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Return to Zero (Part 6)

Mixers

In this month's article we take a look at the most central piece of equipment in any recording studio; the mixer.



The mixing console is the nerve centre of any recording studio, and may be considered as performing two separate and largely independent functions; controlling the input to the multi-track recorder, and mixing the output from tape to produce the final stereo mix. As we are dealing with the basics of the mixer in this article, I'll stick to the simpler kinds of console.

There are many good mixers on the market these days suitable for the home recordist, and if you can afford it, it is worth trying to match your console to the type of recording machine you're using. For instance, if you have a 4-track machine, then a mixer with at least four outputs would be ideal and for an 8-track machine a mixer with eight outputs and so on; hence the terminology 12 into 4, 16 into 8 etc where the first figure specifies the number of input channels whilst the second figure describes the number of outputs for connection to the multi-track recorder. Of course you can plan ahead by getting a mixer with more outputs than you currently need which means that you won't have to change your mixer when you upgrade your recorder.

A typical example of a simple mixing operation would be when recording a drumkit (or drum machine with separate outputs) onto a 4-track machine, where you could have up to six mics on the kit but only one track to spare for drums. These six sound sources must therefore be 'mixed' down to one before being output to the tape machine track of your choice. Since all consoles need to have the capability for mixdown into stereo and monitoring, you also see the term 12-4-2, 16-8-2, and so on; the '2' referring to the final stereo mix on to a 2-track recorder, and with this configuration, the input channels may be routed via the output groups so that the whole drum mix for example may be put under control of a single group fader. This is called subgrouping.

Looking at any mixer, you will see that the majority of space is taken up by a plethora (it's that word again) of pots and faders. Most of this comprises the input channels of the desk, and don't be put off by the sheer amount of controls because each channel is essentially the same. You will find that each channel will have even at the most basic level an input gain, an equalization (tone) section, auxiliary send for adding effects such as reverb and echo, a pan and a channel fader. So on a desk with 12 channels, (for example) a 12-2, you will have 12 identical sets of controls.

Inputs



Mounted on the rear panel, the input to each channel on a budget mixer is usually made via a ¼" jack socket but on the more expensive types a separate XLR connector is usually available for microphone inputs. In a professional studio all inputs are permanently wired up to a patchbay so there's no awkward fiddling around for the socket on the back.

Some mixers have what are called 'Insert Points'; one for each channel, next to the inputs on the rear panel. These usually accept a ¼" stereo jack to which you wire two mono jacks (unless your inserts are wired to a patchbay). This breaks the input before or after the EQ section (depending upon the manufacturer) allowing you to treat that channel with an effects unit; compression or limiting on bass would be common applications.

Meanwhile, lurking somewhere on either the front or rear panel, a switch for Mic or-Line is usually available. Electronic keyboards, synthesisers, active guitars and basses (or those used with a Rockman or similar device), and drum machines will be input when Line is selected; while on Mic you would use microphones or other low level signals. Sometimes you find that an amplified guitar signal is just too much for the Mic input setting and overload distortion may occur regardless of the setting of the gain control. On some mixers with only one input socket you could simply switch to Line which can cope with the higher signal, while on others, a Pad or Attenuation switch is available to cut the input, usually by 20 dB which corresponds to a factor of 10 in terms of signal level.

The input gain knob sets the amount of input to the channel you are using, and if a PFL (prefade listen) facility is available on the mixer, constant reference can be made to it in conjunction with the input gain control so that an optimum working level may be set up. Each channel will have a PFL button and when selected, you can monitor on the master VU or Peak Level meters how much input you have going to that channel. This control monitors the level after EQ but before the channel fader, so altering your EQ will affect the input level and again, reference to the meters would be very useful. If you find that you're boosting or cutting the EQ too viciously then there's probably something wrong with your source sound, or the microphone you're using is wrong for that sound.

So there are three things to do whenever you input a sound to the desk. Decide whether it is a Mic or Line source; select PFL, monitoring the input on the meters and adjusting the input gain pot to avoid overload; and EQ the sound if necessary with reference to the speakers and meters. As far as metering goes, the trend is now for LED peak meters instead of the old style needle VU metering. This is probably because peak meters are easier to see from a distance and also respond faster to transient sounds like drums, which overload very easily. Moving coil meters are however still in common usage.

EQ



Smaller desks may have only two - simply labelled EQ: Treble and Bass which will probably work in a similar fashion to those controls on a domestic hi-fi. Things start getting more interesting when mid controls are added, and on a typical budget mixer you will probably have Treble, Upper Mid, Lower Mid, and Bass controls which may be labelled in Hz and kHz denoting the frequency at which they operate. The audio frequency range is generally considered to extend between around 40Hz and 20kHz, and although all makes of mixer differ, a possible layout would be Treble at 12kHz, upper mid (2kHz), lower mid (600Hz), and bass at 50Hz. Both cut and boost are possible. On the best mixers you should find that each EQ control will have a slight frequency overlap with the one above or below it so that there are no gaps in the frequency range covered, giving you greater control over your sounds with no dead areas.

On more expensive mixers, the trend is to have shelved Treble and Bass controls and parametric Mid equalization which is after all where most of the sound energy occurs. Parametric mids allow you to tune-in to more specific frequencies and boost or cut as required to achieve a more precise degree of control.

Auxiliaries



Below the input gain and EQ sections on each channel you can find two or more auxiliar sends which are usually governed by master auxiliary level controls (one per aux send) to the right of the desk. A simple desk will usually have one effects send and one foldback send per channel with their respective masters.

The Effects is post-fader which means that it is affected by the channel fader setting. Because you have one on each channel routed to the single master output for that send, it is in effect a simple mixer, allowing you to govern the amount of effect you want on a particular channel by the amount of signal you send to be treated. A ¼" jack socket or XLR is usually located on the rear panel and labelled Effects Send, or on older models, Echo Send. From here you take an output to the effect of your choice and an output from the effects unit back to the mixer via another socket dedicated to Effects Return. The amount of effect is governed by the Effects Return (sometimes called Echo Return) volume knob which is fed to the Master faders. On some consoles a Pan control is available to let you place your effect anywhere you want in the stereo image.

If you know what effects you will need in advance, then you can add effects as you record onto tape, but a lot of people prefer to leave their options open and add effects at the mixdown stage because, once committed to tape, they cannot be changed without rerecording the whole track again. Also, concerning the return signal from effects units, many people like to bring this back through a spare input channel on the desk because you have more control over the EQ. This is especially true of reverb where you can alter the nature of the sound completely with judicious use of EQ. For instance, on a drum track you may wish to add reverb to the snare but find that the bass end of the reverb sounds too boomy. If you have brought the effect back through the desk you can simply take some of the bass end out, leaving the top to cut through clearly.

Foldback or pre-fade auxiliary also comprises a separate knob for each channel routed to a Master, but there is no Foldback Return as this facility was intended primarily for live monitoring purposes. The output would go via an amp to onstage monitors for the band who would then have a mix of the off stage sound.

However, it's easy enough to use the Foldback as an extra Effects send simply by taking the output from the effect to a spare input on the mixer. Because of the increase in home recording, manufacturers now tend to make this facility optional as Foldback/Effects by putting a return on the console so you effectively have Aux.1 and Aux.2. You can also use it for monitoring on headphones by connecting the Foldback output on the rear of the mixer to a headphone amp with several outputs. As each channel has its own Foldback volume control you are able to use it as a simple mixer. Remember though that this auxiliary is independent of the channel fader setting and so may be tricky to use as an effects send if the channel level needs to be altered during a mix.

Panning, Bussing, and Channel Faders



The Channel Fader is an output level control which is routed or bussed via the Pan control to the output group faders. On a 6-, 8-, or 12-2 mixer the Pan simply dictates where your sound will appear in the stereo picture as there are only two outputs: left and right - with jack sockets on the back for connection to power amp or tape machine. On mixers with more outputs, say a 16-8-2 the situation is more complicated as you have the option of routing any or all of the sixteen inputs to any or all of the eight or left/right outputs. On a console like this you will have a set of buttons labelled odd and even ie. 1 and 2, 3 and 4, 5 and 6, 7 and 8, L and R, and by selecting one of the buttons and using the pan control hard to the left (odd), or right (even) you dictate which group fader you are sending your sound to (selecting L and R simply routes the sound to the two master output faders). This is what is known as Bussing and whichever output you have sent your sound to will be connected to its corresponding track on the tape machine. These group faders are found to the right of the console and above each should be a monitor volume pot which controls the amount of sound sent to the two output faders for stereo monitoring via an amp and speakers. Most monitor circuits are switchable so that you can listen to the mixer output groups or to the sound coming off the multi-track recorder.

Anyway, after that little detour let's get back to the more basic mixing desk.

Faders are used instead of rotary pots as volume controls because it is much easier to check at a glance what is happening during a mix where the faders may be used a lot. They should also have a smooth action and be free from crackles; nothing is worse than coming to the end of a complicated mix and finding that fader or pot crackle has ruined it. You can get sprays to try and alleviate this problem but they are not a complete solution to the problem.

Buying a mixer



It is worth looking for a console which matches your recording set up in terms of the number of output and monitoring channels or otherwise you're going to have problems with monitoring the sound off tape. If you have a 4-track machine but can't afford either to buy more than a 12-2 or 8-2 desk or upgrade from a similar console, you could try getting a simple 6-2 mixer with no EQ or sends for the purpose of monitoring off tape only, using the 12-2 for recording and mixdown. However, if you intend using your mixer for simple live use, two outputs will probably do. Look for smooth operation of pots and faders (and secondhand buyers should watch out for those crackly controls which can ruin a mix), a clear layout (preferably colour coded), good EQ without any gaps in frequency control, and for the home recordist who collects and hoards effects, as many aux sends as you can get! Also, remember that faulty pots are easy to replace if you know how to solder so you could save a lot of money by buying an old banger of a mixer and fixing it up yourself.


Series

Read the next part in this series:
Return to Zero (Part 7)



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Studio Mains Supplies

Next article in this issue

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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Jul 1985

Donated & scanned by: Mike Gorman

Topic:

Mixing

Recording


Series:

Return To Zero

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 (Viewing) | Part 7 | Part 8


Previous article in this issue:

> Studio Mains Supplies

Next article in this issue:

> Back Issues


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