Handleiding Allen-Heath GL2800 (pagina 25 van 42) (Engels)

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GL2800 User Guide 25

GL2800 Applications and Operating Tips

The following is a collection of brief descriptions, application notes,

hints and tips to help the operator understand some of the technical

terms referred to, and to get the most out of the uniquely capable

GL2800 live sound mixing console. They are written as a concise

reference to spark your imagination to creatively and effectively deal

with the many challenges now faced in modern day mixing.

Gain Structure The term used to describe the gain (level)

matching of the signal through the audio system chain. Each item in

the equipment chain has its own optimum (‘nominal’) operating level

determined by its electrical circuits. If you overload it with a signal that

is too high then its output tries to produce more voltage than its power

rails can provide resulting in harsh sounding distortion as the audio

signal is ‘clipped’. If you work with a signal that is too low then you

are likely to hear the residual hiss of the circuits as the ‘noise floor’ is

amplified along with the signal. The optimum operating level is the

point at which the signal is high enough above the noise floor so that

the hiss is not heard (good ‘signal-to-noise’ ratio, the SNR), yet

provides enough space (‘headroom’) before clipping to allow for the

louder, dynamic musical moments without distortion. ‘Dynamic range’

specifies the maximum range between noise floor and clipping. It is

the sum of the SNR and headroom. The larger the figure the better…

less noise, more headroom, more forgiving to wide ranging signals.

A microphone produces a very tiny signal, for example -50dBu (a few

millivolts). This should be amplified by the channel preamp to the

optimum operating level of the console circuits, around 0dBu

(0.775V), well clear of the noise floor (typically less than -90dBu), and

with good headroom (typically clipping at greater than +20dBu). With

a channel routed at unity gain from preamp input to main output, the

GL2800 boasts a massive 116dB dynamic range. The signal can

pass through many stages which affect its level within the console; the

channel, its EQ, fader, pan, a group mix and its fader, on to the main

LR mix with its master fader, through the matrix, and of course any

inserted equipment. The output XLR provides the professional

standard +4dBu (1.23V) nominal level to the next stage in the audio

chain, typically a speaker processor such as limiter or crossover, or

straight to the amplifier. The amp boosts this signal to tens of volts to

move the speaker cones so producing the audible sound.

To get the best performance from your system, it is important that you

set up the gain structure correctly within the console and also within

and between the connected equipment. Ideally, each circuit should

be set to clip at the same time if the signal became too hot, in other

words each would have similar headroom relative to its nominal

operating level. The resulting dynamic range is the difference

between the highest noise floor and the lowest headroom through the

system. The performance is only as good as the weakest link.

The GL2800 provides comprehensive metering to check every

point within the console signal path. Use the channel meters and

PFL/AFL system to set the gains and mix levels to average around ‘0’

meter reading with usual peaks around ‘+6’. Reduce the gain if the

red ‘+16’ or ‘PK’ indicator lights.

Use the equipment meters to set each item within the system path to

operate at its nominal level. We advise that the speaker processor or

amplifier trims are set so that the console outputs can drive up to their

nominal ‘0’ level. It is a common mistake to set amplifier trims to

maximum sensitivity when this results in the console master faders

being operated at very low positions, with low meter readings, and

reduced dynamic range and therefore audible residual system hiss.

-12

-16

-9

-6

-3

-1

-20

-30

-40

-50

-60

-70

-80

-90

+12

+16

+20

SIGNAL-TO-NOISE RATIO

DYNAMIC RANGE

HEADROOM

CLIPPING

NOISE

NORMAL OPERATING RANGE

SIGNAL

☺ A search on the Internet

can reveal a host of additional

information on setting correct

gain structure and many other

audio topics and educational

resources.

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