Say It Loud!
Power Amps and Loudspeakers
The songs are good, the band is great, but no-one will be unpressed if they can't hear you properly. David Mellor advises on PA hardware.
When you play live you want to get your message, whatever that message might be, across to your audience. This probably means that you want to achieve an adequate volume and a reasonable degree of clarity and intelligibility. An even response across most of the audio frequency band would be an asset too. With modern equipment, this shouldn't really be too much of a problem.
One other consideration, which needs a little more skill and careful planning to achieve, is to cover the entire audience evenly so that everyone can enjoy your wondrous noise; or perhaps you might be content to allow people to choose a high level zone close to the stage or a comfort zone close to the bar! Assuming your music has found its way safely to the main outputs of the mixing console, then it's the job of the amplifiers and speakers to make sure that none of the music's power or subtlety is lost.
Of course, I can't cover the vast subject of public address in one small column, so I'll have to make just a few small assumptions. The types of acts that use PA systems range from solo performers who sing to their own guitar or keyboard accompaniment (or to a pre-recorded or MIDI sequenced backing), through duos to full-blown rock bands. I have a feeling that concentrating on the smaller scale operations will be more appropriate to most Sound On Sound readers, although some of the gear I mention will be suitable for small venue heavy metal thrashers (most probably accountants and bankers by day).
Power amps are fairly easy to deal with, and among the makes that you are likely to encounter are HH, Amcron and Yamaha. Pictured above is a fairly recent power amp, which at last embodies a change from lumpish black boxes to sexy items of hi-tech equipment. The Malcolm Hill Associates Chameleon is one of the biggest and butchest power amps you are likely to meet, and the stereo version turns in a most impressive 1200 watts per channel of honest sinewave power. Used with a typical speech or music source it claims a transient power rating of 2200 watts per channel! All this in 1.75 inches of rack space too (although it is almost 20 inches deep!).
The question of how much power you need in an amp is a tricky one. For PA purposes the bare minimum is 100 watts per channel (into the speaker's rated impedance — check this on the equipments' specifications). This would be about enough to amplify a voice and a guitar for a small venue such as a pub or club. When you start to think about pushing a whole band though the PA then you're usually looking at amps of 300 or 500 watts per channel. Each amplifier will drive, usually, one or two speakers on each channel. Note that two 8 ohm speakers connected in parallel will have a combined impedance of 4 ohms. Two 4 ohm speakers in parallel will give you 2 ohms.
Remember to check the amplifier's specs on minimum impedance and its power output at different impedances. It's OK by the way, preferable in fact, to have an amplifier which is rated at a higher power than the speaker's maximum — just remember that you probably have the potential to blow the cones across the room. Your audience might be impressed by the spectacle but you won't be making much music afterwards.
The smallest PA speakers can be driven from just one amplifier each. Examples would be the Celestion SR series or the Electrovoice S200. These are good for voice and guitar, or for a vocal PA when the other instruments have adequate amplification of their own. The ultra compact SR series uses what is basically a single mid-range drive unit, but with electronic bass enhancement and a specially developed dome to project high frequencies reasonably faithfully. The advantage of having a single driver is that no crossover is necessary (passive crossovers, in high power applications, are a bit of a problem area for a variety of reasons). The S200 has separate bass and high frequency units and is generally a little beefier all round. It can also use electronic frequency response enhancement with an add-on box.
Both the Celestion SR series and the Electrovoice S200 can benefit greatly from the addition of a subwoofer which handles bass frequencies only. These of course, are not the only units of this type by a long way.
Moving a distinct step upwards we come to units like the Turbosound TMS4. This speaker, which has three drive units and measures 48" x 20" x 29", needs to be bi-amplified. This means that not only do you need two amplifier channels for each speaker, you need an active crossover as well. The crossover splits the high frequencies from the lows just after the signal, at line level, leaves the mixer. It's the best way of doing it, but a good active crossover, as made by BSS Audio, costs money. The TMS4 handles 450 watts RMS and gives a good full sound. Big PA rigs are often constructed from a number of TMS4 speakers, and others from the Turbosound range.
"Good speakers, used within their capabilities, will make you sound good on stage and hopefully propel you one stage further in your musical career."
Another way of constructing a loudspeaker system is to go for separate bass bins, mid range and high frequency units. Of course you'll need an amplifier for each unit and an active crossover. An interesting way of implementing this technique is via the Martin F2, which comes as an empty cabinet about the same size as a TMS4 which you can fill with bass, mid, high or ultra high frequency modules as necessary. A short/medium throw stack might consist of an F2 cabinet filled with one F2M mid range horn, one F2H high horn and one F2T supertweeter array, plus an F2B bass bin, which is the same size as the F2. A long throw stack, for getting right to the back of the venue, would have two F2Bs, one F2 fitted with three F2Ms and one F2 fitted with three F2Hs and two F2Ts. A BSX sub bass unit would be optional.
Whatever the scale of your application, remember that there is an incredible variety of equipment on offer. Have a really good look round and go for something which does the job properly. Good speakers, used within their capabilities, will make you sound good on stage and hopefully propel you one stage further in your musical career.
The SOS Guide To Going Live
Feature by David Mellor
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