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

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

  1. simoncroft

    simoncroft Tele-Meister

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    Pickup Up The Meter

    OK, let’s assume that we have a mic connected to Channel 1. We have advanced the Channel 1 fader to the point of unity gain (ie zero) and set the output fader the same. As we turn up the gain control on the input of Channel 1, we can now use the output meters to measure the signal going through the console. As long as the signal is causing just the occasional peak into the red section of the meters – but no more – our input gain is set.

    (In real life, we have to deal with a number of complexities, including the fact that just one drummer can occupy multiple channels of the desk, all of which will need the input gains set individually. Then there is the fact that the same drummer will create colossal peak levels during the encore that we didn’t allow for in the sound check… These are issues we’ll deal with another day, after we’re done with evolving our mixing console.)

    But what if you've already got the channel fader set to a certain level as part of the mix, but you want to check the Gain without moving the fader back to Unit Gain? There's a button marked PFL (Pre Fade Listen) on each channel that will route the channel directly to the main outputs, for the purposes of metering and monitoring. (On some desks, this button is marked Solo. The difference between PFL and Solo need not trouble us at this stage.)
     
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  2. beagle

    beagle Friend of Leo's

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    Mics like the SM58 have a transformer that blocks DC from the coil. Unless you get a shorted cable there's no problem with phantom power.
     
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  3. simoncroft

    simoncroft Tele-Meister

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    For any of this to work, we need to plug in a mic, of course. Easy enough, but before I worked in pro audio, I had no idea why why we might need three-prong plug on the mic cable instead of a jack. I now know it's...

    A Question of Balance


    Our archetypal 60s mixer/amp had high impedance mic inputs on unbalanced 1/4in jack sockets. That worked well enough back in the day but we can’t use them on our ‘evolved’ console, because we’ll get more radio breakthrough than Nigel Tufnel’s guitar in Spinal Tap. Partly it’s because we’re using more channels, partly because we’re using much more amplification, but mainly it’s because the sources of electrical, magnetic and RF (Radio Frequency) interference have increased so much. So, those jack sockets have to go in favor of 3-pin XLRs and low impedance, balanced wiring.

    And while we’re doing the inputs, we might as well put the outputs on balanced XLRs as well. (This assumes we’re using separate power amplifiers. There are some very good modern mixer/amplifiers, especially now that the highly efficient Class D amp designs allow for high output on-board power, with very little weight or heat compared to older designs.)

    Engineering types get into considerable detail over what truly constitutes ‘balanced’ wiring. Good for them, I say. For the rest of us, who mostly want to know the benefits, I’ve drawn Diagram G. What this shows is that the signal to and from the mic capsule is kept entirely separate from the outer screen, which is there to complete the electrical shielding but does not carry the audio signal.

    Because the audio + and – wires (sometimes known as ‘phase’ and ‘anti-phase’) are configured in a ‘twisted pair’ along the length of the cable, they are inherently resistant to outside interference from lighting rigs, the local taxi companies and the rest. It is very similar in concept and benefits to the classic Gibson humbucker.

    Another benefit to the 3-pin wiring scheme is that it makes it possible to deliver ‘phantom power’ to studio-quality condenser mics. In larger venues particularly, there is a further benefit in the use of low impedance mics, because this allows for much longer cable runs without significant loss of high frequencies. (There comes a point where digital is the best solution but that subject is – ahem – somewhat down the line yet.)

    At this point, we all deserve to take a rest! We’ve upgraded our old 1960s-style mixer/amp to the point where we have the basis for a high-spec mixing console. But we’ve got a fair way to go yet. Functionally, we’ll want EQ on every channel, effects sends, monitor mixes and much more in terms of the console’s I/O (Input/Output) capabilities. See you later.



    Screen Shot 2019-08-04 at 18.04.36.png
     
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  4. beyer160

    beyer160 Friend of Leo's

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    As long as you're using balanced XLR cables (standard mic cable) and everything is working properly, phantom power won't damage dynamic mics- this also includes ribbon mics (which are actually just a different type of dynamic mic), internet lore to the contrary. Phantom power is voltage applied to pins 2 and 3, as long as it doesn't connect to ground it won't complete the circuit. The problem is when you have broken equipment or unbalanced cables that provide a path to ground from pin 1 or 2.

    As has been mentioned, most budget mixers have a single phantom switch for all inputs- if this were a problem, you'd have people frying dynamic mics left and right.


    This is kinda OT, but I have to mention ribbon mics- ribbons are almost exclusively used in recording because traditionally, their elements are very fragile and not sturdy enough for the rigors of live audio. There are new ribbons that are much sturdier, but they're relatively rare compared to the old styles that have been around since the '20s. Anyway, up until the '60s, phantom power wasn't a thing. Older ribbon mics had grounded center tapped transformers, which were a problem once phantom power became prevalent. Nearly all of these mics have been modified in the last 60 years, so it's basically a non-issue. If you have a ribbon mic that was made after the early '60s though, you won't have a problem regardless... assuming everything's in proper working order, anyway. How the urban legend persists nearly 60 years later that phantom power is bad for ribbon mics is baffling to me.

    That said though, phantom power does no good to any mic that doesn't need it to operate, so it won't hurt anything to be safe and disengage it whenever possible for any channels that don't need it.
     
    Last edited: Aug 4, 2019
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  5. simoncroft

    simoncroft Tele-Meister

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    You and @beagle have the kind of in-depth professional experience that makes you 'expert opinion', so I'm grateful for the input. When I worked in audio sales, I never had anyone return a dynamic mic claiming it had been damaged by phantom power, but in order to get a real concensus, I've sent out a few of these emails to manufacturers:

    Hi Andrew,

    Since I retired as a pro audio journalist a few years ago, I’ve devoted my time to providing musicians on-line with information that will help them to choose and get the best from their audio equipment. I’d be grateful if you would ask one of your technical people to answer the three questions below, as there is conflicting anecdotal evidence as to whether 48V Phantom Power can in fact damage a dynamic microphone. I have promised to publish my findings on-line, and will of course credit sources.

    Best regards,

    Simon Croft

    1. Have you had dynamic mics returned for repair that have been damaged by connection to a 48V Phantom Power source?

    2. As a manufacturer, what are you findings regarding the possibility of 48V PP causing damage to a dynamic microphone?

    3. If there is a possibility of damage, is there a design feature within the mic that can protect it?
    I've been careful not to include any personal opinion that might skew the result. Responses to follow. :)
     
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  6. Paul in Colorado

    Paul in Colorado Telefied Ad Free Member

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    It's funny. When I was a touring sound guy I mixed on some large rigs (Barns of Wolf Trap for example). Now days I mostly play in bands that use a Bose stick, using a digital stage box mixing on an iPad. For bigger gigs there's usually a house rig.
     
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  7. David Barnett

    David Barnett Doctor of Teleocity

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    There are some transformerless dynamic microphones. If they're designed well, they'll have blocking capacitors to prevent DC from getting to the coil. One example of this done properly is the Shure SM52 bass drum mic. There's plenty of room in there for a proper transformer, but if you open it up all you'll find is the capsule and a small circuit board with some caps on it.

    I once saw two EV RE20 microphones fail during a sound check, where phantom power was present. I cannot make the claim that phantom was responsible, and I don't know if the RE 20 has a transformer or not, but it seemed like an odd coincidence.
     
  8. unfamous

    unfamous Tele-Meister

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    Last edited: Aug 4, 2019
  9. strat a various

    strat a various Friend of Leo's

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    Quote Yamaha EMX2 Owner's Manual:
    Follow the important precautions below, in order to prevent noise and possible damage
    to external devices and the mixer when you operate this switch.
    Be sure to leave this switch off if you do not need phantom power, or when you connect
    a device that does not support phantom power to channels 1 and 2.


    Quote The Nady RSM-2 Ribbon Microphone Owner's Manual:
    "Make very sure that no phantom power ever goes to the RSM-2, even accidentally, as it may burn the ribbon."

    IMO, the biggest threat to a dynamic mic from phantom power is if someone connects or disconnects the mic while phantom power is "on". I can see this sending an unacceptable amount of power through the diaphragm. We try to bias our power tubes on our tube amps around here, but newsflash, most of those Hot Rod Deluxe and PRRI owners just slap in new tubes. I'm not hearing of a pandemic of failed amps, but it happens. I think better to be careful than to be cavalier about 48 VDC directed at whatever mic doesn't need it.
     
  10. David Barnett

    David Barnett Doctor of Teleocity

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    It's even worse if someone plugs or unplugs it while the channel is un-muted. It's not a pretty noise.
     
  11. strat a various

    strat a various Friend of Leo's

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    Some of those loud noises could theoretically damage a speaker or a H.F. horn, but again, someone will come along and say they've never seen that happen, so it can't happen.

    This whole Phantom Power may possibly damage some mic or other is similar to the maxim, "A speaker impedance mismatch could damage an output tranny via flyback voltage." We all do it with impunity with old Fender amps, but old Marshall's? It's just a cautionary statement, better safe than sorry.
     
  12. beyer160

    beyer160 Friend of Leo's

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    Both the mixer and microphone you're referencing are aimed at novice users (although the Nadys are surprisingly good, I have three). The warnings in the manual are simply boilerplate from the manufacturer to protect against consumer claims stemming from misuse by ignorant users. The fact remains- polarized voltage across a dynamic mic element cannot damage it without a path to ground. It's electronically impossible. What is possible is a path to ground being created by damaged cables/connectors or misuse (connecting a mic with an unbalanced mic cable wired pin 3 to ground), which is far more likely. Better then to just tell people not to do it than to have to deal with the aftermath of people trying to blame you for their mistakes.

    In my studio days, I regularly used a Beyer 160 (hey!) ribbon mic with a Daking preamp/eq module that had no phantom power switch- it was always on. Never had an issue. Again though, there's no reason to apply phantom to a dynamic mic if you don't need to, it's just that the Daking sounded amazingly good with that Beyer on guitar cabinets.
     
  13. simoncroft

    simoncroft Tele-Meister

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    I'm not going to re-enter the fray on this one, except to tell you I've contacted Shure, E-V and Sennheiser for comment, which I will post here.

    While I'm happy to share the 10+ chapters of material I've written on the basics of Sound Reinforcement/PA, I am a little concerned that if we keep getting into debates on one issue or another, this thread is going to get a little challenging to read as instructional material.

    That said, I'm grateful for all your input. :)
     
  14. strat a various

    strat a various Friend of Leo's

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    Sorry. Didn't mean to interrupt the lecture. Misunderstood some of the initial disclaimers:
    "This is a thread, not a seminar, so its shape and content is dictated by you, along with other forum members who chose to contribute".

    I already found out that some manufacturers know what they are talking about, while other manufacturers don't, right? So I'm good. Sorry to side-track the subject ... I've dropped it already.

    Pip, pip, cheers. Carry on, lads.
     
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  15. David Barnett

    David Barnett Doctor of Teleocity

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    And some are only interested in good PR.
     
  16. strat a various

    strat a various Friend of Leo's

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    Amen
     
  17. simoncroft

    simoncroft Tele-Meister

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    These were my questions:

    1. Have you had dynamic mics returned for repair that have been damaged by connection to a 48V Phantom Power source?

    2. As a manufacturer, what are you findings regarding the possibility of 48V PP causing damage to a dynamic microphone?

    3. If there is a possibility of damage, it there a design feature within the mic that can protect it?

    This is from Tom Colman who is a Senior Applications Engineer for Shure.

    Answers:

    1 & 2:
    No. All Shure (professional) dynamic mics reject or simply ignore phantom power and are not damaged by it.
    https://www.shure.com/en-US/support/find-an-answer/will-phantom-power-damage-my-sm58

    3. Yes, the way it's wired means it ignores the phantom power.
    https://www.shure.com/en-US/support/find-an-answer/phanton-power-why-is-it-called-phantom
    https://www.soundonsound.com/sound-advice/q-phantom-power-on-dynamics

    This is a good description from Wikipedia:

    Phantom powering consists of a phantom circuit where direct current is applied equally through the two signal lines of a balanced audio connector (in modern equipment, both pins 2 and 3 of an XLR connector). The supply voltage is referenced to the ground pin of the connector (pin 1 of an XLR), which normally is connected to the cable shield or a ground wire in the cable or both. When phantom powering was introduced, one of its advantages was that the same type of balanced, shielded microphone cable that studios were already using for dynamic microphones could be used for condenser microphones. This is in contrast to microphones with vacuum-tube circuitry, most of which require special, multi-conductor cables.

    With phantom power, the supply voltage is effectively invisible to balanced microphones that do not use it, which includes most dynamic microphones. A balanced signal consists only of the differences in voltage between two signal lines; phantom powering places the same DC voltage on both signal lines of a balanced connection. This is in marked contrast to another, slightly earlier method of powering known as "parallel powering" or "T-powering" (from the German term Tonaderspeisung), in which DC was overlaid directly onto the signal in differential mode. Connecting a conventional microphone to an input that had parallel powering enabled could very well damage the microphone.
    https://en.wikipedia.org/wiki/Phantom_power
     
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  18. simoncroft

    simoncroft Tele-Meister

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  19. simoncroft

    simoncroft Tele-Meister

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    Equalization – Not Just Tone Controls

    The typical mixer/amp made from the 1960s onwards that we looked at in the start of this series often came with a couple of tone controls on each channel: ‘treble’ and ‘bass’. It was good enough in its day, partly because expectations regarding live sound were lower, but also because you can successfully mix a bunch of vocal mics with little modification to their basic sound. Once you start adding instruments – and drums in particular – you really need more powerful tools in the box.

    Sadly, the Equalization section (‘equalisation’ with an ‘S’ for us Brits, and often just ‘EQ’) probably has the greatest power of any section on a mixing console to ruin the sound of a live act! That might seem like a dramatic thing to say but too many people look at the EQ, instantly grasp the basics of what it does, but never truly learn how to use it.

    “Hmmn,” we tend to think. “That’s like the tone controls on my guitar amp…” The problem with that line of thinking is that we guitarists are used to tone controls that generally sound better when we turn them up. For reasons I’ll try to explain, the EQ on a mixing desk often sounds better when we use it to turn down some elements of the sound.


    In order to explain why, it’s useful to reintroduce a concept that could formed part of our earlier look at ‘unity gain’. The concept is ‘headroom’. (Technically, ‘headroom’ is the difference between the nominal operating level and the onset of distortion.) To put it another way, the more headroom we have in our audio circuits, the more we can push our luck with the gain before it all starts to sound bad. Sadly, only very expensive consoles offer vast amounts of headroom, so those of us driving the equivalent of a mid-range Ford need to be extra careful about how hard we rev the engine!

    OK, enough with the rubbish analogies, the simple truth is that the more you boost the EQ on any channel, the less headroom that channel has. And if you insist on doing that on say, an entire drum kit, there will be even less headroom left in the mixing desk when you combine those channels. (This is a theme I’ll return to when we get onto operating a mixing console.)


    In order to compete in the ‘specmanship’ stakes, console manufacturers usually include channel EQ with gain of +/- 15dB on each band (sometimes +/-20dB). Not only is that more than enough to use up the headroom in a mixer circuit, it’s more than you probably need to apply in a live sound situation. Seriously, if an incoming signal does not sound at all musical, it is better to go back to source and fix whatever is wrong with the instrument, mic or mic position than it is to try EQing your way out of the problem.
     
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  20. simoncroft

    simoncroft Tele-Meister

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