Understanding Mixers and the Signal Path

Editor’s Note: As I travel the country participating in dozens of state “MEA” shows where training sessions are offered on using live sound mixers in the school, the most common problems that I’ve observed teachers facing is a lack of basic knowledge of the audio signal path, from microphone to speakers. This typically makes for a lot of guesswork by the director, and less-than-optimal sound operation, be it from a major field event, or just running your PA system to power the school talent show. Live sound is both art and science. The art is what you’ll use your ears for, but understanding the science is what gives your ears what they will want to hear. That starts with understanding the mixer, and the all-important signal path. – ML

You must understand everything about your mixer! Most modern mixers offer ample headroom and abundant features. The better your understanding of the primary mixer functions, the better your chances of experiencing creative freedom. The mixer is also called a console, desk, board, front-of- house mixer, or simply FOH in certain writings.

Some manufacturers are offering digital mixers designed specifically for live use. These mixers provide previously unheard of amounts of processing and ultimate control, along with the capacity to recall mix snapshots and to integrate seamlessly with multimedia presentations and other new technology— it’s hard to imagine that all mixers won’t be digital someday.

Whether you’re using a digital or analog mixer—of the simplest or most complex variety—the concepts and principles of sound reinforcement and sonic shaping stand true. If you develop a solid foundation of knowledge about how these devices work, your question won’t be, “What do I do to this sound?” Instead it will be, “Where is the controller that lets me do what I know I want to do?

Signal Path

Signal path is simply the route that a signal takes from point A to point B. For speed and efficiency in any situation, it’s essential that you’re completely familiar with the signal paths involved in your setup. Any good maintenance engineer knows that the only surefire way to find a problem in a system is to follow the signal path deliberately from its point of origin (point A, for example, the microphone) to its destination (point B, the speakers).

There are several possible problem spots between point A and point B. A thorough knowledge and understanding of your signal path lets you deal with any of these problems as quickly as possible.

Many owner’s manuals give a schematic diagram of exactly what the signal path is in a mixer. You may not be totally into reading diagrams, but there’s a lot to be learned by simply following the arrows and words. The basis of electronics is logic. Most complex electronic tasks can be broken down into small and simple tasks. That’s exactly how live sound is. What seems like an impossible task at first isn’t so bad when you realize it consists of several simple tasks performed in the right order.

An example of a typical signal path is as follows: The microphone goes into the microphone input, which goes to the attenuator, which goes into the preamp, which goes into the equalizer, which goes to the group assignment, which goes to the processors or gets routed to the main mix, which often goes through a processor, which goes to the master volume fader, which goes to the main stereo

output of the mixer, which goes to the power amp in, which goes to the speakers, which go to your ears, which go to your brain, which makes you laugh or cry. A thorough understanding of your signal path is the answer to most trying circumstances you’ll come across.

Many owner’s manuals give a schematic diagram of the mixer’s signal path. A thorough understanding of your signal path will help you troubleshoot most audio problems. Review your mixer’s block diagram—follow the path from the mic to the main output. These drawings are easy to follow and, along the way, you’ll see exactly how your mixer routes signals.

Input Levels

The mixer is where your songs are molded and shaped into commercially and artistically palatable commodities. If this is all news to you, there’s a long and winding road ahead. We’ll take things a step at a time, but for now you need to know what the controls on the mixer do. Unless a manufacturer has blatantly copied a successful mixer design, no two mixers are set up in exactly the same way; however, the concepts involved with most mixers are essentially the same.

In this section, we’ll cover those concepts and terms that relate to the signal going to and coming out of the mixing board. These concepts include:

• High and low impedance

• Direct boxes and why they’re needed

• Phantom power

• Line levels

A mixer is used to combine, or mix, different sound sources. These sound sources might be: • On their way to the front-of-house mix

• On their way to effects from instruments or microphones

• On their way to the stereo or multitrack service recording device

• On their way to the monitor system

We can control a number of variables at a number of points in the pathway from the sound source to the house mix and monitors. This pathway is called the signal path. Each point holds its own possibility for degrading or enhancing the audio integrity of your music.

Input Stage

Let’s begin at the input stage, where the mics, instruments, playback, and presentation devices plug into the mixer. Mic inputs come in two types: high impedance and low impedance. There’s no real difference in sound quality between these two as long as each is used within its limitations.

In practical application, microphone connections are almost always low impedance, whereas instrument outputs are typically high impedance. In order to plug a guitar or keyboard into a microphone input, you must incorporate a line-matching transformer.

The main concern when considering impedance is that high-impedance outputs go into high- impedance inputs and low-impedance outputs go into low-impedance inputs.

Output Impedance

There is a difference between input and output impedance. In days when everything was centered on the vacuum tube (an inherently high-impedance device), it was most efficient and financially feasible to match input and output impedances. In the early 1900s, Bell Laboratories found that to achieve maximum power transfer in long-distance telephone circuits, the impedances of interconnected devices should be matched. Impedance matching reduced the number of vacuum tube amplifiers needed. Because these amplifying circuits produced a lot of heat and were expensive and bulky, there was sufficient motivation to do whatever it took to match impedances.

Bell Laboratories invented a small cheap amplifier, called the transistor, in 1948. With the advent of the transistor (an inherently low-impedance device), everything changed. The transistor utilizes maximum voltage transfer where the destination device (called the load) should have an impedance at least 10 times that of the sending device (the source). This concept, known as bridging, is the most common circuit configuration used to connect audio devices.

Because of the load-source relationship, it is possible to simply split one output several times for connection to multiple inputs. Conversely, summing multiple sending signals (sources) to one destination device (load) is not recommended. It’s necessary to utilize a summing circuit to combine multiple sources to a load.

The Preamp

One of the first things your signal from the mic sees as it enters the mixer is the mic preamp (sometimes called the input preamp or simply the preamp). The preamp is actually a small amplifier circuit, and its controls are generally at the top of each channel. The preamp level controls how much a source is amplified and is sometimes labeled as the Mic Gain Trim, Mic Preamp, Input Preamp, Trim, Preamp, or Gain.

A signal that’s been patched into a microphone input has entered the mixer before the preamp. The preamp needs to receive a signal that is at mic level. Mic level (typically 30–60 dB below line level) is what we call the strength of the signal that comes out of the mic as it hears your music. A mic level signal must be amplified to a signal strength that the mixer wants. Mixers work at line level, so a mic level signal needs to be amplified by the preamp to line level before it gets to the rest of the signal path.

Best results are usually achieved when the preamp doesn’t need to be turned up all the way. A preamp circuit usually recirculates the signal back through itself to amplify. This process can add noise, then amplify that noise, then amplify that noise, and so on. So, use as little preamplification as possible to achieve sufficient line level.

Some mixers have an LED (light-emitting diode, or red light) next to the preamp control. This is a peak level indicator and is used to indicate peak signal strength that either is (or is getting close to) overdriving the input. The proper way to adjust the preamp control is to turn it up until the peak LED is blinking occasionally, then decrease the preamp level slightly. It’s usually okay if the peak LED blinks a few times.

Many modern mixers have very clean and transparent input preamps with ample headroom. Headroom, in any circuit, is the operational range between normal and maximum signal level. These mixers rarely utilize a peak LED at the input stage to help set the preamp level. In this case, the channel fader and master stereo are optimized at unity gain. (Unity gain is the state where a circuit outputs the same level it receives at its input.) When the channel fader is set at unity (U), the preamp level is adjusted to produce the proper level for the mix.

Attenuator

It’s a fact that sometimes the signal that comes from a microphone or instrument into the board is too strong for the preamp stage of your mixer. This can happen when miking a very loud instrument, such as a drum or electric guitar amp, or when accepting the DI of a guitar, sound module, or bass with particularly strong output levels. Some microphones produce a stronger signal than others. This is a particular problem when miking drums or loud guitar and bass amplification systems. If the signal is too strong going into the preamp, then there will be unacceptable distortion. When this happens at the input, there’s no fixing it later.

This situation requires the use of an attenuator, also called a pad. This is almost always found at the top of each channel by the preamp level control. Sometimes, especially on condenser microphones, there is a pad between the mic capsule and the microphone circuitry. Try this attenuator before using the mixer attenuator.

An attenuator restricts the flow of signal into the preamp by a measured amount or, in some cases, by a variable amount. Most attenuators include 10-, 20-, or 30-dB pads, which are labeled –10 dB, –20 dB, or –30 dB.

If there’s noticeable distortion from a sound source, even if the preamp is turned down, use the pad. Start with the least amount of pad available first. If distortion disappears, all is well. If there’s still distortion, try more attenuation. Once the distortion is gone, use the preamp level control to attain sufficient input level.

Again, if the input stage of your mixer has a red peak LED by the input level control, it’s desirable to turn the input up until the peak LED blinks occasionally, then back the level off slightly. This way we know we have the signal coming into the mixer as hot as possible without distortion. This is good.

Ideally, we’ll always run electronic instruments through the system with their outputs set at maximum. This procedure results in the best possible signal-to-noise (S/N) ratio and provides a more surefire way to get the instrument back to its original level for a future use.

If you don’t have an attenuator and if you’re reinforcing an instrument such as a bass, keyboard, or guitar through a direct box, you can turn the output of the instrument down slightly to keep from overdriving the input preamp. Be sure to mark or note the position of the instrument’s controls (especially volume) so you can duplicate levels in the future.

1. The channel on the left demonstrates the use of channel peak LED to adjust the input level. With this method, while the source is active, turn the trim up until the peak LED blinks, then back the trim off slightly.

2. The channel on the right demonstrates a unity setting on the channel fader. Set the fader to unity and then adjust the trim for the proper mix level. Most boards that use this system offer a means to meter the input signal. Many mixers display the channel level on the main left-right meter when the channel solo button is selected.

Excerpt from: 

The Ultimate Live Sound Operator’s Handbook, Second Edition 
By Bill Gibson, Publisher: Hal Leonard
ISBN: 987 -1-61780-559-2