and you are not already a professional you may like to watch the video below.