This month, Paul White takes a closer look at the input channel of a typical multitrack console and explains how it all fits together.

Alhough large mixing consoles can look very daunting to the inexperienced, or to those moving up from a Portastudio-type machine, it helps if you remind yourself that most of the console is taken up by identical input channels — and if you understand one, then you understand them all. This is particularly true of in-line designs, where the monitoring controls are also included in the channel strip rather than having a separate section of their own, as is the case in split consoles.

Virtually every console starts out with an input Gain control and a Mic/Line switch at the top of the channel strip. Because the signals from microphones are so much smaller than line level signals, a high-gain, low-noise mic preamp is necessary to bring the mic signal up to a level where it matches the internal circuitry of the mixer. The mic input socket (invariably a balanced XLR type on a serious mixer), feeds into such a preamp, the gain of which can be adjusted using the Gain pot. If the Line input is used and Line selected on the Mic/Line switch, this preamp is bypassed, though the same Gain control is often used for both the Mic and Line inputs, to save on cost and complexity.

The Mic input may also have a 48V phantom power switch, allowing individual mic inputs to be phantom powered for use with capacitor (also known as condenser) microphones, though once again, cheaper desks tend to have a global all-on or all-off phantom power switch to reduce costs. Dynamic microphones don't need phantom power, but as long as they are wired for balanced use and connected via a balanced cable, they can be plugged into a phantom-powered socket without incurring any problems. However, as I have stressed before and will doubtless say again, unbalanced microphones should never be plugged into an input to which phantom power is applied, as sound quality will suffer and there is a possibility of damage to the microphone.

Another button which you may find close to the input Gain control is the mic Pad switch. The purpose of this is to reduce the level of the signal before it reaches the mic preamp, which is necessary when very sensitive microphones are used close to a loud sound source. Under these circumstances, the output signal from the microphone can be high enough to overload the preamp, even when the gain is turned right down. When you switch in the Pad switch, the mic signal is reduced by 20dB, which avoids distortion. The rule is that if you don't need the Pad switch, don't use it, because with quieter sounds, it will cause an increase in background noise. However, if you're miking brass or bass drums at close range, then you may need it.

In-Line consoles usually have some form of switching on the channel strip to reconfigure the mixer for mixdown. This often takes the form of an Input Flip switch, the purpose of which will shortly become evident. While recording, the input channels are fed from the mic or line sources being recorded and the monitor inputs are fed from the outputs of the multitrack tape machine, but when you come to mix, the multitrack tape machine's outputs need to be connected to the input channel. The Flip switch does exactly that, by reversing the normal state of affairs so that the tape machine now feeds the input channels and the line input socket feeds into the stereo mix via the monitor level and pan controls. This is both simple and effective, as it allows the console to be switched from record mode to mixdown mode with no need to repatch any leads. Furthermore, all the line inputs may still be used to feed effects returns or sequenced MIDI instruments into the stereo mix, where they can be balanced and positioned using the monitor level and pan controls. Figure 1 shows the input section of a typical channel strip. Even the simplest in-line consoles usually have one effects send dedicated to the monitor section, and this may be used to add effects to any signals being brought in via the line inputs.

In the case of a split console, there will be no Flip switch, but it may still be possible to use the monitor inputs as extra line inputs, albeit with a little repatching.

Equalisation

Equalisation is really just a fancy term for tone control, though what you find on a studio mixing console is invariably a little more complex than what you get on your typical hi-fi. All an equaliser does is to increase or decrease the level of those parts of a signal in a certain section of the audio spectrum — a treble control turns the level of the higher frequencies up or down, while a bass control does the same for the lower frequencies. On most mid-priced studio consoles, the audio spectrum can be controlled over four bands rather than the two offered by simple bass and treble controls, the usual designation being High (treble), Upper Mid, Lower Mid, Low (Bass). Further to this, the two mid-bands are invariably fitted with sweep controls allowing the user to 'tune' them in to the exact part of the range that needs treatment. This type of control is relatively straightforward to use and is a good compromise in terms of ease of use, effectiveness and cost. This type of four-band equaliser is shown in Figure 2.

High-end studio consoles are more likely to have fully 'parametric' equalisers, where the user can not only select the operating frequency and the degree of cut or boost but also how much of the frequency range (Bandwidth) the equaliser affects. In experienced hands, this type of equaliser is very powerful, but they are rather more tricky to use than the more common 'sweep' type. Parametric equalisers will be the subject of a separate article in the future, so I won't dwell on them here.

Ideally, the equaliser section should have a bypass switch allowing it to be switched out of circuit when not required. This is important, not only because it allows instant comparison between the equalised and unequalised sound, but also because even the best equaliser causes some slight degradation of the audio quality, which means that bypassing it is preferable to simply setting it in its 'flat' position.

Modern in-line consoles frequently feature assignable equalisation, which simply means that the equaliser can either be switched into the input channel or monitor channel signal path. A refinement on this idea is to make the EQ splittable too, so that the High and Low equalisers can be used in one signal path and the two Mids in the other. If this is to be effective, the two Mids need to have a wide range to allow them to cover the whole audio spectrum between them, rather than just the mid-range. Of course, splitting the EQ is a compromise — in an ideal world, it would be nice to have a four-band equaliser in both the input channel and monitor channel paths — but in practice, the compromise is a workable one.

A conventional four-band equaliser would be logically set out with the High control at the top, the two Mids below it and the Low at the bottom, but if the EQ is to be splittable, this isn't the best layout. In this case, it would be far more logical to have the High and Low together, with the two mids grouped together below them — which is exactly what most manufacturers choose to do. Now, when the EQ is split, the controls fall into two logical groups. With split EQs, the bypass options become a little more complicated because there are two different signal paths to consider. In practice, different manufacturers choose different solutions, though arguably the best option is to have separate bypass controls for both the input and monitor channel paths.

Auxiliaries

We've already covered the difference between pre- and post-fade aux sends, the main difference being that pre-fade sends are unaffected by the setting of the channel fader and post-fade sends are affected. For this reason, pre-fade (often known as Foldback) sends are ideal for setting up cue mixes which are quite independent of the channel fader settings, while the post-fade sends (often called Effects Sends) are just the job for feeding effects units where the level of the effect is expected to maintain its relationship with the sound to which it is added.

Some live mixing consoles are arranged so that when the channel is muted using the Mute switch, the pre-fade aux send is unaffected, whereas on most studio mixers, muting a channel mutes both pre and post-fade sends. It is also quite common for some of the aux sends on an in-line console to be assignable in much the same way as described for the EQ. In other words, some of the aux sends may be switched into either the input channel or monitor channel signal paths, to make the deployment of effects more flexible at mixdown. Figure 3 shows the auxiliary section of our hypothetical mixer, though in the interest of illustrating as many options as possible, it does have a rather generous number of aux busses!

In-Line Monitor

There will only be a set of monitor controls in the channel strip in the case of an in-line console so these comments don't apply to split consoles. A typical mid-price in-line console might have a Monitor Level, Monitor Pan and Monitor Aux control, usually located as a group somewhere below the EQ section, and if the console has an AFL/PFL solo system, the monitor section may well have an AFL button. AFL stands for 'After Fade Listen', which means that what you hear solo'd over the monitors or phones when you press the button will be affected by the Monitor Level control. PFL is more commonly used on the Input channels and stands for 'Pre-Fade Listen'. PFL fills a dual role in that it not only allows a channel to be solo'd, but also provides a simple and accurate means of setting the input Gain control on the channel. When PFL is depressed, the channel signal level is displayed on one of the desk's level meters; the input Gain control may then be adjusted so that the signal peaks are in the vicinity of the 0VU mark on the meter scale. PFL, AFL and Solo functions should only affect the monitor and headphone output; they should not change the main stereo output signal being fed to the master stereo recorder. This allows them, for example, to be used during mixdown without interfering with the mix that's being recorded. Figure 4 shows a basic in-line monitor section.

Routing

As explained last month, convention has it that routing the signal to the groups is accomplished by a combination of the routing buttons and the Pan pot. The routing buttons select odd/even pairs of groups while the Pan pot is used to steer between the odd (left) and even (right) groups selected. Figure 5 shows the routing section of a typical eight-buss console.

Again, there are one or two possible variations, the simplest being the Direct channel output. This is simply an additional socket that provides access to the post-fader signal, allowing it to be patched to any desired destination. If all the routing buttons are in their up position, the input channel becomes, in effect, isolated from the mixer's routing system and can be used as an independent 1-input, 1-output preamp. If this seems rather obscure, imagine that you have an eight-buss mixer which you're using with a 16-track recorder, and you want to record, say, nine tracks in one take. This can easily happen when you're recording a live band or one with a complex drum miking arrangement. The eight mixer groups feed eight of the tape inputs, while the Direct output from your 'independent' channel can be patched into the ninth. The only limitation of using direct outputs is that you can only address one signal at a time with no mixing, but this isn't a problem if you want to handle something like a bass, a guitar or a snare drum mic.

As you might have guessed, some designers of in-line consoles have extended this concept even further. Some mixers have an extra Direct button on their input channels that allows the same socket to function as either the group output or a direct channel output. When the button is up, the socket behaves just like any other group output socket and carries the signal from the group fader. However, when the button is down, that particular group output is disabled and the channel signal fed directly to the socket instead. This might sound like an unnecessary complication, but it's really an attempt to make the routing flexible enough to allow you to meet most recording situations without having to physically plug and unplug leads.

Next month we'll be taking a closer look at the master section of a typical mixer to find out exactly what goes on in that department.