ALLEN & HEATH PA20 User's Guide download pdf (Page 27)

PA User Guide 27

SOURCE SELECT Decide how you want to use the

output channels. Set the two switches according to the A and

B sources required. The switches are recessed to prevent

accidental operation once set. Use a pen or sharp object to

change their settings.

 Front-of-House refers to the audience listening area. It

is commonly called FOH or simply ‘house’. The house

speakers provide the PA for the audience.

 Foldback monitors Monitors are typically wedge

shaped floor speakers or stand mounted ‘hotspot’ speakers,

headphones and earpieces used by the performers to hear

themselves and others on stage. Foldback is the term used to

describe the process of returning some signal from the console

back to the performers.

 Different output configurations The PA Series AB

outputs can be used in many different ways. Sound system

requirements vary for many reasons. You may want to run a

stereo house system with separate left and right speakers and

no monitors, or a mono system with a foldback monitor. You

may be using an external speaker processor for a bigger house

system and therefore use your PA Series AB channels for a

pair of equalised stage monitors.

The Output Equalisers Each AB output channel

features a 4-band semi-parametric EQ. This divides the audio

range into four overlapping frequency bands. Each band has a

bell shaped peak/dip response with adjustable gain and

tuneable frequency.

LF, LM,HM,HF The lower control cuts or boosts the

frequencies around the centre point by up to 10dB. The flat

response centre position is detented to help you find it quickly.

FREQUENCY The upper control adjusts the centre point

frequency of the band. The width of the bell, or Q, is 1.8

measured at +10dB.

 Why the parametric output EQ? You are probably

familiar with the 7 or 9-band graphic EQ used on traditional

powered mixers. These have a similar bell shaped response

but set at fixed frequency points. The PA Series parametric

design provides much more accurate control because you can

tune each band to the exact frequency required. It also has a

tighter width per band so reducing the unwanted effect on

nearby frequencies. Used carefully you can deal with poor

room or speaker response and deal with feedback while

maintaining the fidelity of the original sound. In modern sound

engineering the parametric EQ is the professional engineer’s

choice for accurate frequency shaping.

☺ Using the output equalisers The best advice is to use as

little EQ adjustment as possible, and to cut rather than boost.

Before or during sound check, start with all bands set flat with

their gain and frequency controls at mid position. Avoid the

typical ‘smiley face’ EQ that overloads the amplifiers in an

attempt to get more range from inadequate speakers. A small

amount of boost at 70Hz and 10kHz can achieve the desired

results. Using a good test source such as CD or pink noise,

listen for resonant or ‘boomy’ frequencies in the room, and

frequencies that start to feed back or ‘ring’ when microphone

gain is gradually increased. Adjust the controls to tune in to

the offending frequencies and apply only small amounts of cut.

10k1kHz100 30k10

20Hz 200Hz

AB OUTPUT EQ

-15

-10

-5

0dB

+10

+15

120Hz 2kHz

500Hz 10kHz

1.5kHz 20kHz

Experiment with the Allen

& Heath Windows™ EQ

Visualiser software. This

lets you see how the

adjustments you make

with the EQ controls can

affect the signal frequency

response.

Download the Allen &

Heath RTA (real time

analyser) software from

our web site. This is an

invaluable tool to help you

identify and use the EQ to

correct problem room and

ringing frequencies.

L R+ FB2

FB1 FB1L

L R+ FB2

FB1 FB1

L R+ FB2

FB1 FB1L

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ALLEN & HEATH PA20 User's Guide Page 27

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