Setting up a PA System
This article is based on the routine I used to go through when working as one of the house engineers at live music venue. The process doesn't require complex measuring equipment, but there is nothing stopping you incorporating test equipment into the procedure if you wish. Given that this article is based on the set up procedure of a particular PA system, you will need to adapt it to your own requirements.
First of all I'll explain why I followed this set up routine even though the venue had a permanently installed system. I was only one of half a dozen house engineers that looked after the sound. The venue operated seven days a week and thee hundred and sixty five days a year. All genres of music were covered from punk and heavy metal through ABBA tribute bands to folk and acoustic. Some bands brought their own mixing person and some wanted to use their own touring PA. This meant that anything could have happened to the PA since I last used it. Cables unplugged, damaged or repaired incorrectly, drive units blown or distorting and other similar problems.
Try and give yourself enough time to run through the set up without being rushed. I prefer to do it before the band arrives and everything is nice and quiet. The following description is for a four way system and I'll call the different frequency ranges lo, lo-mid, hi-mid and hi.
Ok, so assuming everything is plugged in we are ready to switch on. Start with the mixer, then effects and processing equipment and finally the amplifiers. This will prevent switch on surges being amplified and causing undue stress to the loudspeakers. If they are not already muted, turn off the signal to each amplifier. This can be done with either a mute switch (if it has them) or output level control on the cross over or failing that, the input gain control on the amplifier. It helps to have a cross over by the mixing desk so that you don't have to keep trekking back and forth. If you have an assistant then neither method will be a problem.
Plug a microphone into one of the mixer channels, set the eq flat (if there is a graphic on the main outputs, set that flat too) and adjust the gain so that you have a healthy signal. A healthy signal is somewhere between -3dB and +3dB with a typical input signal. I'll briefly mention gain structures here. Some people get obsessive with setting up gain structures, but with most modern equipment it is not too critical. The aim is to have all the equipment operating above the noise floor without it running into distortion and overload. My method is to set up the input gains as stated above (remember to recheck the levels after applying eq) and then mix so that the output metres are peaking around about 0dB. All processing equipment such as graphics and cross overs are then set to unity gain (ins and out both set to 0dB) and the amplifier input levels then adjusted so that full output is achieved. Unless you have a mismatch of equipment the above should quite easily be achieved. If you are having problems and want some advice, post on the discussion board.
To continue. Un-mute the hi frequency range and turn the microphone up and make a "ch" or "tss" sound into it. This should be heard clearly through the hi section of the PA. The reason for starting with the hi range is that if the connections for the lo and hi have been mixed up you won't damage the drive units. Pan the signal from left to right. Both sides should sound the same. Centre the pan control and check that the sound is clearly centred between the speakers. If something doesn't sound right try reversing the phase/polarity of one side of the PA. Does that make it better or worse? If there is more than one drive unit either side of the stage it might be possible that one of the drive units is either wired incorrectly or not working correctly. If things are really bad you might have to unplug the drive units so that you can compare them individually until you find the problem unit. If there are two units per side, a and b left and a and b right, the procedure would be to compare a left with a right, b left with a right, a left with b right and finally b left with b right. Carrying a few short connecting cables that reverse the polarity helps here. Once you are satisfied that everything is wired up and working correctly you can move on to the hi-mids and repeat the above procedure. You will need to include some of the higher vowel sounds this time "A"s and "La"s instead of the consonant sounds used for the hi section. The aim once again is to check that everything is wired up and working correctly. Repeat again for the lo-mids and los.
The next stage is to un-mute everything. Listen to both channels and decide which sounds best. If they both sound the same then pick either channel. With your microphone panned over to that side keep talking (check one two will do) and if necessary tweak the output levels on the cross over to improve the balance between the different frequency ranges. Try swapping the phase/polarity on the each of the frequency ranges to see what difference that makes. It is technically correct when the output at the cross over frequency sums to a maximum, but it might sound better the opposite way. Don't forget to match any changes you make on the opposite channel. Continue the process of tuning with the graphic equaliser. It is useful to turn the gain up to a point where the system is on the point of feedback and reduce the appropriate frequency on the graphic to prevent the feedback. It can also be useful to turn the system up to slightly below feedback point and excite the system with various plosive vowel and consonant sounds. Listen for any overhang or resonant decay and again reduce these with the graphic. Eventually you should get good clarity and tonal balance. Depending on your own preferences for tonal balance you might reach the feedback level sooner rather than later. If the feedback point is still above the level you will operate at then all is well if not you will need to compromise your settings to achieve a satisfactory amount of gain.
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The diagram shows a couple of PA stacks. The blue cables represent correctly wired connectors and the red ones represent incorrectly wired connectors. You are unlikely to come across systems wired this badly (well not often) but running through the procedures outlined in this article you should discover these defects and correct them |
Once you have set the graphic, mute everything except the hi frequency range. Un-mute the hi frequencies on the other channel and then compare the two. Adjust the level on the cross over and graphic settings until the second channel sound as close to the original channel as possible and with the pan control centred the sound is clear sharp and centred between the loudspeakers. Don't try and match the settings visually, it is the sound that matters not the position of the faders on the graphic. When you are happy repeat the above with the other frequency ranges. When complete have a quick re-run through each frequency range to check everything is still OK.
Sometimes you might find that having matched the hi frequencies, you then mess them up because of adjustments you make to the hi-mid section. This happens because some of the frequency bands of the graphic reach across the cross over point. It is applicable to all the cross over points, I am simply using the hi to hi-mid as an example. To solve the problem simply switch on the frequency ranges either side the cross over point and again go through the comparison adjustment procedure. Try to resist altering the frequency bands away from the cross over point.
Finally turn on everything and listen to each and then both channels. You should have a fine sounding PA with both sides sounding the same and a precise sound image located between the left and right loudspeakers. You might find that with both sides of the PA on together that the lo-mids sound a little fuller than when you were setting up each channel separately. Simply readjust to taste but make the same changes to both channels this time.