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PA — An Introductory Appraisal


Jazz-Funk at Glastonbury Fayre 1981. Venue: Worthy Farm, Pilton, Glastonbury. Date: 11.40 a.m., 21/6/81. Sound: Turbosound Ltd 40kW O/F 10k/M Lights: Power Leasing 20kW


Although instrument amplifiers and drumkits are inherently loud, the energy of music is readily depleted in cavernous halls and outdoors. However much the guitarist, bassist and keyboard player augment their amplification, a good, if sweaty drummer can only (phew!) consistently generate sound pressure levels (SPLs) of around 130dB(A) at 1 metre. This figure implies around 10 to 20 watts of acoustic power, which is in turn equivalent to 100 to 1000 watts of amplified drumkit, using speakers of 5% to 10% efficiency. Bearing in mind that we have to raise our power at least threefold (and preferably tenfold) to produce an audible intensification of SPL, it's clear that auxiliary power amplification (or PA) for drumkits alone demands a bare minimum of 300 watts of amplifier power; with less power, the drums per se will be as loud — or louder — than the PA!

As soon as we add other instruments, our minimum power requirements inflate, though these will be modified according to the efficiency and dispersion properties of the speakers. Unfortunately, it's considered chic for all up-and-coming bands to shove everything through the PA. Yet with a little hindsight it's clear that in the majority of cases where the PA is used as a tool in small venues, only the kick drum and vocals need share the sound system's facilities. And the fewer the instruments we mix together and attempt to reproduce as a unity, the less the intermodulation distortion: The spurious and obnoxious interaction of 'chord' formations between notes emanating from different instruments. Following this line of thought, a novel PA wouldn't mix the instruments, rather it would comprise a number of discrete sound systems with the speakers being tailored to each instrument.

Returning to convention, to mic up a three piece band with two vocalists 1 to 2kW is a ballpark minimum; with less power, the PA is not only contributing little to the overall SPL, but is also likely to muddy the sound (intermodulation again...) and alienate the audience. Bad PA's then can have ironic and destructive results. At the other extreme, it's apparent that good music, cleanly amplified has unexplored physical limitations. In other words, the pursuit of 140dB peak SPL's and corresponding PA meggawattage isn't inherently senseless, dangerous or masochistic. Indeed, possibly the opposite. The experience of cleanly reproduced music at levels above 130dB (theoretically beyond the pain threshold), particularly when presented in harmonious environments along the lines of the Glastonbury Fayre is commonly considered to lie beyond words, but the many who have deemed it "beautiful" will echo the sentiment "Good music reproduced cleanly can never be loud enough".

But PA is not merely an instrument for ensuring that a large audience can experience majestic emotional intensity or voluptuous Dubb and gain access to the inner space generated by loud electric music: It's also a tool in its own right. In common with all creative people, be they inventors or artists, musicians are relieved to delegate the crude mechanics — the infrastructure — of performing music on stage to others.

Having a mixing desk out front and stage monitoring enables musicians to exclusively direct their energies into musical expression, virtuosity and the regulation of higher energies within the music through the medium of themselves and the instrument in their hands; and this aside from the profanities of stage dramatics and stage showmanship, which are also given rein once musicians have relinquished the need to worry about their sound balance as perceived by the audience.

Meanwhile, at the out front mixing desk, the sound engineer's role is as a translator and balancer, striving to ensure that the music is displayed as a whole in a manner which suits musicians and audience alike. Backstage, the monitor man's task is to ensure that each musician can hear exactly what he needs to hear to play with attunement — often an impossible aim in the context of a loud and improvised set.

Of course, sound systems, like most things are imperfect, yet within their shortcomings lies the opportunity to use the PA as an instrument in its own right. This perverse potential for creativity is something to be explored in forthcoming articles. At the sametime, we'll aim to seek out the means of achieving essentially accurate reproduction (viz. clean sound) if only because experience suggests that Rock music has the capacity to become greater than the sum of its constituent sounds under these conditions. And inclosing, remember that when — as a musician — you invest in a PA, you depend on this instrument to communicate with your audience. So it really deserves just as much attention as your Strat's pickup wiring or the brand of drumsticks you choose.

Y'realtime country orientated scenario


Angus Mac'leod writes from the Isle of Skye as follows:
"I'm a drummer in a country orientated band, seeking advice on how best to set up our gear on stage. The guitarist's set-up consists of an Aria Diamond semi-acoustic amplified by a Sound City 120watt valve amplifier, feeding a 1 x 15" Fane speaker, whilst the bassist uses a Fender copy being amplified by a WEM Dominator 50 watt valve amp driving a 1 x 15" Fane speaker."

On the basis of the information supplied, you need have no worries about the bassist's gear, and the Sound City amp, assuming it's well maintained would be hard to improve upon without expending large sums of money. However, the 15" Fane (indeed, almost any 15" driver) isn't capable of reproducing the upper harmonics of the guitar, and your guitarist should audition some 12" speakers, beginning with the 2 x 12" in your PA; well worn 4 x 12" cabs a la Marshall can often be picked up cheaply, but whatever you choose, be sure to compare it to your 15" Fane to gain a feeling for the direction in which 12" driver(s) will enhance the tonality of the instrument.

"Can you take more than one slave off one slave input and can a slave output be taken off a speaker?"

The multiple slaving (A.K.A. "daisy chaining") of ten or more amplifiers is quite in order, and I hope we can look at subtle aspects of this topic in future editions of E&MM. A more immediate problem is presumably the lack of a 'link out' socket, and if you're not prepared to remedy this by drilling a hole to accommodate a socket, which is simply wired in parallel with the input, then a 'Y' lead is the simplest alternative. This is essentially a Jack to Jack lead with a paralleled line jack socket hanging off one end. If several amplifiers suffer from 'lack of input socket', then a more robust and possibly cheaper cure is to mount a number of chassis jack sockets in a diecast box. All are wired in parallel, one being connected to the source amplifier, and the others leading to your slave amps.

Deriving line level signals from across your speakers is also in order but do it only if it results in a useful or desirable sound, viz, to amplify the effect of a good valve instrument amplifier. Otherwise, you can expect unnecessary and unpleasant distortion, hum and hiss. The connection is always in parallel and an attenuator (Figure 1) is essential to avoid severe and involuntary overdriving of the succeeding amplifier's input stage.

Figure 1. Attenuator for slaving off speaker outputs.


"Would a graphic equaliser make a worthwhile difference if used with either guitar or accordion? Would a preamp make any difference to the guitars?"

A graphic equaliser would certainly make a difference to your guitarist's sound, but it's most unlikely to be worth pursuing until you can afford expensive toys without feeling guilty about the status of your PA system. The preamp is also strictly unnecessary, if more worthy, unless your music suffers significantly from boring hum and noise.

"Our PA consists of an HH MPA 100 feeding two 1 x 12" Mackenzie and a Selmer 'Treble 'n' Bass' 50 watt valve amp running into two 1 x 12" Fane drivers. We have three Shure Unisphere B Mies, an Acoustic guitar with transducer pickup and an accordion with strip mic fitted. What is the best way to get all the instruments and mics into a monitor system as I can't hear the band very well with all the speakers in front of me, and what kind of power monitor system would you advise? Is there any easy way of avoiding feedback from the mics, and what kind of PA to guitar power ratio is necessary for clean vocals?"

Tackling your howlround problems is best begun by scrapping your existing microphones, and looking for vocal microphones with a more uniform frequency response. In order of ascending cost, audition a Calrec CM654D, an Electrovoice PL80 and a Shure SM58. The cost of this improvement in itself will probably approach your budget, but look out for secondhand versions of the above mics, and bear in mind that the upgrading should make a significant difference to the sound levels you achieve on stage, not to mention vocals clarity and quality.

The next task is to relinquish the Selmer amp, because whilst it's a commendable instrument amplifier, it's not likely to make a very good slave amplifier. You could either aim for a cheap 100 watt slave (e.g. Carlsbro M130, Custom Sound 702) or go a step further with an elementary 6 channel mixer (e.g. Canary 6/2, Custom sound 701, MM MP180). In the latter case, you'd attain useful equalisation facilities, which with skillful application would ameliorate the problems of vocals, v. guitars you imply. The problem here is not so much one of power, but of technique — fitting the guitar into gaps in the time-amplitude-frequency dimensions of the vocals, and vice versa. Compare Ronnie Montrose's classic 'Space Station No. 5' with the maladroit New Wave HM to gain a feeling for the way intelligible vocals can exist amongst the wildest lead guitar. EQ would also give you some control over the feedback threshold in acoustically nasty venues; but best of all, it would enable you to derive a monitor signal, and along with the now abandoned Selmer amplifier and the 1 x 15" driver (ex-your guitarist) you'd have a rudimentary monitoring system to help your drumming.



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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - May 1982

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