Sound And Light Training SALT Manual by Ken Ellis

6. Components of a Sound System - Mixer

The Mackie SR40*8 mixer - 40 channels, 8 subgroups, per-channel metering
(This link is worth clicking on!)

• A mixer is an electronic device which combines the electrical sound signals from microphones, instruments, Tape, CD, etc.



• With the mixer, you can adjust the volume and tonal quality of each input source to achieve a harmonious and pleasing blend ("mix") of all the sound sources.



• The output of the mixer goes through the compressor (optional), the digital delay (only for large distributed speaker systems), the 31 band graphic equalizer (or parametric equalizer), the cross-over (only for 2-way or 3-way speaker systems), to the amplifier(s), and then to the speaker(s). If you are using a stereo speaker system, all the equipment is doubled!



• Most mixers provide separate controls for the main speakers and one or more sets of monitor speakers. Each set of monitor speakers also needs a compressor (optional), 31 band graphic equalizer, amplifier, and speaker(s). Sometimes, a separate mixer is used for the monitor mixes.



• Usually, the mixer provides the ability to connect effects processors to add reverb, echo, delay, etc. to the sound.



• It is very desirable that the house mixer and sound technician be located somewhere near the middle rear of the audience area so that the sound technician will hear the same sound the audience hears (but not block the view of the audience). It is best if the mixer is located on the main floor with the audience, but sometimes it is necessary to locate it in a balcony. The house mixer should not be located in another room or behind a glass wall.



• If a separate mixer is used for the monitor mixes, it should be located to one side of the stage where the monitor mix technician can have eye contact with all the performers (yet not be seen by the audience).



• The microphones are connected to the mixer either directly via mike cords or through a "snake" if the mixer is too far from the mikes.



• Instruments (guitars & keyboards) are connected to the mixer through a special adapter called a "direct box" (used to convert un-balanced high impedance 1/4" cord connections to balanced low impedance XLR microphone connections).



• A "snake" is a multi-conductor cable that caries a number of microphone lines (usually 16 or 24) from the performance area in the front to the mixer location in the back.



• If a separate monitor mixer is used, the snake must have splitters on each channel to feed both the Monitor Mixer and the House Mixer.



• A mixer usually handles 8, 16, 24, 32, 40, or 48 channels or inputs.



• Below, we will attempt to describe most of the controls found on the average GOOD quality mixer that might be found in a church environment:




---

• There are separate controls on each channel for:



• Trim Pot - The Trim Pot or channel gain control is used to compensate for the difference between the various input sources. Generally, you should start with the Trim Pot for each channel set at the 12:00 (or straight up) position. During practice, you should adjust the Trim Pots to compensate for channels with louder or quieter signals. For example, the keyboard may put out a stronger signal than a guitar. In this case, the trim pot for the keyboard may need to be turned down or the trim pot for the guitar turned up.
  
  During practice, listen to each channel one at a time with the headphones (using the PFL or AFL switch). Adjust the Trim Post so that each channel sound about the same loudness. If your mixer has per-channel meters, they can make it a lot easier to adjust the Trim Pots. On most mixers, when you operate the PFL or AFL switch, it shows that one channel on the main L/R meters. This can also be used to help you adjust the Trim Pots for nearly equal levels from all channels.
  
  When adjusting the Trim Pots, you need to be careful not to over-drive the channel. By setting the Trim Pot too high, you can cause the channel to over-drive and distort or clip. This is not a pleasant sound! Some mixers have a little red LED on each channel to let you know when it is set too loud. You can also use the per-channel meters or press the PFL or AFL button and look at the main L/R meters. If the meter goes into the red, turn the Trim Pot down a little.



• Mike / Line switch - The Mike / Line switch is not included on all mixers. It is used to select between the low level mike signal connected to the XLR jack or the high level line signal connected to the 1/4" jack. If this switch is not present, usually both of the jacks are active.



• Phantom Power switch - The Phantom Power switch is not included on all mixers. It is used to turn Phantom Power on or off. Sometimes one switch controls Phantom Power for 4 or 8 channels. Normally, the Phantom Power switch should be left in the off position unless you are using a microphone that requires Phantom Power to operate. Sometimes the Phantom Power switches are beside the channel input jacks.



• Equalizer or Tone Control - The per channel Equalizer controls are used to adjust the tonal quality of each input and to reduce or eliminate feedback. Low end (low cost) mixers may only have treble and bass tone controls. Better mixers have high, mid, and low frequency controls. Top of the line mixers have additional equalization controls which include frequency adjustment capability and sometimes even an adjustment for the width (or Q) of the control. Often low-cut and sometimes high-cut switches are also provided.



• High-cut - The high-cut switch should usually be left turned off. It is only needed if the input to that channel has an unusual amount of high frequency energy (i.e. a low quality guitar pickup).



• High - The high frequency rotary tone control is often useful to smooth out a channel which is particularly shrill. Seldom should it be used to boost high frequency sounds because this can quickly cause feedback. However, when a keyboard or guitar with a pickup is connected to the channel, the high frequency control can be used to brighten the instrument without danger of feedback.



• Sweepable controls - Sweepable tone controls are very helpful in eliminating feedback. The best mixers have several sweepable controls on each channel.
  
  Before practice starts, you should check each channel for potential feedback. Start with all the tone controls set to null (straight up or 12:00 position). Turn the channel on to normal operating level. Then slowly move the channel fader up the whole way. If you get feedback before you reach the top, move it down just a little to keep the feedback quiet and stable.
  
  Decide whether the feedback is a high frequency or a low frequency. If it is high, use the higher sweepable tone control. If it is low, use the lower sweepable control (if you have one). Turn the level control down about 2 hours (i.e. from the 12:00 position to the 10:00 position). Then "sweep" the frequency control until the feedback goes away. It takes a little gettin' used to, but with practice you will get the hang of it.
  
  Then move the channel fader up a little more and see if you get feedback at another frequency. If you have another sweepable control, you can eliminate that feedback too.



• Low - The low frequency rotary tone control is useful in eliminating "boomy" sound in mikes. It can also be used to add or reduce bass for keyboards or guitars. Occasionally, it can be used to eliminate a really low frequency feedback.
  
  If you turn the bass up, you need to be careful not to over-drive the channel and cause it to distort.



• Low-cut - The low cut switch should usually be enabled (pushed in) on most channels except keyboard, bass guitar, drums, and CD. This switch takes out low frequency rumbles (like breathing noise and air conditioner sounds).



• Monitor or Auxillary send(s) - The Monitor or Auxillary sends are most often used to provide a separate "mix" for the monitor speakers.
  
  This allows the performers to hear a somewhat different "mix" than the audience hears. Often a performer will want to hear more of their own voice or instrument than is in the house mix. Possibly the instrument which sets the tempo for the whole group needs to be louder in the monitor mix than in the house mix. Sometimes different members of the performance have different monitor requirements. This can be solved by providing multiple monitor mixes and setting up several monitor speakers (each with its own equalizer and amplifier).
  
  The Monitor or Auxillary controls are used to produce one or more Monitor mixes. Low end mixers may only have one set of Monitor or Auxillary controls. High end mixers often have as many as 8 Monitor or Auxillary sends, sometimes even more.
  
  To set up the Monitor mixes, first you must decide how many Monitor mixes you are going to use. You must connect each Monitor or Auxillary send through a compressor (optional), through a 31-band graphic equalizer (highly recommended for feedback control), to the monitor amplifier, and then to the appropriate set of monitor speakers (which have been placed in front of the correct person or group of people).
  
  Start by turning the main Monitor or Auxillary send controls off. Then, for each Monitor mix, you must decide which of the inputs should appear in that mix. Start by setting the monitor control for each input you want in that monitor to the 12:00 position. If some inputs should be louder in that monitor, start them at the 1:00 position and for those that should be quieter, start them at the 11:00 position.
  
  Next, turn the main Monitor or Auxillary controls up slowly. SLOWLY find the point at which the monitor begins to feed back. Reduce the control at least 9db below the feedback point or an eighth of a turn if the control is not labled in db.
  
  While the group is practicing, go on stage, listen to the monitors. Ask the performers if they are hearing what they want to hear. Adjust the Monitor or Auxillary controls for each set of monitors to suit the performer's needs.
  
  Generally, Monitor, Auxillary, and Effects sends do not have meters or overload warning lights on them. Generally, you won't have trouble with overload if you keep most of the controls near the 12:00 position.



• Pre / Post switch - The Pre / Post switch near the Monitor or Auxillary sends determines whether the signal sent to Monitor controls is affected by the main Channel Fader (slider). When the switch is in the "Pre" position, the Monitor signal is taken from a Pre-Fader position. The Monitor level is NOT affected by the Channel Fader. When the switch is in the "Post" position, the Monitor signal is taken from a Post-Fader position. The Monitor level IS affected by the Channel Fader. Generally Rock Music groups prefer the "Pre" position and choir type groups perfer the "Post" position.
  
  If the performance contains solos where the sound technician must move the Channel Fader up for the solo, then if the switch is set in the "Pre" position, the monitor for the solo will NOT get louder when the house mix gets louder. If the switch is set in the "Post" position, the monitor for the solo will get louder the same as the house mix gets louder for the solo.



• Effects send(s) - The Effects sends are used to select certain channels that should be sent to effects processors such as reverb, chorus, or digital delay. Generally, processed sound is returned from the Effects processor to the mixer's Effects Return inputs. If you want to have better control over the effects sound, you can connect it to an unused channel.
  
  Low end mixers generally only have one set of Effects sends. High end mixers usually have multiple Effects sends. Monitor, Auxillary, and Effects sends are all similar in nature. The main difference is whether the send is "Pre" or "Post" Fader. It is best to read the manual on your mixer to determine whether its Monitor, Auxillary, and Effects sends are Pre or Post fader or whether you have a switch to make the selection yourself.



• Subgroup Select switches - The Subgroup Select switches (not included on all mixers) are used to group several channels into a Subgroup (i.e. Subgroup 1 or S1 = soprano, S2 = alto, S3 = men, S4 = instruments). Sometimes there are Subgroup switches labled L/R which bypass the Subgroups and send the signal directly to the main L/R output.



• Left / Right Pan control - The Left / Right Pan control is used to select whether the signal should be sent to the odd numbered Subgroups (Left), even numbered Subgroup (Right), or both (center or 12:00 position). If the signal is sent directly to main output, then the pan control determines the location of the signal in the stereo mix.
  
  If you are just sending the sound to the house speakers, usually you will use a mono send (unless the house speakers are set up for stereo). However, if you are also making a recording of the sould, it may be desirable to make a stereo mix where the sould on the left and right are somewhat different. By turning the Pan controls part way to the left or right, you can position each sound within the stereo field. When you mix for stereo, generally you need to use the Subgroups in pairs.



• Channel Fader The Channel Fader, slider, or level control is used to adjust the level of each channel in the main mix.
  
  During practice, many sound engineers like to setup the mixer so that they have a good well balanced general mix with all the Channel Faders are in a straight horizontal line. This way, after making any changes in Channel Fader levels needed for a particular song or solo, they can return all the Faders to a straight line. This is easier than having each Channel Fader set at a different level and after a solo having to remember where that Fader was before you changed it for the solo.
  
  Generally, you should start with the main house L/R control off and all of the Channel Faders up at the position marked either "0" or "U". This is usually about 3/4 of the way up. Then, slowly raise the house L/R controls to their "0" or "U" mark. If you get feedback before you get there, then move the house L/R controls down to about 9db below the feedback point.
  
  While the group is practicing, adjust the Trim Pots on each channel so that you get a good blend or mix. It is helpful to listen to each channel individually with the headset by pressing the PFL or AFL button.
  
  If there are solos or other areas in the performance where one or several performers need to be accentuated, move their Channel Faders up for that portion of the program and then return them back to the "0" or "U" mark.
  
  With most mixers, there is about 10db of gain left between the "0" or "U" mark and the top or maximum position of the Channel Fader. With some performers, this is not enough extra gain to accentuate a solo adequately. You may find it works best to setup the mixer with all the Channel Faders normally set at "-5" or "-10" so that you have 15db or even 20db of room left to accent a solo. It is all a matter of what works best for you!



• Pre-Fader Listen or After-Fader Listen - The Pre-Fader Listen (PFL) or After-Fader Listen (AFL) button allows the sound technician to listen to one channel at a time using a headset. If the button is labled PFL, then the level heard in the headset is independant of where the Channel Fader control is set. However, if the button is labled AFL, then the level heard in the headset is dependant on the setting of the Channel Fader.
  
  During practice, you should listen to each channel using the PFL / AFL button. The Trim Pot on each channel should be adjusted so that the volume (and meter readings) on all channels appear nearly the same.
  
  The PFL / AFL button can also be used to look for troubles, like the source of a hum or buzz. Pressing PFL / AFL on a Subgroup allows you to hear that group alone without the rest of the mix. If your mixer has a PFL / AFL button on the Monitor, Auxillary, or Effects sends, you should listen to each of them to see that there is no distortion (overload, or clipping) and to see that the mix sounds like what you expected.



• Mute or Channel On switch - The Mute or Channel On switches are used to enable / disable individual channels. Not all mixers have these switches. Mute switches are more prevelant than Channel On switches. If the Mute switch is pressed, the channel is turned off (muted). Generally, a Mute switch has a light which illuminates when the channel is Muted.
  
  Some mixers provide Mute Automation. These mixers include a computer which can remember a number of snapshots of mute switch settings and recall them at will. This is particularly useful in large drama presentations with many FM microphones worn by all the performers. All the FM microphones can normally be muted (to eliminate background noise and minimize feedback possibilities) and the system can be programmed to un-mute the correct mikes for each scene.




---

• If your mixer has Subgroups, there are separate controls for each Subgroup:



• Subgroup Left / Right Pan control - The Subgroup Left / Right pan determines where this subgroup is positioned in the main L/R stereo mix.



• Subgroup Left / Right Select - Some mixers have Subgroup Left / Right select buttons instead of Subgroup Left / Right Pan controls. The L/R select determines whether the Subgroup is sent to the Left main, Right main, or both outputs.



• Subgroup Fader - The Subgroup Fader (level control, or slider) is used to adjust the level of the subgroup within the main mix. This allows the Sound Technician to raise or lower the level of a whole Subgroup of inputs with one control. For example, if the altos have a verse by themselves and need to be a little louder, you can raise the alto Subgroup for that verse.



• Subgroup PFL / AFL - The Subgroup Pre-Fader Listen (PFL) or After-Fader Listen (AFL) button allows the Sound Technician to listen to one Subgroup at a time in the headset. If it is a PFL button, the level in the headset is independent of where the Subgroup Fader is set. If it is an AFL button, the headset level is controlled by the Subgroup Fader.



• Mute or Subgroup On switch - The Mute or Subgroup On switches are used to enable / disable individual Subgroups. Not all mixers have these switches. Mute switches are more prevelant than Subgroup On switches. If the Mute switch is pressed, the Subgroup is turned off (muted). Generally, a Mute switch has a light which illuminates when the Subgroup is Muted.




---

• The main L/R output usually has only one control:



• Master L/R Faders - The Master L/R Faders are used to control the main output of the mixer to the house speakers. Most Sound Technicians prefer to normally operate the Master L/R Faders at the "0" or "U" position. In order to do this, the volume or gain control on the house amplifier should be adjusted so that the system operates at a normal comfortable level with the faders set at "0" or "U".




---

• Depending on the type of mixer you have, there may be additional controls, but since these vary greatly with different mixers, we won't try to address them all here.
  
  The mixer controls listed above are used to adjust, blend, and tune the sound, but it is the input and output jacks that are used to connect the mixer to the various other components of the sound system:



• Channel Jacks - Each channel of the mixer usually has two input jacks, one is usually a low impedance XLR jack and the other is usually a high impedance 1/4 inch jack. Normally, microphones are connected to the XLR jack and instruments, keyboards, tape decks, and CD players are connected to the 1/4 inch jacks.
  
  Frequently, each channel will also have "insert" jacks. Better mixers usually have two 1/4 inch insert jacks labled "send" and "return". Some mixers use a single three conductor 1/4 inch jack for both functions. The purpose of the insert jacks is to send the signal from one channel to an effects device or a graphic equalizer that is dedicated to that one channel.



• Effects Jacks - If the mixer has one or more Effects Sends, each of these will have an output jack. Each Effects Send has a corresponding Effects Return with an input jack.
  
  Generally, the Effects Send output jack is connected to the input of the Effects Unit and the output of the Effects Unit is connected to the Effects Return input jack. If the Effects Unit is stereo, you have two choices: You can use only the left channel output jack (which makes the unit operate in mono mode) and connect it to one Effects Return input jack. Alternatively, you can use both the left and right output jacks and connect it to two Effects Return input jacks, but only do this if you are doing a stereo mix.



• Monitor / Auxillary Jacks - If the mixer has Monitor or Auxillary Sends, each of these will have an output jack. The Monitor and Auxillary outputs are generally connected through an Equalizer to the Power Amp for stage monitor speakers or auxillary speakers.



• Sub-Group Jacks - Usually, if the monitor has Sub-Groups, each will have its own output jack. The Sub-Group outputs are most often used when doing multi-channel recording. In this case, each Sub-Group is sent to a different track on the recorder.



• Main Output Jacks - All mixers have output jacks for the main mix. Usually, they have left and right jacks. Frequently, they will have both 1/4 inch and XLR jacks (it doesn't matter much which type of jack you use). Some mixers have a Mono or Sum output which is a combination of the left and right outputs.
  
  The main output jacks should be connected through an Equalizer to the Power Amp for the main speakers.
  
  If you are doing a Mono mix, you can use just the left output jack to feed the main speakers (because the left and right are identical). You might want to use the right output jack to feed the Narthex, Cry-Room, or cassette recorder.
  
  If however, you are doing a Stereo mix (for the purpose of making a stereo recording), then it is best if your mixer has a Mono or Sum output to use to feed the main Equalizer and Power Amplifier. If you MUST use the main left and right outputs, you need a way to "sum" them together. If you simply use a "Y" cord, your recording will become Mono also. The best way to "sum" the left and right channel is to build a special "Y" cord with a resistor network (probably about 1Kohm in series with each side and a 1Kohm shunt).
  
  

             >------1Kohm-----O-----1Kohm-------<
                              |
  Left Output               1Kohm                Right Output
                              |
             >----------------O-----------------<
                          To Mains
  

  If you are doing a Stereo house mix (and we recommend against that), then you will need to connect the left and right main outputs through two separate Equalizers to two separate Power Amps.



• Recording Jacks - If you are recording the mix, you have a number of options: Some mixers have a separate set of RCA jacks labled Record Out, If your mixer has both 1/4 inch and XLR main output jacks, you can use the 1/4 inch jacks for recording and the XLR jacks to drive the house, If you are running a Mono mix, you can use the left main output for the house and the right main output for recording, you could also use an effects or monitor send for recording (but that requires you to set up a separate mix for recording), and finally, you could use "Y" adapters to split the main output for recording.



• Headset Jacks - Most mixers have a headset output jack. If the mixer has PFL buttons (Pre-Fader Listen), then you can listen to one channel at a time or the whole mix. You can also listen to each effects and monitor send.




---

• Finally, there are a few special items that are provided on some mixers:



• Talk-Back - Some mixers support a "Talk-Back" microphone which allows the Sound Technician to talk through the main and/or monitor speakers.



• Clear-Com - Some mixers support a "Clear-Com" system which is an intercom that can be connected between the Sound Technician, Lighting Technician, Monitor Mixer, Stage Manager, Director, etc. as needed.




---

• Before we leave our discussion of mixers, we will go over some basic Mixer Operation principles:



• Mixer Pre-set - Before starting to connect a mixer, you should go over all the controls and pre-set them so that when you turn the system on, you won't get disasterous feedback. Make sure all the Trim Pots are set all the way off (left), set all the equalization controls straight up, start with all the effects, monitor, and auxillary sends turned off (left), turn off (down) all the channel faders, sub-groups, and mains.



• Equalizer Pre-set - Set all the sliders on the main house equalizer and all the monitor equalizers to their mid-point.



• Power Amp Pre-set - Turn the volume control on the main and monitor power amps all the way off (left).



• Mixer Connections - Next, connect all the mixer inputs and outputs, equalizers, main amplifier, monitor amplifiers, main speakers, and monitor speakers.



• Power Connections - To minimize the chance of encountering "ground hum", it is best to connect the entire sound system to a single AC outlet (assuming your system isn't too powerful for one circuit breaker). Select an AC outlet near the power amplifiers and put a multi-outlet strip there. Run 3-conductor extension cords as needed to reach the other equipment. Never cut off the third prong! However, you should have a supply of ground lift adapters (3 prong to 2 prong) handy.



• Power-Up - Turn on the mixer, tape or CD player, the effects units, the equalizers, and finally the main and monitor power amps.



• System Check-Out



• Start by bringing up the main faders to normal (0 or U) position.



• Then bring up the sub-groups to normal (0 or U).



• You should have a tape or CD player connected to the system. Turn the trim pot on the tape / CD channel all the way off (left) Now play some music (but don't expect to hear it yet).



• Make sure a sub-group is selected for the tape / CD channel and bring the tape / CD channel fader up to normal (0 or U).



• Slowly turn the tape / CD channel trim pot up until the main left / right meters read high in the normal (green) range without going into the red.



• Now, with the amplifier volume turned all the way off (left), turn on the main amplifier power. Slowly increase the volume until you hear the music out of the main speakers.



• Check that all the main speakers are working and increase the amplifier volume setting until the music is at the desired performance level. When you are satisfied, turn off the main faders to turn off the music in the main speakers.



• Next, turn the tape / CD channel control for the first monitor mix up half way (to the 12:00 position). Also set the first monitor send level to half way (12:00).



• Now, with the amplifier volume turned all the way off (left), turn on the first monitor amplifier power. Slowly increase the volume until you hear the music out of the first monitor speakers.



• Check that all the first monitor speakers are working and increase the amplifier volume setting until the music is at the desired performance level. When you are satisfied, turn off the tape / CD channel first monitor send to turn off the music in the first monitor speakers.



• Repeat this process for each set of monitors.



• You still have the effects units to set, but we'll leave that to you to figure out!



• System Tune-Up during Practice



• (this section not written yet)



• System Operation during Performance



• (this section not written yet)