Sound Reinforcement (PA) - A Guide to Making Live Sound Work For You

Discussion in 'Bad Dog Cafe' started by simoncroft, Aug 2, 2019.

  1. Henry Mars

    Henry Mars Tele-Afflicted

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    It is really easy to over think all of this. Back when I started playing professionally in 1963(?) all we had was Shure Vocal Master ... it worked great and it still does. If you are playing a large auditorium or a stadium the requirements are bit different. For the average bar you really only need 300 or 4ooW and a powered mixer.
    I go to some of the local watering holes and these guys are carrying around enough PA gear for a stadium.
    Like we gotta mic the drums in a 1200sqft room and have a 5000W PA with a 32 Channel Mixing Board two 16 channel snakes and 32 mics, sub woofer, monitors and a sound man and an 18ft truck for a 5 piece bar band making 5 bills a night. Insane.
    With all that is going on on your average gig I don't really think the PA is a big factor.
     
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  2. Chud

    Chud Poster Extraordinaire

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    Yep, that’s who it usually was. Occasionally a bassist who wanted his sound more brown, or rounder. Lol


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  3. strat a various

    strat a various Friend of Leo's

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    Not only that, when we were doing gigs with a 100 watt PA head and two old fashioned columns, we were playing considerably louder than most of the bar gigs you see today, with their sub-woofers, 2000 watts FOH, and three or four 700 watt powered speakers for monitors ... all tuned down low enough for the waitress to hear the drink orders in front of the stage. Except for the bass drum, that has to be loud for some reason.
     
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  4. simoncroft

    simoncroft Tele-Meister

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    While I agree that a lot of bands playing small venues today are probably carrying more gear than is needed, I can also remember going to big-name gigs back in the day where the sound was so bad, it was hardly worth being there. Same with smaller rock gigs, where the back-line was just too loud, the vocal PA distorted, and the singers' pitch pretty dubious because they couldn't hear themselves.

    The Evolution of the Mixing Console (part 1)

    This post is primarily intended to explain why mixing consoles (aka ‘mixers’, ‘desks’, or ‘boards’) took on what can seem like a bewildering array of additional features compared to the simple volume and tone controls found on a 1960s mixer/amp. In a later post, I’ll be running through the mixing console from a more operational perspective – ie how to use it in a concert situation. For now, I’m simply trying to give an overview of the functions a desk performs, so that you can look at one and feel confident that you understand it.

    Before we get stuck in, there is a point you might want to consider. For the most part, the input channel strips on even the biggest mixing console are identical, so if you understand how Channel 1 works, you’ll also understand Channel 128, and any channel in between. Well, that’s cut our learning curve down to size!

    Let’s go back to that mixer/amp I drew in Diagram A (earlier post) and see what simple improvements we can make, so that it’s more suited to the modern world. The first thing we can see (with a liberal dose of 20/20 hindsight) is that as the mixing system gets bigger, it becomes less suited to vertical operation. (Apart from anything else, as the channel strips get longer, there would come a point where we couldn’t see the band we’re supposed to be mixing!)

    All mixer input channels start with a preamp. That's because the output from a microphone is relatively tiny, so we need to boost it to a level the rest of the mixer circuitry can use.

    The output of a mic varies wildly, depending its design and what sound source is in front of it. If we stick the same mic in front of a female country singer and a snare drum, we can expect very different levels at the desk. For that reason, each preamp has a Gain control.(Diagram D).

    When whoever is operating the mixing desk sets the Input Gains, they do so with two related objectives: to set the level high enough to minimize unnecessary circuit noise/loss of resolution, and to set the level low enough to avoid overloading the circuitry, aka over-modulation, as this will result in distortion.

    Achieving this means we have the 'optimum operating level' going through the input channels. the difference between the optimum operating level and the maximum level before overload is called 'headroom'.


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  5. Dacious

    Dacious Poster Extraordinaire

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    images (59).jpeg
     
  6. archetype

    archetype Fiend of Leo's

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    Like the "producer switch" on Lee Sklar's basses. Same situation. The producer said too bright, too thick, too fat, too whatever. Lee would flip the toggle switch which was wired to absolutely nothing and say "How does it sound now?"
     
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  7. beyer160

    beyer160 Friend of Leo's

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    One of my old bands used to rehearse in a warehouse that'd been converted to 15' x 15' rehearsal rooms. There was a band of idiots down the hall that used to mic all the instruments (including drums!) and run through their PA in one of these rooms. They were ungodly loud in our room down the hall, to the point that we had a hard time hearing ourselves over them (I was playing bass through a 400 watt head into a 2x15, the guitarist had a 100 watt JCM800 halfstack), I imagine all those chuckleheads are deaf today.

    Part of the problem is that this equipment didn't used to exist- when I started playing in the '80s, all anybody had were small Peavey systems for vocals, the only guys who owned anything more complex were working bands who played 5 nights a week. Now though you can go into Banjo Mart and walk out with a stupidly powerful rig that 90% of bands don't really need. And once you have it, everyone thinks you need to use it. I played plenty of gigs as a young'un where we cobbled two bands worth of practice PAs together and had a couple Peavey speakers on stands and a couple more as monitors, all run off small mixer heads and we sounded... well, as good as we were going to. Blowing out the room by being louder wasn't gonna make us sound better, though.
     
  8. simoncroft

    simoncroft Tele-Meister

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    I supported a 'big name' band when I played bass in the Monochrome Set back in 1978. The venue was a top London club. The main act told us we could have three channels on the board only, so we could only mic the vocals. (Quite how you take up 21 channels on a three-piece band...) We went on, and the audience were quite appreciative. While we were outside getting some air, the main band took to the stage. They were mind-bendingly loud! Within a few minutes about half the audience were outside with us, because they couldn't stand the volume. Poetic justice. :twisted:
     
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  9. strat a various

    strat a various Friend of Leo's

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    In the 70s. a Rock/R&B/Blues band I worked with opened for a famous guitar hero who had been in a well known 60s band, much loved around here, so no names. He was not nice to us. Ouch.
    So at the sound check we saw his song list which was all old Blues tunes and two of his hits. We played his exact song list for our opening set. He did not appreciate our homage to his wonderfulness.
     
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  10. simoncroft

    simoncroft Tele-Meister

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    Going back to Diagram D above (why was I here again?), it might help if I briefly run through what the various switches do. The one marked 48V supplies phantom power to condenser mics. It has an accompanying indicator light.

    Although some condensers will run on less voltage than 48, none of them will work with no power at all. If you come across an exception to this rule, it's because there's a battery inside. Knowing this, and checking the battery, can save you from unexpected mic failure.

    The symbol that looks a bit like a diagram of Saturn actually represents 'phase reverse'. Sometimes, in a multi-mic set-up such as on drums, different mics are picking up the same things from different distances. Just like the pickups on a Strat, this can cause a hollow out-of-phase sound, meaning the sound is actually weaker with both mics on, compared to just one. If that seems to be the case, it's always worth trying the phase reverse.


    NB – You may wonder if it is possible for the entire rig to be out-of-phase with the sound on stage. The short answer is yes! Let's just imagine someone has mis-wired the output of the mixing desk, so the + and – are reversed. Every time the bass player hits a note and the speaker in his/her amp jumps forward, the ones on the main rig will jump backwards. It's one of those things worth checking any time there has been a significant change in the system configuration.

    The 100Hz button has a symbol above it showing that this is a bass roll-off. This means that it will progressively diminish the frequencies below 100Hz. Take a vocal mic for instance. There is nothing in the singer's range that goes down to 100Hz, but if the mic is on a stand, and the stage is hollow, the mic is probably picking up rumble from the drums, bass guitar etc. The bass roll-off will help to reduce that rumble, helping to clean up the mix.

    That just leaves us with a little note below the gain that says 'LINE = -20'. Some mixing desks have a Mic/Line switch on each channel to select the appropriate input socket (Line level being the output of a keyboard instrument, another mixing desk and the like). I believe this particular desk automatically switches to Line when something is plugged into the Line, and the level reduces to -20dB to prevent the preamp from overloading.
     
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  11. strat a various

    strat a various Friend of Leo's

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    The Phantom Power generated into the mic line will damage a Dynamic mic. We should all make sure it's only on if we are powering a Condenser mic ... and what do if the band wants to mix dynamic and condenser mics? Use Condenser mics that take a battery.
     
  12. simoncroft

    simoncroft Tele-Meister

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    Let’s go back to that mixer/amp I drew in Diagram A (earlier post) and see what simple improvements we can make, so that it’s more suited to the modern world. The first thing we can see (with a liberal dose of 20/20 hindsight) is that as the mixing system gets bigger, it becomes less suited to vertical operation. (Apart from anything else, as the channel strips get longer, there would come a point where we couldn’t see the band we’re supposed to be mixing!)

    Once we’ve placed our controls at a near-horizontal angle, we could consider replacing the rotary volume controls with linear faders. (Diagram E.) That way, not only would we be able to see at a glance where they are set, we’ll be able to judge their position by touch alone – very useful when you’re also trying to watch the stage in a darkened venue.

    Our 60s mixer amp had a volume control on each channel, but now we’re entering the hi-tech world of modern sound mixing, we need to up our game a bit. It’s a good point at which to review the difference between ‘gain’ and ‘volume’.

    Fortunately, we guitarists are already used to thinking of ‘gain’ as being how much drive we put into the preamp stage and ‘volume’ as being how loud the sound is coming out of the speakers. That’s exactly how it is on a mixing console, but with one major difference. The last thing in the world we want is distortion! No, no, our aim here is to get the maximum level before distortion sets in.

    With a guitar amp, we’re normally only using one guitar at a time. With a mixing console, we have multiple inputs. One channel could have a really quiet acoustic guitar on it, and another a very loud snare drum. We need an input gain control on every channel of the mixer, so that we can set each input to its optimum operating level.

    The difference between the operating level and the onset of distortion is called 'headroom'. That headroom is your safety margin, so it's as well to spend it wisely.

    As we'll see as we go along, the are several ways in which we can use up the headroom without meaning to.


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  13. beagle

    beagle Friend of Leo's

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    No Pad (26dB) button?
     
  14. simoncroft

    simoncroft Tele-Meister

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    I've often heard that repeated but I have never, in decades of involvement in the pro audio industry, encountered someone who has said it's happened to them. As most consoles over a certain size have individual phantom power switches on each channel, all you have to do is make sure the power is only on for the channels that have a condenser mic connected.
     
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  15. simoncroft

    simoncroft Tele-Meister

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    Not on this particular desk, as it has such a wide-ranging Gain control, but thank you for nudging me to mention what the Pad does. Some signals (mic in kick drum, for instance) run so hot that even with the Gain all the way down, the signal level is too high for the input channel's circuit. The Pad provides a pre-set gain cut, usually in the order of -26dB, to prevent overload.
     
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  16. beagle

    beagle Friend of Leo's

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    Sorry I was out of sync, I think the TDPRI carrier pigeon is in serious trouble today. I used phantom power for condenser mics and DI boxes on the same mixer as dynamics without any problems. With smaller mixers it was always the kind of the normal state of affairs. I'm not right up to date with the more modern stuff of the last 10 years or so though.
     
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  17. simoncroft

    simoncroft Tele-Meister

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    Screen Shot 2019-08-04 at 16.48.07.png

    Although we’ll cover this in more detail when we look at how to actually operate a mixing console, it seems appropriate at this point to explain how the meters on a console are used as an aid when setting input gain. If you take another look at Diagram E, you’ll notice that there is a graph along the side of the fader, on which one of the values is ‘0’. That zero point is known as ‘unity gain’.

    If the fader is higher than this point, the signal leaving the fader is higher in level than it was when it entered it (it’s boosted). Intuitively enough, when the fader is below the zero point, it leaves the fader at a lower level. (You could say it has been cut, or ‘attenuated’.) Unity gain is an important concept when operating a mixing console, as the following example will show.

    But before that, we’ll need to add some meters to our mixer. Diagram F shows the ones on the output section of the Soundcraft GB2 I'm using for my example. In real life, they’re color-coded illuminated bar-graphs that show the levels above unity gain in orange and the onset of overload in red. We’ll also need an output fader at this point. (No diagram, because it will look pretty similar to the input faders.)

    NB – Some of you may be more familiar with the older-style meters with a moving needle. Although the function is much the same, the electronic version are nearly always set to acts as PPMs. Peak Program Meters react a lot faster than the older VUs, so glive a clearer indication that 'clipping' is about to occur. 'Clipping' is a momentary erosion of the headroom, that may indicate a reduction in channel gain is advisable.
     
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  18. simoncroft

    simoncroft Tele-Meister

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    Before I get pulled up on this, yes you can have old-style mechanical meters that are PPMs, but they are face-meltingly expensive, so you tend to find them only in radio and TV applications. :)
     
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  19. strat a various

    strat a various Friend of Leo's

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    Many mixers that the typical member here would use on a small/medium gig has one phantom power on or off for the whole board ... Mackie, Yamaha, etc. There is info on the net regarding this issue, it's not hearsay. I suppose it is actually like a proper ground vs a two prong AC cable ... I've heard of two fatalities ever because of that, and I am not even certain they would have been saved by a ground prong. Since it's mic threatening, not life threatening, then I say ... go for it. Some dynamic mics are pricey. If any singer or sound man is certain they won't mind being the first verified case you personally hear of, OK. Or, better, if you have a system with a global phantom power on/off, and use a mix of mic types, do some research on the phenomenon ... start with the manuals that come with your mixer, they nearly all warn of this issue.
     
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  20. simoncroft

    simoncroft Tele-Meister

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    I think your points are fair, and just because I've never personally come across anyone who has been struck by lightning, it doesn't mean it's never happened. Same with 48V damage. My thinking is that I should ask a few microphone manufacturers what their finding are. I'm sure they have run practical tests on this, on the basis that if they get products returned, they'll know whether or not to honor the warranty.

    I've written user manuals for mixing desks and included the same cautionary note about 48V as all the others. Even though I didn't really believe in the phenomenon, I was protecting the manufacturer from any liability!

    When I had some manufacturer response, I'll post it here.
     
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