Finding and Fixing PA System Issues in Live Event Productions
Sometimes problems will surface during set-up or sound-check, and occasionally during the performance itself. Even in fairly basic PA systems there are plenty of opportunities for something to go wrong: leads can get pulled loose, batteries (even new ones!) can die unexpectedly, and amplifiers can overheat and shut down. Often the cause of signal loss is trivial, although catastrophic component failure can happen.
- Stay calm! Panicking or losing your temper with people or equipment will not make the problem go away.
- Focus your attention: is the signal reaching its destination? If not, why not?
- Be methodical. Don't change connections, leads and components at random: work your way along the signal path, eliminating possible causes of failure - see below - as you go. If you carry spares (as you should!) you don't need elaborate test equipment: you can always find the culprit by substitution. If a microphone isn't working, replace it with one you know to be OK. If it still doesn't work, replace the lead with one you know to be OK (this also gives you an opportunity to check you have it connected in the right place). Obviously your options are more limited if something happens during the performance - you need to be as inconspicuous as possible - but that means you should be more methodical, not less: don't mute or unplug anything that might interrupt what is happening on stage.
- Be safe! Electricity kills. Never take the covers off live equipment (always disconnect from the mains before you take the cover off anything, especially in a non-workshop environment). Never replace fuses with any other conductor (wire, nails, screws): if fuses keep blowing, the equipment is faulty and potentially dangerous. Never bodge mains connections. Never attempt repairs that need both hands while you are hanging off a ladder.
- Mark faulty equipment. If you find a lead is faulty, don't just put it back in the bag: mark it in some obvious way (red tape is good for this), put it to one side, and don't take it out with you again until it has been repaired and tested. The same goes for everything else, from microphone to mixer to monitor.
Look for Common Explanations First
Use existing system facilities as diagnostic tools: PFL meters will tell you whether a signal is reaching the desk (most desks also have a headphone socket, so headphones are useful for this too). Most offboard equipment and most power amplifiers have at least a ‘Signal Present’ LED. Signals can get lost because:
- The signal lead isn't plugged in, or has pulled free, or is connected in the wrong place. Labelling both ends of every signal lead with a unique identifier - ours are all coloured and numbered - makes it easier to tell which lead is connected where. It is easy to make mistakes in the dark or in a hurry, and labelling is invaluable when it comes to finding the mistake. Again, if the input or output is pulled out of a processor on a channel insert, the signal path will be interrupted. Radio signals (radio mics or in-ear monitors) need the correct ‘virtual connection’: transmitter and receiver must both be set to the same frequency.
- Something in the signal path has no power. Inputs can disappear because of flat batteries, power leads pulling out of preamplifiers or foot pedals, or phantom power being off when it should be on. If you are using processors on inserts, check the processor (it needs power too). It should be fairly obvious if the desk or a power amplifier is switched off. Although fuses do occasionally fail for no apparent reason, a blown fuse should always be investigated further. Always replace fuses with the correct type and value. If you haven't a spare (why not?!), borrow one from less important equipment as a last resort. With equipment-specific fuses it is vital to use the correct type: equipment fitted with anti-surge fuses (marked with a ‘T’) will normally have a high initial switch-on current, and a quick-blow fuse (marked with an ‘F’) of the same value will blow at switch-on. If none of your equipment seems to be working, check the venue power supply (this isn't as inane as you might think: a lot of venues have power-control policies that might mean all the stage sockets are off!).
- Something is muted or not routed. If the channel, group, main busses or crossover outputs are muted, the signal stops there. If the signal isn't routed anywhere it will go nowhere. If a signal that was there a minute ago has disappeared, check you haven't done anything to the channel (like pressing the Mute button when you meant to press PFL, or assigning the channel to the wrong mute group). If the signal is routed through a group or assigned to a VCA, the group or VCA fader needs to be up! If you have lost it during the performance, use your headphones to check it is still reaching the desk: some musicians use tuners that mute the signal when the tuner is engaged, and you don't want to mess with the mixer connections or settings just because the guitar is muted on stage!
- Gain is too low. If the Pad is down on the channel, or the gain is at minimum, a weak signal may be effectively unnoticeable (you may also need to check the attenuation switches on the DI box if you are using one on that channel). If there is a noise gate on the channel insert, the signal may be lower than the gate's threshold. If the channel, group or main faders are down (or muted), or the power amps are turned down, you probably won't hear much from the speakers.
- A cable is faulty. 99% of actual system faults (as opposed to errors, as above) are caused by faulty cables. Cables may fail at the solder points (bad soldering and poor strain-relief are the most common causes of this: cheap cables and connectors are a false economy), and although this normally results in loss of continuity, sometimes a loose end can make contact with one of the other conductors, causing short-circuit conditions. In the worst case, this can cause failure of the preamplifier or amplifier stage driving it, or console damage where phantom power is involved. Mains plug and ‘Speakon’ terminal screws can become loose, and core wires in mains cables subject to frequent flexing can eventually break under the cable clamp.
- Equipment has failed. Fortunately, most modern touring equipment is rugged and reliable, and failure is rare. If it worked the last time you used it, it probably still works, unless:
- It has been dropped;
- It has been wet (from spills, immersion, rain, or condensation. Garages in midwinter are not ideal storage);
- It has been hot (car boots in midsummer are not ideal storage);
- Somebody's two-year-old nephew has been pushing paper clips into the ventilation slots.
If it is new, read the manual: although equipment straight out of the box is occasionally faulty, it is far more often supplied with an array of controls and an impenetrable menu that make it impossible to use without first wading through 100 pages of poorly-translated gobbledegook.
If you can establish beyond doubt that a unit has failed (for example, if the end of a speaker cable produces sound from one loudspeaker but not from another), your options are limited by what else you have available.
- Failed Microphone/DI Box. If you have no spares (why not?!), decide whether you can dispense with one: is one of the backline instruments that you would normally mic loud enough to get by? Is the mic on the snare underside essential? Or consolidate existing mics: use a single mic to cover two sources. For example, a pair of rack-mounted toms (or two backing singers, or two trumpets) could share one mic. If you only have one mic (for vocals with solo guitar, for example), it looks like an ‘unplugged’ session is in order. If your only radio mic has failed, use a wired mic: your singer's dance routine is probably less important than the whole gig.
- Failed Processor. If you have no spare processor channels, consolidate. Do without compression/EQ on the least vital source. If a main or monitor EQ has failed, decide which is the most vital to the performance. It is possible that one or more members of the band can do without monitors, or perhaps two or more can share a single monitor send. Some system controllers (e.g. the dbx Driverack range) include EQ and compression algorithms, and if these are not already in use they can be used for all Front-of-House processing. If the monitor feeds need a lot of EQ because of feedback considerations, consider backing everything off by a few decibels to make up a bit of headroom.
- Failed Effects Unit. Don't do the song with the timed delay on it (or do it without the delay). The venue probably has enough reverb of its own!
- Failed Mixer. Most PA systems with a separate monitor desk (a ‘spare’ in an emergency) travel with experienced crew, who will know what to do. Channel failure is of minor consequence if you have spare channels. If the whole mixer has failed and it is your only mixer, you are in trouble. Your best bet in that case - if you can get away with it - is to let the backline cope as well as it can, while trying to get the main vocals up to a usable level for the power amps. Most of the plastic-box speakers that have built-in power amps will accept mic inputs, so if you are using these you can plug the mic straight into one, and link to any others. A separate microphone preamplifier (e.g. ART Micromix, Behringer Shark or something similar), if you have one, will get you from mic to line-level. Otherwise, try running the vocals through a DI box (you'll need a battery-powered or passive DI if the desk has gone down) into anything that can accept low-level inputs and provide a useful amount of gain. Some compressors and other processors can achieve this. Daisy-chain processors, if you can't get enough gain from one on its own.
- Failed Crossover/Controller. Unless you have active monitors or multiple amp racks you probably don't have a spare. In a standard two-box (subs and tops) front-of-house system, if you have anything with a sweepable high-pass filter you can use this to roll the bass off the feed to the mid/high speakers. Some graphics and parametrics have sweepable shelving high and low filters that will do for this. If in doubt, set the filter to a slightly higher frequency than your normal crossover point. A lot of bass bins have limited high-frequency output by design, so you can probably get away without a filter on the bass feeds in an emergency (you might need to cut a bit of the low-mid on your equalisers to compensate). Also, some (e.g. our Martin Audio S15s) have inbuilt passive crossovers, which can be switched in to act as low-pass filters. If you don't have Z-leads or splitters, run everything in mono and use the left mixer channel for the bass feed and right channel for the mid/high feed. Otherwise, split the signal from the mixer's outputs, running one half of the split straight to the sub amp and the other half through the high pass to the mid/high amp. If your crossover/controller has limiters that are normally in use, remember to use more caution than usual with mixer output levels.
- Failed Power Amp. There are several ways to go. Dispense with a couple of monitors, and use the monitor amp for Front Of House (unless it is a monitor amp that has failed, in which case just dispense with the monitors). Or run everything in mono, using one half of a power amp to run the subs and the other half to run the mid/high speakers (you can also do this to restore your monitors if a monitor amp fails and - unaccountably - the musicians think what they hear is more important than what the audience hears). Watch speaker impedances when doubling up: some subs are 4Ω, so running two off one power amp channel is Not Recommended. If necessary, run a single sub, and back the mid/high off by 3-6 dB. If you are already running in mono and don't have enough amplifiers, try dispensing with the subs and restricting the PA to vocals.
- Failed Speaker. If you use the same speakers for monitors and Front of House, use one of the monitors to replace it if is a main speaker, or do without it if it is a monitor. If you use different speakers for each application, you might still be able to use a pair of monitors as Front Of House speakers as long as their HF pattern is more or less acceptable (with speakers at left and right of the stage, a matching pair is preferable).
- Failed Sub. Back off the mid/high by a few decibels and manage with one less sub. If your subs are normally placed together you may need to back off as much as 6dB from the mid/high speakers to compensate. If you normally place them either side of the stage, 3dB will probably be enough (but see the section on bass speaker position).
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