MIXERS: HOW THEY WORK

Bruce Bartlett © 2006

The heart of your recording studio is the mixer. It’s a control center where you plug in all sorts of signals, mix or blend them, add effects, EQ and stereo positioning, and route the signals to recorders and monitor speakers. To some people, the myriad of knobs on a mixer is intimidating. Actually there are only a few controls you need to know – they are duplicated for all the channels.This article explains how mixers work. Each mixer is a little different from others, but you can read your mixer’s manual to learn its unique features.

Mixers come in many formats:
• An analog mixer is a mixer that works with analog signals.
• A digital mixer is a mixer that works with analog or digital signals. It converts analog signals to digital, and processes the signals in digital format.
• A software mixer exists only in your computer as part of digital recording software. You control it with a mouse. The recording is done on your hard drive.
• A controller surface resembles a standalone mixer with faders and knobs. It plugs into your computer’s USB or FireWire port and controls its software mixer.
• Some mixers have a USB or FireWire connector built in, ready to connect to a computer for use with recording software.
• A recorder-mixer or portable studio combines a mixer (analog or digital) and a multitrack recorder in a single chassis.

A mixer can be specified by the number of inputs and outputs it has. For example, an 8-in, 2-out mixer (8x2 mixer) has 8 signal inputs, which can be mixed into 2 output channels (buses) for stereo recording. Similarly, a 16-in, 8-out (16x8) mixing board has 16 signal inputs and 8 output channels for multitrack recording. A 16x4x2 mixing board has 16 inputs, 4 submixes or groups (explained later) and 2 master outputs. There also are connectors for external equipment, such
as effects devices and a monitor power amplifier. The more inputs your mixer has, the more instruments you can record at the same time. If you're recording only yourself, you may need only two inputs. Let’s look first at the analog mixer. Knowing how it works will help you understand the other types.

ANALOG MIXER
Any mixer can be divided into three sections: input, output and monitor. Here are the main parts of each section and what they do:

Input Section
• Inputs connect to your mics, electric instruments and recorder outputs.
• Faders are sliding volume controls that affect the loudness of each instrument. This lets you control the balance among instruments in the mix.
• Equalization (EQ) knobs adjust the tone quality of each instrument (bass, treble, midrange).
• Aux knobs set the amount of reverb and special effects, and also can be used to set up a monitor or headphone mix.
• Channel-assign buttons route each input signal to the desired recorder track.
• Pan pots place each instrument or vocal where desired between your stereo speakers. In some mixers, the pan pot is also used with the channel-assign buttons during recording to send signals to the desired tracks.

Output Section
• Master faders set the overall level of the mix.
• Outputs connect to your recorder inputs and to the monitor power amplifier.
• Meters help you set the correct recording level (to prevent distortion and noise).
Monitor Section
• Monitor-select buttons let you choose what you want to listen to.
• Aux knobs or faders control the monitor mix.
Let’s look at each part in more detail.

INPUT SECTION
A mixer is made of groups of controls called modules. An input module (Fig. 1) affects a single input signal -- from a microphone, for instance. The module is usually a narrow vertical strip, one per input. Several modules are lined up side-by-side. Each input module is the same, so if you know one, you know them all.

Fig.1. A typical input module

Let’s follow the signal flow from input to output through a typical input module (Figure 2). Every
mixer is a little different, but you are likely to find features like those described here.

Fig. 2. Signal flow through a typical input module.

Input Connectors
On the back of each input module are input connectors with these labels:
MIC: accepts signals from a microphone or direct box.
LINE: accepts signals from an electric musical instrument, or a track output of a multitrack recorder.
TRACK: accepts a track output of a multitrack recorder. Some mixers do not have separate track connectors.

A mic-level signal is typically about 1 to 2 millivolts. A line-level signal is about 0.3 to 1.23 volts.

Some units use a single jack for both mic and line inputs; others have separate jacks for each. The mic input is either an unbalanced 1/4" phone jack (a 1/4" hole) or an XLR-type connector (with three small holes). The line input is either a 1/4" phone jack, an RCA (phono) jack (such as that on a stereo system), or an XLR-type connector. You can plug a synthesizer directly into a phone-jack line input without using a direct box if the cable is under 10 feet; a longer cable may pick up hum. In that case, use a direct box plugged into a mic input. In some mixers, a phone-jack input is switchable between low impedance (for a microphone) or high impedance (for an electric-guitar pickup). Other mixers might have a separate lowimpedance input and a high-impedance input.

Input Selector Switch
This switch lets you select the type of signal you want to work with. It might have some of these labels:
• MIC (for mic-level signals: microphones and direct boxes)
• LINE (for line-level signals: a synthesizer, drum machine, electric guitar, or multitrack recorder’s track output)
• INPUT (mic or line)
• TRACK (for line-level signals from a multitrack recorder’s track output)
• MUTE: Silences the signal – turns it off. During mixdown, it's a good idea to mute tracks that have nothing playing at the moment to reduce noise and leakage.

Using the input selector is simple. If you plugged in a microphone or direct box to record its signal, set the input selector to MIC or INPUT. If you plugged in a synth, drum machine, or electric guitar, set the selector to LINE or INPUT. When you're ready to mix, select TRACK. Some mixers have no input selector. The mixer processes whatever signal is plugged in, and you adjust its level with the TRIM control (explained later).

Phantom Power (P48, +48)
This switch turns on phantom power (48V DC) for condenser microphones. The microphone receives phantom power and sends audio along the same two cable leads.

Phase (Polarity Invert)
This feature is found only in advanced mixers. Used only with balanced lines, this switch inverts the polarity of the input signal. That is, it switches pins 2 and 3 to flip the phase 180 degrees. You might use it to correct a miswired mic cable whose polarity is reversed. If you mike a snare drum top and bottom, you need to invert the polarity of the bottom mic.

Mic Preamp
After entering the mic connector and input selector switch, a microphone signal goes into a mic preamplifier inside the mixer. The preamp boosts or amplifies the weak microphone signal up to a higher voltage, making it a line-level signal.

Trim (Gain or Input Attenuation)
This knob controls the amount of amplification done by the preamp. When would you use it? If a microphone is picking up a loud instrument or vocal, the mic signal is very strong. This signal can overload the mic preamp, causing distortion -- a gritty sound. Turning down the TRIM control prevents this distortion.Set the TRIM control as follows: First set the master fader(s) within the shaded portion of its travel (sometimes at 0, about 3/4 up). Do the same for the input fader (discussed next). Assign the signal to a multitrack recorder track. Have the musician play the loudest part of the song, and adjust the TRIM control so that the meter peaks at the optimum recording level.In some mixers, in each input module is a tiny light (LED) labeled "clip" or "peak." It flashes when the mic preamp is distorting. If this light flashes when you're picking up an instrument or vocal, turn down the TRIM control just to the point where the light stays off, then turn it down another 10 dB for extra headroom.Some inexpensive mixers do not have an overload LED or a trim control. The input fader serves
this function.

Input Fader (Channel Fader)
After the preamp amplifies the mic signal, the amplified signal goes to the input fader. This is a sliding volume control for each input signal. During mixdown, you use the faders to set the loudness balance among instruments.

Direct Out
Found only in fancier units, the direct out is an output connector following each input fader and equalizer. The signal at the direct-out jack is an amplified version of the input signal. The fader controls the level at the direct-output jack. You can use the direct-out jack when you want to record one instrument per track (with EQ) on an external multitrack recorder. Connect the directout jack to a multitrack recorder track input. Since the direct output bypasses the mixing circuits
farther down the chain, the result is a cleaner signal. If a mixer has eight inputs with a direct-out jack for each input, you can use the mixer with an 8- track recorder--even if the mixer has only 2 or 4 outputs.

EQ (Equalization)
The signal from the input fader goes to an equalizer, which is a tone control. With EQ you can make an instrument sound more or less bassy, and more or less trebley, by boosting or cutting certain frequencies. We covered EQ in detail in past issues of Singer.

Insert Jacks
On the back of each input module, these connectors let you break the signal path and insert an external device there, before the fader and EQ section. You might plug a compressor in series with an input module's signal for automatic volume control. Or insert any other signal processor (reverb/delay, for instance) into the signal path of one track. This way, if all your aux sends are tied up, you can add another signal processor. On the reverb/delay unit, set the dry/wet mix
control for the desired amount of effect. Another use for insert jacks is to send each input module's signal to a multitrack recorder track. The output of each track returns to the insert jack and continues though your mixer. In this case, you use the trim controls to set recording levels, and use the faders, EQ and aux sends to set up a monitor mix. Some insert jacks use a single TRS connector for send and return. The tip connection is the send; the ring is the return, and the sleeve is the common ground or shield. Other insert jacks use a separate jack for send and another jack for return. Inexpensive units omit insert jacks. Some units have insert jacks on only two inputs.

Channel Assign Switch
The equalized signal goes to the channel assign switch (sometimes called track selector switch). It lets you send the signal of each instrument to the recorder track you want to record that instrument on.
Some units have an assign switch labeled 1, 2, 3, or 4. If you want to record bass on track 1, for instance, find the assign switch for the input module the bass is plugged into, and push switch 1.
If you want to record four drum mics on track 2, push assign switch 2 for all those input modules.
Some mixers assign tracks with two controls: a selector switch and a pan pot (described next).

Pan Pot
This knob sends a signal to two channels in adjustable amounts. By rotating the pan-pot knob, you control how much signal goes to each channel. Set the knob all the way left and the signal goes to one channel. Set it all the way right and the signal goes to the other channel. Set it in the middle and the signal goes to both channels. Here's how you might use a pan pot to assign an instrument to a track. The track-selector switch (assign switch) might have two positions labeled 1-2 and 3-4. If you turn the pan pot left, the signal goes to odd-numbered tracks (either 1 or 3, depending on how you set the assign switch).
If you turn the pan pot right, the signal goes to even-numbered tracks (2 or 4).
Suppose you want to assign the bass to track 1. Set the assign switch to 1-2, and turn the pan pot far left to choose the odd-numbered track (track 1).
During mixdown, the pan pot has a different function: it places images between your speakers.
An image is an apparent source of sound, a point between your speakers where you hear each instrument or vocal. Set the pan pot to locate each instrument at the left speaker, right speaker, or anywhere in between. If you set the pan pot to center, the signal goes equally to both channels, and you hear an image in the center.

Aux (Effects or FX)
The aux or aux-send feature (Fig. 3) can be used in at least two ways:

• To set up a monitor mix--a balanced blend of input signals you hear over speakers or headphones
• To set the amount of effects (reverb, echo) heard on each instrument in a mix.

Figure 3. Auxiliary signal flow.

Some mixers have no aux sends; some have one aux-send control per module; some have two or more (labeled aux 1, aux 2, etc.). The more aux sends you have, the more you can play with effects, but the greater the cost and complexity. The aux number (1 or 2) is not necessarily assigned a specific function; you decide what you want aux 1 and aux 2 to do.

During recording and overdubbing, the aux knobs of all the input modules can be used to create a monitor mix. The monitor mix that you create with the aux knobs is independent of the levels going on tape. You use the gain-trims during recording to set recording levels, and you use the aux knobs to create an independent mix that is heard over your monitor system.

In Figure 3, the aux 1 send control is just before the fader. In this mixer, the signals from all the aux 1 knobs in the mixer combine at a connector jack labeled "aux 1 send" or "monitor." You can connect that jack to your power amplifier, which drives monitor speakers and headphones.

In this mixer, the aux 2 send control is just after the fader. During mixdown, each aux 2 knob controls how much effects (reverb, echo) you hear on each track. The aux knob controls the level of each input signal sent to an external effects device, which adds echo, reverb, or some other effect. The effects signal returns to the mixer's aux-return or bus in jacks. Inside the mixer, the effects signal blends with the original signal, adding spaciousness or ambience to an otherwise
"dry" track.

A few mixers have an aux return control (also called effects return or bus in) that sets the overall effects level returning to the mixer.

Follow these steps to use the aux controls to adjust the amount of effects heard on each track:

1. Patch an effects unit between your mixer's aux-send and aux-return (bus-in) jacks.
2. On the effects unit, set the dry/wet mix control all the way to "wet" or "effect."
3. If your mixer has aux-return (bus-in) knobs, turn them about 3/4 up and pan their signals hard left and right.
4. Turn up the aux-send knob for each input, according to how much effect you want to hear on that input signal.

Suppose you're using reverb as an effect. You might turn it up by different amounts for the vocals, drums, and lead guitar, and leave it turned down for the bass and kick drum. As you're setting the aux levels, check the overload indicator on the reverb unit. If it's flashing, turn down the input level on the reverb unit just to the point where the overload light stops flashing. Then turn up the output level on the reverb unit (or turn up the aux return on the mixer) to achieve the same amount of reverb you heard before. There might be a pre/post switch next to the aux-send knob. When an aux knob is set to pre (prefader), its level is not affected by the fader setting. You use the pre setting for a headphone mix during recording or overdubbing because you don't want the fader settings to affect the monitor mix.
The post setting (postfader) is used for effects during mixdown. In this case, the aux level follows the setting of the fader. The higher you set the track volume with the fader, the higher the effects level is. But the dry/wet mix stays the same.

OUTPUT SECTION
The output section is the final part of the signal path; the section that feeds mixed signals to the recorder tracks. It includes mixing circuits, submaster or group faders (sometimes), master faders, and meters (Fig. 4).

Figure 4. The right half of this diagram shows the output section of a mixer.

Mixing Circuits (Active Combining Networks)
The mixing circuits are near the right side of Figure 4. Recall that you use the assign switches to send each input signal to the desired channel or bus, and each bus feeds a different recorder track. A bus is a channel in a mixer containing an independent mix of signals. The bus 1 mixing circuit accepts the signals from all the inputs you assigned to bus 1 and mixes them together to feed track 1 of the recorder. The bus 2 mixing circuit mixes all the bus 2 assignments, and so on.
Mixing circuits also accept the effects-return signals, such as the reverberated signal from an external digital reverb unit.
A four-bus mixer provides 4 independent output channels or buses; each bus carries a signal which may contain the sounds from one or more musical instruments. The four buses feed a 4-track recorder. A mixer with only two output buses can be used with a 4-track recorder by recording two tracks at a time.

Master and Group Faders
Located toward the right side of your mixer, the stereo master faders are one or two sliding volume controls that affects the overall level of the stereo output channels or buses. Usually, you set the master fader(s) within design center, the shaded area about 3/4 up on the scale. This setting minimizes mixer noise and distortion. You can fade out the end of a mix by turning down the master faders gradually.
Elaborate mixers also have faders that control the level of each bus independently, called group faders. You might create a drum mix, or keyboard mix, and control the overall level of each mix with a group fader. Group faders can serve double duty as master faders if you use only two of them. Then you can take your stereo mix from the bus 1 and 2 outputs. The noise is less there than after the master faders.
You feed the multitrack either from group outputs, direct outs or insert sends. If you're mixing several instruments to track 5, for example, assign those instruments' signals to Group 5. Connect the Group 5 output to recorder track 5 in. If you're recording one instrument on track 5, however, connect that instrument's direct out or insert send to track 5 in. The signal is cleaner at the direct out or insert send than at the group output.

Meters
Meters are an important part of the output section. They measure the voltage or level of various signals. Usually, each output bus has a meter to measure its signal level. If these buses feed the recorder tracks, you use the meters to set the recording level for each track. Your mixer will have one of two types of meters:

- VU meter: A voltmeter that shows approximately the relative loudness of various audio signals. Set the record level so that the meter needle reaches +3 VU maximum for most signals, and about -6 VU maximum for drums, percussion, and piano. That's necessary because the VU meter responds too slowly to show the true level of percussive sounds.

- LED bargraph meter: A column of lights (LEDs) that shows peak recording level. For all instruments, set the level to peak a few dB below maximum.

MONITOR SECTION
The monitor section is used to control what you're listening to. It lets you select what you want to hear, and lets you create a mix over headphones or speakers to approximate the final product. The monitor mix has no effect on the levels going to your recorder.
During recording, you want to monitor a mix of the input signals. During playback or mixdown, you want to hear a mix of the recorded tracks. During overdubs, you want to hear a mix of the recorded tracks and the instrument that you're overdubbing. The monitor section lets you do this.

Monitor mixer controls
The monitor mixer is a row of AUX knobs, one in each input module. Each knob controls how loud each input signal is in the monitor mix. The AUX knobs of all the input channels are used to set up the monitor mix you hear as you are recording and overdubbing. During recording, the monitor mix goes to headphones and to an AUX output jack that feeds external stereo amplifier and speakers. You choose to monitor over headphones or speakers. Some monitor mixers also have monitor pan and monitor effects controls. The pan knob controls the left-right position of the instrument or track between your stereo speakers or headphones.
During overdubs, the monitor mixer blends recorded tracks and live microphone signals into a cue mix that is sent to headphones. You overdub new parts while listening to the cue mix. In some mixers, the monitor mix and cue mix are identical.
During mixdown, you use the AUX controls as effects sends instead of monitor controls. You monitor the 2-channel output of the mixer.

Monitor Select Buttons
These buttons let you choose what signal you want to monitor or listen to. Since the configuration of these buttons varies widely among different mixers, they are not shown in Fig. 4. Some units have no monitor-select switches. Instead, you always monitor the 2-channel stereo monitor mix. In some mixers, you can use the faders to set up a monitor mix. Here’s how: Connect your multitrack recorder ins and outs to the insert jacks (Fig. 5). Connect the insert-jack 1 tip (send) to
track 1 in; connect track 1 out to the insert-jack ring (return). Make similar connections for the other tracks. With this setup, use the trim controls to set recording levels. Use the faders to set up a monitor mix, cue mix, or mixdown with EQ and effects.

Figure 5. Using insert jacks to send each input signal to a recorder track. The track signal returns
to the mixer, where you adjust level, panning, EQ and effects.

Solo
The SOLO button in an input module lets you monitor one instrument or vocal at a time so you can hear it better. By pressing two or more solo buttons, you can monitor more than one input ignal at a time.
Suppose you hear a buzz in the audio and suspect it may be in the bass guitar signal. If you push the SOLO button in the bass guitar's input module, you'll monitor only the bass guitar. Then you can easily hear whether the buzz is in that input.

DIGITAL MIXING CONSOLE
So far we’ve looked at the analog mixing console, which works entirely with analog signals. A digital mixing console accepts analog or digital signals. It converts the analog signals to digital, and processes all signals internally in digital format. The signal stays in the digital domain for all mixer processing. Level changes, EQ, and so on are done by digital signal processing (computer calculations) rather than by analog circuits.
In a digital console, each analog input signal goes through an analog-to-digital (A/D) converter so that the mixer can process the digital signal. Most of the digital output signals from the mixer goes thru digital-to-analog (D/A) converters. The resulting analog signals feed a power amp, effects unit, and so on. Analog and digital consoles operate differently. To use EQ in an analog console, you find the channel you want to EQ, and adjust its EQ knobs. To use EQ in a digital console, you press a button to select the channel you want to EQ, then press an EQ button, then the EQ settings for that channel show up on an LCD screen. You press buttons and turn a knob to adjust the EQ frequency and boost/cut for that channel.
Since one knob controls the EQ for all the channels, digital consoles have fewer controls than analog consoles. One knob or switch can have several functions. This makes digital consoles harder to operate than analog ones because, with a digital console, you can’t just reach for an EQ knob for a particular channel. You have to do several button presses to set the EQ
parameters.
On the other hand, digital consoles have built-in effects and automated mixing. You can set up different mixes, store each mix in the mixer’s memory, and recall each mix with the press of a button. When you recall a mix, some mixers make the faders move into the positions you set up. This feature is called “flying faders.” Other mixers do not move the faders when you recall a mix. You have to set them manually by looking at a display, which is a disadvantage.

Digital Mixer Features
Look for the following features in digital mixers when making a buying decision:

• Number of mic inputs
• Number and type of digital inputs and outputs (S/PDIF, AES/EBU, TDIF, ADAT)
• Number and type of option card slots (extra I/O, extra DSP, SMPTE sync, effects)
• Number of effects processors
• Ease of use
• Automation: flying faders or non-flying faders.
• Surround-sound monitoring.

SOFTWARE MIXER
A software mixer is a simulated mixer that exists only in your computer as part of digital recording software. You control it with a mouse or with a controller surface (explained next). The recording is done on your hard drive.
The input and output connectors for a software mixer are in a computer sound card or in an external audio interface. Sound cards and interfaces are 2-channel or multichannel. The effects that the mixer controls are either plug-ins (software) or external (hardware).
Controller surface
Using a mouse to set up a mix can be fatiguing and slow. As a solution, you might buy a controller surface ($550-$3000). It looks like a mixer with faders and knobs, but it controls the virtual controls you see on the computer screen. The controller plugs into your computer’s USB or FireWire port.

RECORDER-MIXER
A recorder-mixer or portable studio combines a mixer (analog or digital) and a multitrack recorder in a single chassis.

Now that you understand the basics of mixers, you should have a better idea of how to operate them.


Recording engineer Bruce Bartlett is the author of Practical Recording Techniques 4th Ed. published by Focal Press.

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