audiobasicmanual - mixer_the channel strip

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Coursera.org

The Channel Strip Lesson 3 Assignment

Tiago Matias | 2014

Introduction to Music Production

Mixer:



2

Lesson 3: The MIXER

Introduction Page 3

The Mixer Page 4

The Signal Flow Page 5

The Mixer Sections

Input / Preamp Page 6

Equalization Page 8

Auxilary Bus Sends Page 9

Fader Section Page 10

Conclusion Page 11

Index



3

Introduction

This document is my personal assigment for the lesson 3 of the Introduction to Music Production course on

coursera.org.

This document can also be used as a beginers first basic steps manual.

In this assignment I will exemplify what it is and how a channel strip of an audio mixer works.This document will be devided into two parts: First we will analyse what is the basic concept of a mixer and the

signal flow into a channel strip. After that we will study each section of a channel strip and how it works.

I hope you find this document to be helpfull and informative.



4

The Mixer

The mixer is the piece of gear that is the most used in all audio applications. Even a DAW is a combination

between a tape recorder and an audio mixer. A mixing board is an equipment that receives various diferent

signals and sum all of them into diferent outputs as you wish. Usually all the tracks (sounds) are mixed into a

single stereo output.

An audio mixer is a group of diferent types of gear all in the same enclosure. Those generally are:

Microphone Preamp;

Line Level Trim;

Equalization;

Auxilary bus sends;

Stereo imaging address (Pan or Balance);

Volume control (fader);

Bus addressing (Sub-Group);

Depending of the model of a certain mixer, there could be more or less sections then those listed above.Professional mixing desks usually have matrix buses, VCA controllers, Tape Outs and Ins and a few other

sections.

MIDAS® Verona mixing desk



5

The Signal Flow

The signal flow of a mixer really depends on the mixer itself. Every manufacter chooses how their product

addresses the signals. There is no standard signal chain and anyone who says the opposite may not understand

very much of the topic.

I will exemplify below in this page two possible diferent types of signal chain in a channel strip of a mixer. Please

notice that there are even more possibilities. Type 1 is one of the most adopted by most mixer manufacturers.

Type 1 Type 2

Signal Signal

+48V +48V

Ø Ø

Pad Pad

Gain / Trim Gain / Trim

Low Cut Filter

Insert External Devices Insert

Mute

EQ Section

AUX Sends

Mute Low Cut Filter

EQ SectionAUX Sends

PFL Control Room PFL

AFL AFL

PAN PAN

Stereo Output Bus



6

The Mixer Sections

Most people are overwhelmed when looking at large

format mixing desks but the concept of a mixing desk is very

simple.

Besides the secondary funcionalities such as VCAs, matrix

buses and those more professional sections, all mixingboards are nearly the same.

Every each of them have what is called a Channel Strip and

this strip is made for each channel, then it repeats the

process to all other channels. Once you’ve learned one

channel strip and how it works on that specific mixer, you

will know how most of the mixer works.

4 channel mixer electronic esquematic

Input / Preamp

The input section starts in the input plugs such as XLR, TS or TRS.

The signal that is injected in this plug is addressed to the input gain stage which is controlled by a

potentiometer usually called Gain or Trim. This stage is the Preamp device / section.

Gain: defines the value of the gain (signal boost) we want to apply to the received microphone signal.

Trim: lets the user to ajust more or less gain to a line level signal. At position Zero or U (unity) the input level of

this channel is exacly the same level of the signal that is coming from the cable.

Pad: is a switch that attenuates the input signal before the gain stage. If a signal is to hot (loud) when the trim or

gain pot is all the way down, it’s better to engage the pad. Depending on the mixer, this pad could attenuate the

signal by 10dB, 20dB, etc. Usually the value of the pad is showed in negative values such as

-20dB or -15dB. This reminds the user that the input signal will be decreased by 20dB or 15dB.

Phantom Power: the phantom power is a DC voltage that can be sent via the input plug, thru the entire cable

until it reaches the microphone or DI box. Condenser microphones and active DI boxes need to be

powered on so the electronic circuit works. Phantom power is an alternative of using a battery in the

microphone, which could make them bigger and not as consistent with voltage, this DC current can be sent thru

the cable. The mixer has a phantom power switch (+48V) which allows the DC current to flow thru the cable

until reaches the mic.

Usually this voltage is 48V but it really depends on the mixer and microphone. Some microphones only require

9VDC to properly work. However, they are prepared to receive 48V because most of the desks send 48VDC.

Unexpensive mixers send a 48V phantom power to all the channel XLR plugs with one switch. This configuration

is not very as usefull as each input having it’s own switch because not every microphone can handle that

current. Ribbon microphones, for example, can be damaged by this DC current.

Intermedium mixers usually send phantom power every 4 or 8 channels.

Professional desks have a phantom power switch per channel, so you can apply that voltage only to the

channels you wish.



7

Low Cut / Hi Pass Filter: most of the mixers have in the input section a filter. This filter is engaged with a switch.

The hi pass filter is very usefull to eliminate lower frequencies which are not needed in that specific track. For

example, in a vocal track, people usually insert a hi pass filter because it can eliminate low informations such as

noises or bottom end that does not define the timber of a human voice. A human voice does not propagate

frequencies like 60Hz or 80Hz, so those frequencies captured by that microphone can fight in the mix with the

bass drum and bass guitar. If you apply this filter into the instruments or other sound sources that do not

reproduce below the frequency of the filter, you will find that the sound of that track will be more clear, cleanand sometimes even brighter.

This button can be frequency fixed or in most high end desks you can choose the frequency you wish using a

potentiometer.

PLEASE NOTICE: in some desks, this button is on the EQ section and not in the input section.

Phase Ø: this button inverts the polarity of a signal (180°). This option is used to correct phase issues when

capturing a source with multiple microphones. For example, when capturing a snare drum with top and bottom

mics the bottom mic must be 180° phase inverted. Why? Well, as the drummer hits the snare in the attack skin,

the response skin will move in the same direction, so, when the top mic is capturing the downward movement,

the bottom mic is also capturing the downward movement. While the bottom mic capsule is pointed up, the top

mic capsule is pointed downwards, so each one is moving in oposite directions.

This means the two capsules are 180° inverted and when you sum 2 mono signals with diferent phases, the

signals cancel each other, especially when 180 degrees inverted. The final sound you might have when not

flipping the phase is either no signal or a weird, ugly and comb-filtered snare drum which sounds horrible.

TIP: if you do not have a phase button on you mixer, try both mics on the snare drum. Listen carefully both and

then only the top mic. If both sound good, use them, if only the top one sounds better, that means the phase

invertion of the skins into the microphones is making it to sound worse. In this case use only the top one.

This technique is used in a lot of diferent mic applications.

Yamaha MG 24/32 input sections



8

Equalization

EQ On/Off: this button is more common in large format desks or on those quality small but very expensive

mixers. This button turns on or off the EQ section of the mixer.

EQ: there are 2 types of mixer equalization sections: Semi-Parametric and Parametric.In example I will start from top to bottom describing a parametric EQ because it is the most complex.

High: usually is a shelf type of filter in which you can increase or decrease the level of

all frequencies above the threshold frequency.

High Freq.: is the threshold frequency you choose start to augment or attenuate.

Hi-Mid: is a bell type of filter that is applied on the frequency band of the Hi-Mids.

Hi-Mid Freq.: is the frequency of this band you choose to augment or attenuate.

Q or Bandwidth: it defines the bandwidth you wish to work with being the center

frequency, the frequency of the last potentiometer.

Low-Mid: is a bell type of filter that is applied on the frequency band of the Low-

Mids.

Low-Mid Freq.: is the frequency of this band you choose to augment or attenuate.

Q or Bandwidth: it defines the bandwidth you wish to work with being the center

frequency, the frequency of the last potentiometer.

Low : usually is a shelf type of filter in which you can increase or decrease the level of

all frequencies below the threshold frequency.

Low Freq.: is the threshold frequency you choose start to augment or attenuate.

A Semi-Parametric equalizer is very similar to a Parametric. The diference is that the

semi-parametric does not have the bandwidth selector, so the response curve (slope)

of the EQ is fixed.

Midas® Heritage 3000 eq section

Yamaha MG24 eq section Behringer Xenix 802 eq section



9

Auxilary Bus Sends

Auxilary sends are used for parallel sending of signals to diferent outputs. Aux sends are very common used to

send monitor mixes in recording or live situations. They can also be used to send signals to external devices such

as effect units, parallel compression, secondary P.A. systems and so on.

Aux Send Potentiometer: controls the amount of signal level you wish to send to a secondary output.

Aux Send On/Off: in some professional mixers there is the possibility of turning on or off an aux send by

engaging a secondary switch or pressing the potentiometer up and down.

PRE: this switch allows to address the aux send before or after the fader section. When PRE is engaged it means

the aux send will be a Pre-Fader audio signal. When not engaged the aux send will be a Post-Fader audio signal,

this means any volume changes you make in the fader will affect the aux sends.

In most complex mixers, auxilary sends can be sent in stereo. In this case the mixer has 2 pots* for every aux.

Aux Send Level: controls the amount of signal level you wish to send to a secondary output.

Aux Send Panning: controls the panoramic of signal level you wish to send to a secondary output for Left,

Center and Right.

In these mixers, often if you disengage the stereo aux mode, the two potentiometers work as 2 independent aux

level controls to 2 diferent auxes.

For example in the Midas® Heritage 3000 model:

Mono Aux Mode: Pot1 = Aux send level for AUX1

Pot2 = Aux send level for AUX2

Stereo Aux Mode: Pot1 = Actuates as PAN knob for AUX 1 and 2

Pot2 = Aux send level for AUX 1 and 2

Heritage 3000 aux sends

Allen&Heat® GL2400 aux send section Midas® Venice 320 aux send section

* Pot = Potentiometer



10

Fader Section

Finally at the end of the channel strip we can find the fader section. This section is the channel’s output control

zone before it goes to the Master bus. The more common components we can find are described below.

Mute: this button cuts the signal of the particular channel.

PFL: means Pre Fader Listening. This solos the channel muting all the others so we can listen to only that

particular channel before the fader, so we listen to the original volume level of this channel. Some mixers have

AFL ( After Fader Listening) and some have both, letting you to choose between both.

Pan: this knob controls the ammount of the mono signal we send to the left and right master outputs for getting

a stereo image position of that track.

Bal (Balance): does the same effect as the Pan but it is a very diferent technology. This option only exists in

stereo channels. The channel receives both Left and Righ signals, then the Balance knob controls the volume of

those two. If you turn the knob all way to the left, the electronic component will completely attenuate the right

signal. If you turn the knob all the way to the right, the component will completely attenuate the left signal. This

provides a similar effect as the Pan control, but is individually changing the volume of both left and righ input

signals of a channel.

Fader: the most reconized piece of a mixing desk. It changes volume. That’s it!

The fader is the final control of the channel, it is a volume control for that particular signal.

PAN vs. BAL

Mono Sinal Stereo Signal

L R

x x

L Master Bus R L Master Bus R(PAN) (BAL)

Pan: takes a mono signal and addresses it to left or right buses of the master;

Balance: takes a stereo signal and controls the volume of left and right channels;

Fader bay of an Allen&Heat® ZED14 mixer

Mono faders with Pan knobs Stereo Faders with Balance knobsMono Channels Stereo Channels



11

Conclusion

This was my attempt to explain what is a channel strip and how it works. Once we learn the basics of a channel

strip we instantly understand that a 72 channel mixer is almost exacly the same as a 4 channel mixer. The only

things that change are the number of functionalities whitin the channel strip and in other mixer sections.

All the content in this document is my own knowledge. No other documents were used in parts on in whole.

I hope you enjoyed and that you have learned a bit more about this topic of sound and music which may

probably be your new passion.

I would like to thank to Jonathan Teysko from Sweden for helping me to correct the english words and

sentences in this document.

Images :

Mixer user guides / manuals

www.google.com

Tiago Gouveia Matias May 19, 2014 Azores

http://www.google.com/



audiobasicmanual - mixer_the channel strip

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