Technical Live Sound Paper. Number 2

Gain Structure and System Setup. - How to Make it Louder, Cleaner and Protect Your Speakers.

1.) Intro

Most P.A. systems are made up of a series of components. Mics, mixers, outboard equipment, amplifiers and speakers is the usual signal path. As per diagram below.

This paper is concerned with ensuring that these components are correctly matched which will in turn ensure that you get the very most from your system. As much level as possible, with minimum distortion and minimum ‘noise’ or hiss. These ideas, when combined with our ideas on feedback reduction in paper 1 will ensure you squeeze the absolute maximum from your P.A. system.

2.) Gain Structure

‘Gain structure’ is a phrase many people have heard of but don’t fully understand. Gain structure refers to the aligning of all units that make up a P.A. system so their levels are matched, or to put it another way, they all reach distortion at the same point. At every point on our P.A. system
diagram where two items are joined (with the exception of the amp-speaker connection there is a potential for a ‘gain mismatch’)

A serious ‘mis-match’ at any of the points shown above will result in either more noise, or distortion and just as importantly it will mean that you have not maximised your system’s level or as some people say, ‘headroom.’

a.) Microphone Matching and Channel Gain Matching.

Lets start by taking the first link in the audio chain from mic to mixer. (Actually, this could just as well be the output from a D.I. box, a keyboard, CD player or drum machine.) The most important control here is the gain control at the top of the mixers input channel. This is used to match the level of input signal to the mixers input circuit and achieve a level of 0db. (Maximum gain before distortion) Many larger live and studio mixers have metering on every channel or a ‘meter bridge.’ This makes the job of setting the channel gain control very easy, apply a signal (i.e. get someone to sing into the mic) and get it to peak around 0db.

b.) PFL Metering.

With smaller desks this is a little more complicated. In order to make cost and size savings smaller desk manufacturers use whats known as PFL metering to set up channel gain. In terms of metering this facility allows the input level of any channel to be set using just one set of meters. The process is as follows:
1.) Get the source of the channel to sing or play at ‘performance level.’ In other words, the same level they would sing/play during the show.
2.) Depress the channel’s PFL button. (Ensure that no other channel PFL buttons are down. Many mixers have a PFL/AFL light to tell you if any PFL’s are down. More on AFL later.)
3.) The PFL meters will go up and down with the channel gain control. Set the channel gain control so that the meters are peaking at around 0db.
Note that all of the above can be done with the channel or master faders being down.
PFL stands for ‘Pre Fade Listen’ and as well as providing individual channel metering, also allows the user to listen to the input signal of a channel ‘raw’ before the EQ and the fader, via headphones or a monitor ‘wedge’ speaker.
The above procedure will ensure there is a ‘healthy’ level of signal at the mixers input section (in other words, maximum signal without distortion.)

c.) Mixer and Amplifier Gain Matching.

One of the most important and misunderstood areas of gain structure is the relationship between the output of a mixer and the input section of the power amp(s).
I, like many other musicians, until comparatively recently thought the best way to achieve the most output from a P.A. system was to turn the power amplifier gain controls up to full. However, this is not necessarily correct. The gain controls on power amplifiers are not ‘level’ or ‘volume’ controls. They are gain matching devices. They are used to match the output level of a mixer with the input section of the amp. Important: power amps are fixed gain devices - turning down the amplifier gain controls does not change the potential output of the amplifier, it only requires more input voltage to achieve it.
So why not just turn up the amplifiers gain controls up to full anyway? Well, the problem is that many amplifiers will clip with an input signal of +6dbu when the gain controls are all the way up whilst most mixing consoles can output +18dbu of output level before clipping. This means that you could be sacrificing 12db of the available output of your system!

I know it sounds perverse, but you can actually increase the available output of your system by turning the amps down. But the reality is that it is not a question really of turning the amps up or down, but a question of matching the output gain of the mixer with the input gain of the amp.
So how do we achieve this? You will need your mixer and amps for this as well as a test CD (if your lucky enough to have a mixer that can generate pink noise or has a sinewave generator this is not necessary.) Actually for most practical applications some modern music with lots of different
frequencies and a regular bass drum will probably do just as well.

The process is as follows:
1.) Connect the main output from your mixer to the amp(s). At this point do not connect the speakers (2kilowatts of pink noise is not fun!) - Actually, whilst most modern amps will happily run without a loudspeaker load, some older amps may not like it - best to check with the manufacturer (if there still around.)
2.) Set the amplifiers’ input attenuators to ‘infinty’ or ‘zero.’
3.) Generate pink noise via the mixer, test CD, or music CD. Pink noise is best because it contains the complete audable spectrum in equal amounts and is constant, music may not have all frequencies present and will have a rhyhmic ‘pulse.’ (If your using a test or music CD follow the guidelines above concerning channel gains.) Make sure no PFL or AFL buttons are depressed (so the mixers meters are showing the output gain of the mixer, not the input gain of a channel)
4.) Bring the master faders of the mixer up until your meters are just into clipping (usually the red portion) then back them off slightly so the clip lights are no longer on.
5.) Bring the input attenuators of the amplifier(s) up until it’s meters are also just into clipping, then once again back it off slightly.
The output gain of the mixer and the input gain of the power amplifier(s) is now matched. Performing this process not only maximises the systems headroom (output capability), but you can also tell from the mixer if your amps are clipping (distorting), since the amps now clip at the same level as the mixer. This will help protect your speakers. Anyone who has been through the expensive business of reconing speakers and replacing compression driver diaphragms know the potential damage a distorting amplifier can cause.
6.) Plug your speakers in and listen to some of your favourite music. Not only will it sound cleaner but your system will go louder before it distorts (i.e. it now has more headroom.)
So, if you think your system isn’t giving you what it should and your 2,000 watts per channel amp is sounding less than impressive, it may be a gain mismatch between the mixer and the amp - follow the above process and I’m sure the situation will improve.
Of course, this doesn’t just apply to your front of house system. The same process can be applied to any monitor and/or matrix systems you might be running.

d.) What About Active Speakers.

The process for active speakers (speakers with amps built in) is much the same, but is a made a litle bit harder by the fact that you cannot disconnect the speaker from the amp section for pink noise. (At least I don’t know of any models of active speaker that allow you to do this - at least not without opening the box!) But the priciple is the same - getting the clip light on the speaker to light at the same time the mixer’s meters ‘clip’. Perhaps the best way of aligning active speakers is by using music rather than pink noise.

e.) Gain Matching Graphic EQ’s and Processors.

Of course, the more complex a system becomes, the more gain matching you have to do but the principles are the same. Matching the output of one device with the input of another.
Graphic EQ’s are often placed between pre-fade auxiliary sends and the monitor amps. Many mixers do not allow you to meter the output level of an aux send, but many mixer manufacturers are now employing an AFL system. (AFL, you won’t be surprised to hear stands for After Fade Listen.) It works much the same way as PFL, but allows monitoring and metering of a mixers outputs rather than it’s inputs (e.g. groups, aux sends etc.) Selecting AFL for aux send 1 will switch the mixers meters to show the output level for that aux so you can carry out alignment in the same way as your main outputs following the procedure above.
If you have a graphic in circuit then match the input gain of the graphic with the output gain of the mixer in the same way as the amplifiers, then do the amplifiers in turn. (The slight difficulty here is that you really need to do this after the graphic has been set because the settings of the EQ will affect it’s output level.)
Exactly the same thing applies to crossovers. If your using an active crossover, set it’s parameters, then match it’s input gain to the mixers output level and then do the amplifiers in turn.

3.) Conclusion.

Most people are using the same equipment night after night so it’s not necessary to carry out gain matching before every gig. It is perfectly possible to align the system in a rehearsal room, make a note of the settings (in case anyone tampers with them) and stick with that. Of course mic gain
settings are usually carried out channel by channel at the sound check.
A little time spent looking at and adjusting the gain structure of your P.A. system can at best result in a spectacular improvemant in system performance or, at worst, satisfy you that you are getting all you can get from your system. It may also tell you that, if, after digesting and carrying out all of the above information and procedures, your system still isn’t loud enough, you need something bigger!