Freak Accident:

[fdew]

Re: Freak Accident:

Hi I am a new guy here and I am not an electrician. I signed up because I am on a sound guy Forum (Churchsoundcheck.com) It is a Forum for church sound guys and as you can imagine, we have been very interested in the tragic accident of the pastor who was electrocuted in a baptismal.

I am writing to provide a little information on the sound system side of things.

First Phantom power. Phantom power was correctly pointed out as power provided to condenser mics. It is from 11 to 48 volt DC It is current limited (suppled from the power supply often through a 6810 ohm resistor. It is suppled equally to BOTH sides of the audio pair.
http://www.tangible-technology.com/power/Phantom_Power_connect.html
It must be suppled through high value resistors because otherwise it will short out the audio. A conventional microphone (Non condenser) has a coil across the pair of audio lines running in a shielded cable. Because it is across this pair with + 48 VDC on both wires it doesn't see it' and therefor is not damaged. If the mic is condenser then it gets it's DC power from the difference between the shield and the two audio wires.

A great many mics used in churches are Condenser and need phantom power. In the church where I run sound all the mics need phantom power (about 14 mics ) The only exceptions are the two wireless mics we use.
A good Condenser mic can cost as little as $35

Most (but not all) equipment that plugs into a microphone jack is not harmed by phantom power.

The less expensive boards will have only one switch to supply phantom power to all mics.

Grounds. On the Churchsoundcheck Forum we are nuts about safety grounds. You don't get to mess with them, You will get yelled at big time if you do. Safety grounds do cause us problems because they can form a ground loop (current flow in the shield of audio cables. We solve this problem by installing isolation transformers. When the transformer provides impedance matching and isolation it is called a Direct Box (often used between a guitar pickup, and a microphone line. These boxes allow us to couple the audio but not the ground. We will also some times use a cable with the shield disconnected from the plug at one end. (Remember, this shield is not a safety ground but a audio signal ground.)

In some cases it is necessary to wire or rewire a facility with a isolated star safety ground system. (The safety ground from a outlet doesn't connect to the separate ground on the box until they get back to the panel.) For this, of course, we need to use the services of an electrician. It can be interesting to find one who understands why we are so nuts not just about grounding, but about how the outlets are grounded.

I hope this info helps you thick wire guys understand some of us thin wire guys. I would like to be able to come back and ask you some questions when we get into safety issues on our forum

BTW We use a wireless mic mounted on a stand for our baptismal. Before this accident we had the pump and heater on a big knife type disconnect, with directions to turn them off before people entered the baptismal. The baptismal is grounded (The church was built in the early 60s) now we have that plus GFI
Frank DeWitt

 

[jklein]

Re: Freak Accident:

Hi all,
First post here, got referred by some of the good folks over at ChurchSoundCheck.com, where this was recently a hot topic of discussion (we're the "sound guys" if you haven't figured that out yet).

Our main goal is educating the DIY'er church sound/video/lighting tech who usually just happens to be the poor slob, or slobette, who wandered too close to the sound booth one fine Sunday morn. A lot of what you're discussing here runs in the same vein as what we try to instill in our members...know your limits, get professional help when required (regardless of cost), and keep the mythology out of the discussion.

Along those lines I wanted to clear up a few misconceptions about sound systems floating around this discussion, as the electricians in our group do for us with electrical topics (don't worry, it's always prefaced/finished with "get a professional in there!").

Condenser mics are in fact in wide use in the church today, since their prices have come way down in recent years. They typically run on 48vdc nominal, although many can operate in the range of 9-52vdc. One of you was correct in theorizing that it affects signal level that can be handled; lower phantom power = earlier onset of distortion. Also, applying phantom power to a non-powered (dynamic) mic will do absolutely nothing to it (that's where the "phantom" term comes in, it won't "see" it). If it does, something's wrong with the mic or the phantom power supply.

Most mixers or external phantom power supplies will supply 48vdc phantom power (some globally, some in banks, some on individual channels). 48vdc is the defacto standard, although there are exceptions. Some battery powered mixers, small format mixers, or small power adapters may only supply a fraction of that. I've got a small battery powered Peavey mixer that supplies 18-36vdc but also have an inline AC powered adapter that only supplies 24vdc via a wall wart.

Another thing with the phantom power is that the circuit resistor design limits current to somewhere in the teens up into maybe the 30's of milliamps, which while it can conceivable cause cardiac arrest, it would be a rare case indeed. It is conceivable that an internal fault could apply voltage to a sound system signal line, but as long as all the safety grounds are in place and connected properly the system breaker should trip before someone gets shocked.

Even so, we are the first to warn ANYONE against touching any part of the sound system, or any other "live voltage" carrying conductor/equipment of any type while immersed or standing in water. This topic of baptistry micing comes up now and again and the first order of clarification is to use a good distance mic out of reach of the speaker, or use a wireless mic of some sort.

We are also the first to pummel someone just about to death for even suggesting to defeat the safety ground with AC "cheater" plugs or breaking off the ground pin on a 3 prong plug. One of those situations where if your system buzzes, fix the buzz instead of taking life endangering shortcuts.

Some of you guys are indeed correct that there are entirely too many untrained and unsuspecting church technicians who don't know any better and who don't have anyone to tell them how to properly take care of and maintain their systems. I would ask all of you to point any and all church technicians, both clued and clueless, to ChurchSoundCheck.com so we can help them learn and give them the resources they need to make intelligent decisions, especially concerning when they should seek professional help (and not psychological, we're all already way past that ). It is totally cost/ad/myth free, email based, hosted by a well respected audio system consulting firm, and frequented by many professionals in the field that help keep volunteers like me on the straight and narrow.

Thanks for listening, enjoyed the posting.
Jeff K.

 

[charlie b]

Re: Freak Accident:

Frank and Jeff: Many thanks for your information and your support of us "big wire guys (and gals)."

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by hurk27:
There have been a few articles wrote on the dangers of sound equipment and the hack jobs roadies/sound men do when wiring the equipment, one of the most common is to lift the grounding conductor, This is done to remove the 60hz hum that is caused by parallel neutral currents flowing in the grounding systems, and shielding between audio equipment, This is a big danger that many (Roadies/Sound men) don't realize that someone can be killed as a result.

While it is true that electrical system miswires can cause excessive current to flow in cable shields, leading to hum and buzz, that is not the most common cause of such problems.

The single most common cause of hum and buzz in audio systems is a design flaw in the audio equipment called the "pin 1 problem". This was first publicly identified in the June 1995 issue of the Journal of the Audio Engineering Society, and has more recently been the basis of the AES48 Standard. Even though more than 10 years has passed, a very large percentage of the audio equipment on the market still has this design flaw.

Another very common design problem is with the physical construction of audio cables. Currents that flow in cable shields (mains frequency or RF) should not get coupled into the wires inside the shield. This coupling mechanism is called Shield Current Induced Noise or SCIN, and again was first identified in the June 1995 AES Journal. Virtually all cables sold for permanent installation work in the audio field have bad SCIN problems, and the cables that don't have bad SCIN problems don't have the required fire ratings for permanent installation work.

Last, there are all too many unbalanced interconnects used in the audio business, particularly in inexpensive equipment. Such unbalanced interconnects are just begging for hum and buzz problems since the cable shield is carrying the audio signal.

The "fix" you mentioned done by the uninformed of lifting the safety ground is of course the easy but very dangerous way to "solve" such problems. There are completely safe ways of making an audio system hum and buzz free, but too many just want a "quick and dirty" fix and don't think or care about the loss of safety.

Originally posted by hurk27:
I find that en-lite of this very common problem that equipment manufactures have not made changes to isolate their signal shields from the power supply grounding.
There is no reason to have the two tied together period!

Here I must disagree with you. Most audio equipment needs shielding. By far the most affordable way to provide such shielding is to use the metallic case. Since the metallic case is exposed to human touch and is powered from the Mains, it usually must be grounded to the electrical system's ground conductor. If the case is not used as the signal shield, a separate shielding enclosure inside the equipment case would be required that is insulated from the equipment case. This would be a huge expense, and would provide no benefit unless all the equipment interconnected in the audio system was made the same way. If any equipment or the connectors used in the system allowed signal shield to be touched at any place in the system, then the signal shield would have to be grounded for safety reasons. Since virtually no audio equipment on the market is made the way you propose, it would be impossible to introduce your system.

Originally posted by hurk27:
I have found many times 50 volts or more on the shielding to a known good ground that was the result of a complaint of someone getting a tingle from a Mic or a guitar or other equipment.

Here you are proving my point. Such shocks would not happen if the shields were properly grounded. The reason they happen is often that folk have lifted the safety grounds on equipment. Couple this with high leakage currents from the Mains, and people get shocked.

Originally posted by hurk27:
GFCI protection of stage equipment should be a requirement and enforced, This would stop many of the circulating neutral current when they keep tripping the GFCI's they have to be repaired.

I agree GFCIs are great ideas. Unfortunately in the road sound world there is an attitude of "the show must go on" even if things are unsafe. A GFCI makes it much more likely that the show will be interrupted. If a sound company used GFCI they would quickly lose business because the show promoters that hire them only care of the shown went on without interruptions.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 11:02 AM: Message edited by: bill addiss ]

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by busman:
From my old days in a garage band, I remeber a "phantom" voltage that was required for condenser mics. See the following excerpt:

"Because the condenser microphone must have a continuous, stable DC voltage to bias the membrane, it is common practice to supply that voltage from the sound mixing board. The voltage is applied via one of the microphone leads, typically 48 volts, and is commonly referred to as "phantom power". Since the alternative is a battery supplied bias, with the risk that a battery can go out in mid performance, the phantom power provision from mixing boards is useful."

This quote is inaccurate in one detail. Phantom power is not "applied via one of the microphone leads". Instead the positive leg of the phantom power is applied through a pair of matched resistors to both the signal leads. The return for the phantom power is through the cable shield.

Originally posted by busman:
I would guess that this is what got the subject shocked.

Mark

While it is possible, it is highly unlikely. The phantom power feed resistors limit the maximum current that can be delivered to 14 mA into a short circuit. Phantom powered microphones have internal resistance that further limits the current. Typical current is on the order of 3 mA.

The only way for a shock to happen would be if the cable shield that provides the return path for the phantom power to open up. This would be noticed immediately since the mic would not work without power. More importantly in the context of shocks from phantom power, the maximum current will be on the order of 3 mA even before it is further limited by body resistance. You might feel a "nip" from phantom power, but getting a fatal shock is highly unlikely.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 11:03 AM: Message edited by: bill addiss ]

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by hurk27:
Wow 48 volts for phantom power? All the mixers I have seen use 16 volts DC?

There are International Standards that define phantom power at 48, or 24, or 12 volts. Today 48 volt phantom (P48) is by far the most common, with 24 volt phantom (P24) almost unknown, and 12 volt phantom (P12) used in some battery powered equipment. Of course nothing requires a manufacturer to follow the Standard ...

Originally posted by hurk27:
I'm looking at a Tascam 16/32 studio mixer and it says right at the mic inputs 16 volts and there is a switch to turn it off at each XLR channel. Something new?

Nope, just something non-standard.

Originally posted by hurk27:
Even the snake lights that light the controls are 16 volts they just plug into an extra XLR port on the top side and run off the phantom supply.

Look again. The jacks for lamps are 4 pin (mics are 3 pin) to prevent anyone from plugging a mic into them. They run from a much higher current supply (often 12 volts) and not from the phantom supply.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 11:01 AM: Message edited by: bill addiss ]

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by realolman:
I don't think too many people use condenser mics.

Not at all. Condenser mics are very popular. I have a little side business selling mics, mostly into the church market. The majority of what I sell are phantom powered condenser mics. See:

(website address removed per forum rules)

Originally posted by realolman:
I don't think it's anything new. The power supply can go between the mic and the mixer. I think some are 52 volts. I sure wouldn't get in a baptismal font with one.

48 volt phantom power is allowed by the Standard to vary +/- 4 volts. As a result you often see mics marked that they can run on phantom as high as 52 volts.

Using any wired mic in a baptismal is not a good idea.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 11:01 AM: Message edited by: bill addiss ]

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by hbiss:
If there is a difference in potential between a guitar and amp powered by an on-stage circuit and the mike connected through the audio equipment powered by another circuit there is a problem with the electrical system.

-Hal

I would say there may be a problem with the electrical system, but often there is not. Most older guitar amps had two wire power cords. Further many of them had 3 position power switches (ON-OFF-ON). The two different ON positions connected the chassis to one side or the other of the power cord through a capacitor. The idea was you could flip the switch and choose the quieter position. The danger was that often the chassis would wind up connected to the hot side of the Mains, and as these amps got older the capacitors got leaky or shorted.

Many devices used by musicians on stage use wall wart type power supplies, which are usually equipped with 2 wire plugs. Often these wall warts have enough leakage current to induce hum into the audio system.

If all the equipment involved had 3 wire power cords with safety ground connections, the potential differences in a properly wire electric system should be small. However not everything has that safety ground connection.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 11:00 AM: Message edited by: bill addiss ]

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by hurk27:
There is no reason the audio shield need to be connected to the power grounding, this was a left over from the old tube amps that used to power the main PA system.

Tube amps have nothing to do with it. In fact modern digital audio equipment requires shielding even more than old analog equipment. In part this is to meet the legal requirements as to how much RF noise equipment is allowed to emit.

Originally posted by hurk27:
All new amps have power supply's that would isolate the audio section of the amp, and the shields connection to the chassis can be removed with out any problems. This connection is not needed and the amp will function with out it.

While amps with balanced inputs may function without a cable shield connection, they will be much more susceptible to hum, buzz and RFI problems.

Amps with unbalanced inputs will not work without the cable shield connection.

Originally posted by hurk27:
Remember when they came out with the XLR balanced audio system they removed the signal negative connection from the shields but left the shield connected to the chassis grounding.

Balanced interconnects for audio system far predate the introduction of the XL connector.

Originally posted by hurk27:
This was to extend the max length of runs and to help in reducing the hum and noise on the audio cables. Well it did help in removing some but since there was still a parallel path with the shielding it still cause the hum.

Ground loops should rarely cause problems with balanced audio interconnects. The reason they often seem to cause problems is because equipment has the "pin 1 problem" and cables have SCIN. Eliminate these two problems and most "ground loop problems" suddenly go away.

Originally posted by hurk27:
I use to keep a bag of XLR isolation transformers on hand because we had that exact setup at a park here, ...

Transformers can be used to solve many audio problems. Good transformers are expensive which is why they are not used that often. In extreme situations such as you described, they are often the only way to solve the problems short of getting the power fixed as they eventually did.

Originally posted by hurk27:
The only common denominator is to stop connection the audio shields to the power supply grounding and reconnect them to the input signal negative, this will isolate it from the power system keeping the shields/equipment safe to touch.

Only if you had a totally double insulated sound system. Lots of items including microphones and connectors have exposed metal that connects to the audio system shields, so the only safe solution is to ground the shields.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 11:00 AM: Message edited by: bill addiss ]

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by hbiss:
First not all PA or sound equipment has the capability of providing phantom power.

True, but all but the very cheapest gear today provides phantom power. Even Behringer mixers provide phantom power.

Originally posted by hbiss:
The higher end stuff would because it could be used with high quality (read$$) mics that might require phantom power. It's not likely that you would find these mics used for general sound reinforcement, the cost wouldn't make sense.

Tell that to my customers. I sell microphones as a small side business, and my most popular sound reinforcement vocal mic

(website address removed per forum rules)

is a $71 condenser mic.

Originally posted by hbiss:
If you enabled phantom power on a mic channel to which a "regular" mic was connected you would either damage the mic, or the mic would be a dead DC short to the phantom power which is current limited.

Nope. The reason it is called "phantom" power is that it "disappears" when it is not needed. With extremely rare exceptions (some old broadcast ribbon mics) any balanced mic can be connected to a mic input with phantom power applied, and the mic will just ignore it if the power is not needed.

Originally posted by hbiss:
Also the days of 48 volt phantom power are long gone. Today it's around 16 volts DC.

I am on the Audio Engineering Society Standards Committee dealing with phantom power and related issues. By far and away the most common variant of phantom power today is 48 volt. 16 volts is not even a Standard for phantom power.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 10:59 AM: Message edited by: bill addiss ]

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by Jon Livingston:
Regarding the P.A. microphone, almost all modern microphones use field effect transistors to boost microvolt-level signals into volt-level alternating currents suitable for transmission over 600 ohm impedance 3-wire 'XLR' standard circuits.

A few minor items.

It would be correct to say that almost all modern condenser microphones use field effect transistors.

The FETs in condenser mics are mostly used to lower the output impedance to enable the mic to drive a long mic cable. This usually does not involve any significant voltage gain. The output voltage of the typical condenser mic with typical acoustic input levels is in the millivolt range.

600 ohm circuits have not been used for 30 years or more in professional audio. Modern microphones usually have nominal output impedances of around 150 ohms, and most mixers have microphone input impedances of 1,500 ohms or higher.

Ray A. Rayburn
(website address removed per forum rules)

[ January 07, 2006, 10:58 AM: Message edited by: bill addiss ]

 

[roger]

Re: Freak Accident:

Ray, you seem to know your stuff, but you really need to leave the commercials out of the posts.

From the forum rules;

Please note that advertisements, chain letters, pyramid schemes, and solicitations are inappropriate on this forum.

Roger

 

[Audioconsultant]

Re: Freak Accident:

Originally posted by roger:
Ray, you seem to know your stuff, but you really need to leave the commercials out of the posts.

From the forum rules;

Please note that advertisements, chain letters, pyramid schemes, and solicitations are inappropriate on this forum.

Roger

Fair enough, except that I was not aware of the forum rules when I signed up. Someone should fix the sign up page, since I had to click on an agreement to abide by your rules, and the rules I agreed to said something like "Please insert your rules here."

Someone sent me a message saying that URLs were not allowed in posts, and I noticed all the URLs had been removed from my posts. This is fine and I would not have done that if I had known what the rules were.

The FAQ page likewise makes no mention of the forum rules. Where could I find these rules? Their location is not exactly obvious.

Ray A. Rayburn

[ January 07, 2006, 12:27 PM: Message edited by: Audioconsultant ]

 

[bill addiss]

Re: Freak Accident:

Ray,

The page that is being referred to has a link on the 'main' Forum page. Here it is:

http://www.mikeholt.com/codeforum/about/about_forum.htm

Bill

 

[bill addiss]

Re: Freak Accident:

P.S.

Ray,

I figured you weren't aware of the rules, that's why I sent you the message. Don't take the removal of the urls personally, we have to be as consistant as we can.

Bill

 

[realolman]

Re: Freak Accident:

Originally posted by Audioconsultant:

Originally posted by realolman:
I don't think too many people use condenser mics.

Not at all. Condenser mics are very popular. I have a little side business selling mics, mostly into the church market. The majority of what I sell are phantom powered condenser mics. See:

You do in fact seem to know your stuff, but I would say there are a lot more dynamic mics out there than condenser mics.

It probably has to do with your market. I bet you would sell a lot more organs than banjos

[ January 07, 2006, 02:15 PM: Message edited by: realolman ]