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The SOS Guide To Going Live

Vive La Difference!

Choosing Mixers For Live Sound

All mixers are fundamentally the same, aren't they? Dave Lockwood explains the differences between mixers designed for studio and stage.


The Live variant of Soundtracs' Megas console.


Surely choosing a mixer for live sound applications is not going to be that different to choosing one for recording? Well, not quite; the most fundamental difference between a recording desk and a PA desk lies in the signal routing options.

For live work it is normally only necessary for a number of microphone and instrument sources to be combined into a stereo (or mono) output, which is then used to drive power amplifiers and speaker stacks. In a recording desk, the multitrack and monitoring circuits complicate things a little — channels can be routed not just to the stereo output, but to anywhere between four and 24 sub-group outputs. These are the feeds to the individual tracks on the tape recorder. 'Monitor channels' are provided to feed these group output signals into the dedicated 'monitor output' circuits so you can hear them. The 'stereo master' output of the recording desk, the equivalent of the main outputs on a PA desk, is actually only used at mixdown, when it feeds the 2-track mastering recorder with a mix of all the input channels that are selected to the stereo, or left/right bus.

So if you take a recording desk and switch all its input channels to left/right, have you then created a PA desk? If you simply ignore the sub-grouping and monitoring, you've certainly got something that will perform the same job as a PA desk, but there are other areas in which you will find it is not quite so well adapted, for real PA desks have their own specific features.

LEVELS



One essential point to bear in mind when choosing a PA mixer to be used with a 'professional' grade power amplifier is its output level. Power amps are specified to give their full rated output for a given input level; a 'pro' power amp with a nominal +4dBm interface level will never perform wholly satisfactorily with -10dBm output, as offered by the majority of home recording desks (although this is sometimes internally switchable to the higher level).

Leaving aside the vast desks used in professional rigs, which are starting to resemble studio consoles ever more closely in some respects, the average PA mixer has a simpler channel strip than a studio desk. This is neither by accident nor totally down to reasons of economy — a clear unambiguous layout and simple operating procedures are essential features in a live desk. An over-sophisticated EQ will not help you kill feedback on the lead vocal mic the moment it starts.

EQs



EQs on a PA desk need to be, above all else, smooth; anything that introduces a significant peak into the response will compromise the feedback threshold of the whole system. A common EQ configuration for a small PA desk is a simple fixed-frequency 3-band (or even 2-band) channel equaliser, combined with more sophisticated EQs on the main and monitor outputs. The purpose of the output EQ should be solely to notch out troublesome frequencies.

On-board graphics tend to have fairly broad bands, sometimes over an octave, whereas a true parametric with a bandwidth control will be more selective and can be 'tuned-in' to cut a specific frequency without unduly affecting others. Output EQ cannot really be used to compensate for fundamental system response limitations, ie. you can't get a full-range response out of cone drivers alone just by ramping-up the top end on a graphic. With the channel EQs it is as wise in live mixing as in the studio to optimise the sound at source, with careful mic choice and placement, before you reach for the EQ. Even then, begin by trying to achieve what you want by cutting unwanted frequencies rather than by boosting anything. Smooth, shelving-characteristic EQ bands fixed at 100Hz and 10kHz are perfectly adequate for this sort of work. If there is a mid control, a 'bell' curve fixed at about 1kHz will do fine. Remember, you can't compensate for a poor microphone with EQ, no matter how sophisticated it is, so you are better off not trying. Indeed you can often be better off overall leaving some instruments un-miked, if it means you can avoid using any mics of significantly inferior quality to the remainder on stage.

AUXILIARIES



Just as in a recording desk you never seem to have enough post-fade auxiliaries for F/X, in a live desk it is pre-fade sends that are at a premium. It is these that are used to create individual foldback mixes that will not be affected by any mix activity going on out-front. For any reasonably sized stage, a minimum of two foldback circuits is recommended, one for the drummer and one for everyone else; no drummer jokes here, it's just that they tend to be stuck towards the back of the stage and will want a mix that nobody else will want, probably including some of the back-line amplification signals.

Go easy on the F/X for live mixing; most venues are over-reverberent already, and even where they are not the level of spill on a small stage will often ruin the intended effect by putting it on everything. Be particularly careful with delays; a loud drummer's snare drum will get everywhere, and inadvertently delayed snare can wreak havoc with everybody's timing.

HOW MANY CHANNELS?



In my experience more live sound mixes-have been ruined by over-miking than by any other cause. Every extra microphone on stage adds to spill and reduces system headroom. Using a modest rig, you certainly don't need a mic on every drum, and you should only mike up back-line where strictly necessary, ie. when low-power amplification is being used. Ultimately, of course, the number of channels will depend on the size of your band and the type of venue that you are playing, but a clean balance at a slightly lower level from just a few mics will always have more impact than an uncontrolled wash of sound with everything miked up.

Mic inputs on live mixers really should always be low impedance and balanced, which normally means XLR connectors. They might be a bit more expensive than jacks, but they are really the only connector that is sufficiently rugged for the job, and you can easily extend them with another cable, unlike jacks. With powerful lighting systems often in use, stages are electronically hostile environments, making the immunity to external interference offered by balancing practically essential. Don't worry too much about phantom powering on the mic inputs — most condensor mics are actually rugged enough for stage use these days, but their extended frequency response will be totally wasted by the limitations of most amplification systems. On the other hand the 'tailored' frequency response (presence peak, bass roll-off) of many dynamic mics is an asset for PA work.

One thing you won't find on a PA desk is a 'destructive' solo facility (one which achieves solo monitoring by muting all other channels, as on a recording desk; 'solo-in-place'). A conventional PFL (pre-fade listen) circuit will solo signals only into the headphone output, and give a solo reading on the metering which can be used to check gain levels. Good metering is just as important as on a studio desk, although you don't need as many meters — a pair on the main outputs, switchable to auxiliary circuits, and following PFL, is quite sufficient.

SUMMING UP



A PA desk must be physically rugged in order to withstand constant transportation. Modular designs in particular must have an extra-rigid frame to avoid flexing that might damage internal connections, and a non-modular design is actually no real disadvantage in this application. High-spec mic amps are not quite so essential as in a recording desk, for PA signals are always going to be close-miked or DI-ed. The noise spec may not be quite so critical, for there will always be a fairly high level of ambient noise wherever there is an audience, but headroom is, if anything, more important; there are no 'second-takes for level' in live performance. The consequences of highly amplified clipped signals also go beyond the audible, and damage to high-frequency drivers is all too easy.

There is no such thing as the ideal PA desk — the requirements of different situations are just too diverse. The most important principle is perhaps to avoid unnecessary complexity. Where compromises have to be made, go for quality rather than quantity. A simple EQ is no real handicap, but a few auxiliaries will increase flexibility a great deal. Just as in the recording process, the PA signal chain is exactly that — a chain — and therefore only as strong as its weakest link. However, any limitations in the mixer, as the first stage in the system, may be ruthlessly exposed by high quality amplification. The requirements for PA desks are therefore no less stringent than for their recording counterparts, just slightly different.


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Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Mar 1992

The SOS Guide To Going Live

Topic:

Live

Mixing


Feature by Dave Lockwood

Previous article in this issue:

> Are You Insured?

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