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'[EE] Isolating a Microphone for Safe Use in Wet Ar'
2011\05\16@093618 by Mark E. Skeels

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All,

I want to configure a microphone to eliminate the possibility of electric shock when it is used in (very) wet areas.

The most important thing is that it not endanger the user via any possibility of electric shock.

It needs to eventually feed a standard type of audio mixing console; XLR balanced type, low impedance connection.

It needs to be of reasonable fidelity.

This does not have to be a condenser type mic, but the option would be nice; that means it would ideally provide the phantom voltage supply, if necessary. It could be battery powered. This I guess is probably not possible given the safety requirement and would probably limit the application to dynamic mics.

It would be nice to have a "black box" sort of thingy that could be used with different mics.

Assuming a non-RF solution, how do you go about doing this?

Mark
-- Mark Skeels
Engineer
Competition Electronics, Inc.
TEL: 815-874-8001
FAX: 815-874-8181
http://www.competitionelectronics.co

2011\05\16@100957 by Olin Lathrop

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Mark E. Skeels wrote:
> This does not have to be a condenser type mic, but the option would be
> nice; that means it would ideally provide the phantom voltage supply,
> if necessary. It could be battery powered. This I guess is probably
> not possible given the safety requirement and would probably limit the
> application to dynamic mics.

That doesn't make sense.  The battery would be low voltage and contained in
the microphone.  You are going to have to keep the water out of the mic
itself and any electronics anyway.  The battery should be well isolated from
the user.

Even if it does get wet inside the mic and there is some electrical path to
the outside, then that will still be only a single connection.  Current from
the mic case to the ground or wherever has noplace to flow.  If course if
this happens, the mic won't be working well or at all in the first place.

To put this another way, put a battery inside a metal box.  You can safely
hold the box whether there is any leakage from either or both sides of the
battery to the box or not.  This may kill the battery, but there won't be
any current thru your hand or thru your body to elsewhere since you're only
connected to a single node.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\05\16@102104 by Brendan Gillatt

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On 16/05/11 14:37, Mark E. Skeels wrote:
> All,
>
> I want to configure a microphone to eliminate the possibility of
> electric shock when it is used in (very) wet areas.
>
> The most important thing is that it not endanger the user via any
> possibility of electric shock.

> Assuming a non-RF solution, how do you go about doing this?

Have you considered optical fibre as the transmission media?
Alternatively there are 'optical microphones' on the market which do not
use electricity (at least at the pickup) at all. They tend to cost $$$,
however.

You may have some luck running a condenser mic at a lower voltage than
the nominal 40v. This may increase the safety somewhat.

Regards,
Brendan
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2011\05\16@102912 by Michael Watterson

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On 16/05/2011 14:37, Mark E. Skeels wrote:
{Quote hidden}

Electret is only 1.5V AA cell or button cell.
True Condenser mic is 100s of Volts I think.

Do you REALLY mean Condenser or Electret?

Use a 600 Ohm isolation transformer if concerned about connection to mixer desk earth via wet mic.

Electrets and Condenser mics don't like damp never mind wet. You can buy weather proof and water proof mics with XLR off the shelf though

2011\05\16@103254 by Michael Watterson

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On 16/05/2011 15:20, Brendan Gillatt wrote:
> You may have some luck running a condenser mic at a lower voltage than
> the nominal 40v. This may increase the safety somewhat.
>

That would be external power, not the actual internal operating volts. the actual operating volts depends on if Electret or true condenser mic.

You don't want to run the 48V desk power into the wet at all. Use no phantom power and isolation transformer with self powered mic if electret or condenser

2011\05\16@113420 by Oli Glaser

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On 16/05/2011 15:28, Michael Watterson wrote:
> Electret is only 1.5V AA cell or button cell.
> True Condenser mic is 100s of Volts I think.

I'm not sure about the above - what makes you say this?
I am aware of a couple of unusual types but AFAIK, the vast majority of condenser mics use the standard 48V phantom power present on most preamps/mixing desks. The current is pretty low, generally being supplied through pins 2 and 3 via ~6K65 resistors.
On researching this for our preamp, I discovered that some (generally cheaper) equipment does not supply the full 48V, taking advantage of the fact that many microphones work fine with less. Some go down to as low as 9V IIRC. This can cause issues with mics designed to work with the full phantom voltage.
Of course there are tube mics that require a high voltage supply, but that's different.



> Do you REALLY mean Condenser or Electret?
>
> Use a 600 Ohm isolation transformer if concerned about connection to
> mixer desk earth via wet mic.
>
> Electrets and Condenser mics don't like damp never mind wet. You can buy
> weather proof and water proof mics with XLR off the shelf though.

2011\05\16@114555 by graham foulkes

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Hi Mark
I would make the comment that your main source of concern is shock hazard
from the mixing console or any other mains powered equipment that you are
using rather than from a battery to power the condenser mike or electret
mike. It is important to have any mains powered equipment earthed or ground
bonded to avoid shock hazard under damp or fault conditions. This earthing
is to connect any conductive user accessible part of the system to a common
ground with respect to the power input supply ground. Also transformer
isolation of the equipment is usually standard in most currently
manufactured equipment but must never be assumed and can be compromised
under damp/fault conditions. People sometimes assume that using a microphone
matching transformer is enough to give isolation when in reality the cable
screening and the microphone body are connected back to the input of the
source equipment, extending the potential for contact with a live surface.
Earth loops can be very tricky things to avoid in audio systems. Finally a
ground fault interrupter device on the mains powered equipment is strongly
recommended to ensure that a shock hazard current does not ever flow through
a user of the equipment.

On Mon, May 16, 2011 at 9:37 AM, Mark E. Skeels <
spam_OUTmskeelsTakeThisOuTspamcompetitionelectronics.com> wrote:

{Quote hidden}

>

2011\05\16@115312 by Michael Watterson

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On 16/05/2011 16:33, Oli Glaser wrote:
> I'm not sure about the above - what makes you say this?
> I am aware of a couple of unusual types but AFAIK, the vast majority of
> condenser mics use the standard 48V phantom power present on most
> preamps/mixing desks. The current is pretty low, generally being
> supplied through pins 2 and 3 via ~6K65 resistors.
That's the standard desk power.

It's nothing whatsoever to do with what the raw capsule needs.

An Electret only needs an FET with 1.5V supply as isolation. Via resistor from 48V desk instead of internal AA cell or button cell(s) it would be 0.5mA to 10mA max depending on circuit.

A true condenser mic needs high voltage on the plates and an amplifier. Newer designs may only take a couple of mA. Older ones might take more than 20mA.
Most modern cheaper so called "condenser" mics are just electret with a fet and big resistor to cut down voltage.

2011\05\16@121925 by Oli Glaser

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On 16/05/2011 16:52, Michael Watterson wrote:
> On 16/05/2011 16:33, Oli Glaser wrote:
>> I'm not sure about the above - what makes you say this?
>> I am aware of a couple of unusual types but AFAIK, the vast majority of
>> condenser mics use the standard 48V phantom power present on most
>> preamps/mixing desks. The current is pretty low, generally being
>> supplied through pins 2 and 3 via ~6K65 resistors.
> That's the standard desk power.
>
> It's nothing whatsoever to do with what the raw capsule needs.

I'm aware of that - that's why I mentioned that some equipment takes advantage of the fact that most mics don't need the full 48V.

> An Electret only needs an FET with 1.5V supply as isolation. Via
> resistor from 48V desk instead of internal AA cell or button cell(s) it
> would be 0.5mA to 10mA max depending on circuit.
>
> A true condenser mic needs high voltage on the plates and an amplifier.
> Newer designs may only take a couple of mA. Older ones might take more
> than 20mA.

Are you saying then that a "true" condenser mic will not work with 48V?
What voltage do they need? Do you have any links to information on this?

> Most modern cheaper so called "condenser" mics are just electret with a
> fet and big resistor to cut down voltage.
>

Yes. I was thinking of the more expensive, studio quality mics though. Not your average electret PC mic that only needs the supply for FET bias.

2011\05\16@124249 by William \Chops\ Westfield

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On May 16, 2011, at 6:37 AM, Mark E. Skeels wrote:

> Assuming a non-RF solution

Why are you assuming that?  Wireless mics are readily available OTS  technology (from cheap toys to $1000+ professional systems, and even  rentable), and it seems like a really obvious solution.
The local high-school drama group adds condoms over the electronics  for additional sweat-resistance.  (this is common practice in  theater.  Supposedly.)


> tend to cost $$$

Alas, true of microphones in general, and you get what you pay for  with the unfortunate usual logarithmic transfer function (pay  exponentially more for linearly better performance.)  Previously  mentioned HS drama group had problems with their mics.  (problems of  an obviously electronic/rf nature, rather than the usual audio tech  missed cues, badly positioned mics, and untrained actors.  Sigh.  It  really shouldn't take $1000 per mic to do better than that.  Makes you  want to go up and fiddle with knobs and passive re-radiators and such,  even if you know nothing about theater micing.)
(It was a great show anyway.)

BillW

2011\05\16@133857 by Michael Watterson

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On 16/05/2011 17:18, Oli Glaser wrote:
> Are you saying then that a "true" condenser mic will not work with 48V?
> What voltage do they need? Do you have any links to information on this?
>

They have an internal PSU running off 48V.

modern condenser microphones use low voltage RF and demodulate the effect of the changing capacitor. Older ones used a DC bias and are more sensitive the higher the voltage
http://www.articlespan.com/article/76202/the-mechanism-and-functioning-of-a-condenser-microphone

Some may have worked direct from the 48V, but certainly I have seen higher voltage studio models when I was in BBC.
I think some had valve in mic, then box on floor with  PSU running off the 48V desk...

you can have any voltage you want inside something now in a very small space. In 1983 I was designing ultra-miniature SMPSUs running at 1MHz to generate isolated +12V, -12V and 5V (isolated) from a external 5V DC supply and various voltage from -48V telecom supplies.

Even in 1950s there were "vibrator packs" about valve size to run a 50W mains valved amplfier off 12V DC battery.
In 1930s and 1940s they used rotary converters / dynamotors to make 250V DC and 500V DC for transmitters in aircraft off 12V. (not small!)

Electret via phantom power
http://www.coutant.org/evcs15/index.htm

2011\05\16@145629 by Oli Glaser

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On 16/05/2011 18:38, Michael Watterson wrote:
> On 16/05/2011 17:18, Oli Glaser wrote:
>> Are you saying then that a "true" condenser mic will not work with 48V?
>> What voltage do they need? Do you have any links to information on this?
>>
> They have an internal PSU running off 48V.
>
> modern condenser microphones use low voltage RF and demodulate the
> effect of the changing capacitor.

Yes, some do, these are known as RF Condenser Mics. There are plenty of modern condenser mics that use DC though.

>   Older ones used a DC bias and are more
> sensitive the higher the voltage
> http://www.articlespan.com/article/76202/the-mechanism-and-functioning-of-a-condenser-microphone

That link states the basics, but no info regarding capsule voltages.

> Some may have worked direct from the 48V, but certainly I have seen
> higher voltage studio models when I was in BBC.
> I think some had valve in mic, then box on floor with  PSU running off
> the 48V desk...

Yes, but was the higher voltage actually used for the plates, or simply present as it was needed to power the valves?
I have used plenty of tube mics with their own separate power supplies that plugged into wall.

What I am getting at is that I thought a definitive statement about condenser mics needing 100s of volts could be a bit misleading. That is why I was asking for some solid confirmation of this, as in my experience working with condenser mics (plus fixing the odd one), many have used the 48V with no step up (IIRC from schematics I have seen previously - even on the ones with converter the voltages are often not noted)
I am not aware of any that charge the plates to several hundred volts, so I would be interested to see an example/ schematic.


{Quote hidden}

2011\05\16@153841 by Mark E. Skeels

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Putting aside the idea of a phantom powered condenser mic for a moment, suppose I use this:

http://www.fullcompass.com/product_image.php?imageid=89714

If you look closely you'll see a schematic in which the two grounds (pin 1 XLR) on either side of the transformer are isolated (if I am reading it right.)

If that is not correct, then imagine a transformer that is like this (isolated ground connections).

Assuming sufficiently high breakdown voltage, if I use this the mains hazard would be isolated and nullified, wouldn't it?


Mark Skeels
Engineer
Competition Electronics, Inc.
TEL: 815-874-8001
FAX: 815-874-8181
http://www.competitionelectronics.com

On 5/16/2011 10:45 AM, graham foulkes wrote:
{Quote hidden}

>> -

2011\05\16@172328 by Michael Watterson

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On 16/05/2011 20:39, Mark E. Skeels wrote:
> Putting aside the idea of a phantom powered condenser mic for a moment,
> suppose I use this:
>
> http://www.fullcompass.com/product_image.php?imageid=89714
>
> If you look closely you'll see a schematic in which the two grounds (pin
> 1 XLR) on either side of the transformer are isolated (if I am reading
> it right.)
>
> If that is not correct, then imagine a transformer that is like this
> (isolated ground connections).
>
> Assuming sufficiently high breakdown voltage, if I use this the mains
> hazard would be isolated and nullified, wouldn't it?
>

Yes. That's the gadget I was thinking of earlier. You need a procedure etc that prevents it being bypassed.

Also both Electret and RF energised Condenser Microphones are available in self powered AA battery outdoor/weather proof versions, also shotgun mics.
Though for "close" work (person holding mic near mouth rather than studio controlled placement) the dynamic mic is often preferable.

2011\05\16@175343 by IVP

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> I want to configure a microphone to eliminate the possibility of
> electric shock when it is used in (very) wet areas.

Surely it's what the microphone is plugged into that's going to be
the source of any lethal voltages ?

RCD

2011\05\16@185918 by Michael Watterson

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On 16/05/2011 22:53, IVP wrote:
>> I want to configure a microphone to eliminate the possibility of
>> >  electric shock when it is used in (very) wet areas.
> Surely it's what the microphone is plugged into that's going to be
> the source of any lethal voltages ?
>
> RCD ?
Indeed. Isolation. RCD is only a last defence really... If you KNOW you have an issue you isolate. e.g. isolated 110V transformer in bathrooms and Centertapped 110V on building sites in 230V countries.

As long as it's not phantom powered the microphone itself isn't an issue.

barefeet in salt water + wet hands  and  48V DC is marginally possible to be needing immediate medical assistance.

There can be issues with RF cordless microphones:
Interference, battery life, security (easy to eavesdrop at distance), range.

2011\05\16@191713 by IVP

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> barefeet in salt water + wet hands  and  48V DC is marginally
> possible to be needing immediate medical assistance.

How about LVDC (5V ? 12V ?) down the mic cable and a small
SMPS step-up at the mi

2011\05\16@194543 by Oli Glaser

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On 17/05/2011 00:16, IVP wrote:
>
>> barefeet in salt water + wet hands  and  48V DC is marginally
>> possible to be needing immediate medical assistance.
> How about LVDC (5V ? 12V ?) down the mic cable and a small
> SMPS step-up at the mic

Or just use an electret mic (or a dynamic) and leave the electronics until after the isolation transformer.
You could use a 1:1 into a mic input (RS/Farnell etc should have plenty to choose from), or maybe a passive DI box into a hi-z input.

This kind of thing might work okay (might want to ask about the voltage rating though - seems to be designed mainly with ground problems in mind)
www.audiospares.com/product.php?productid=1755
www.epanorama.net/documents/groundloop/audio_isolator_building.html

2011\05\16@200624 by IVP

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> barefeet in salt water + wet hands  and  48V DC is marginally possible
> to be needing immediate medical assistance.

I've no experience with 48V mics but it appears from this

http://en.wikipedia.org/wiki/Phantom_power

that under normal conditions it is current-limited to 10mA max (IEC).
I haven't found a figure for the let-go threshold, ie when you're being
shocked and can't voluntarily let go, at 48V. An insulated mic would
be a start, although perspiration could make a conductive path when
conditions are very wet

Off the top of my head the two likeliest candidates for hazard would
be HV getting onto the mic cable and a live chassis + ground faul

2011\05\16@202635 by IVP

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> Or just use an electret mic (or a dynamic) and leave the electronics
> until after the isolation transformer.

'isolation' in this context refers to impedance isolation doesn't it

2011\05\16@203634 by Lyle Hazelwood

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Old news, but relevant to the discussion:
http://www.chron.com/disp/story.mpl/metropolitan/3844589.html

Lyl

2011\05\16@211838 by Oli Glaser

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On 17/05/2011 01:26, IVP wrote:
>> Or just use an electret mic (or a dynamic) and leave the electronics
>> until after the isolation transformer.
> 'isolation' in this context refers to impedance isolation doesn't it ?

Not sure quite what you mean here - if you mean impedance matching then it could do that too (e.g. the DI box would), but a 1:1 transformer would just be for galvanic isolation purposes. I can remember using DI boxes with the ground lifted and similar to help with issues with dodgy ground and dangerous old equipment (I can remember getting a small shock from the mic whilst singing plenty of times)
As long as the transformer had a suitable voltage rating across its windings, then it probably shouldn't matter much what exact form it comes in. I was thinking of something roughly along these lines (or whatever ratio is needed to match the equipment the OP is intending to connect to):
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2106431 <uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2106431>
Seems to be tested to 1kV according to datasheet. With shield between windings.
Search page it came from with other options:
http://bit.ly/jKbMph

2011\05\16@234813 by Josh Koffman

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On Mon, May 16, 2011 at 12:42 PM, William "Chops" Westfield
<westfwspamKILLspammac.com> wrote:
> The local high-school drama group adds condoms over the electronics
> for additional sweat-resistance.  (this is common practice in
> theater.  Supposedly.)

It is. Just be sure to buy non lubricated ones. An alternative is to
use standard balloons and cut off the neck. That can end up cheaper.

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2011\05\17@085936 by Mark E. Skeels

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[SNIP]
Old news, but relevant to the discussion:
> http://www.chron.com/disp/story.mpl/metropolitan/3844589.html
>


Yep, I knew about this....in part, this is what prompted this thread.

Silly Baptists. They insist on standing in 3 feet of water while needing to be heard by hundreds of people.

If only I were Presbyterian........

But really, see why a wireless (with body pack and wearable mic) would not be really that good? Plus battery considerations, range, interference, and the need to hear more than just the person wearing it, and the inconvenience of somebody actually falling into the water, plus the fact that at least one person is going to be submerged..

In the end a RF link might still be the best, safety wise, but since I have to use some other form of micing anyway, I just thought it'd be good to kick the subject around here.


Mark Skeels
Engineer
Competition Electronics, Inc.
TEL: 815-874-8001
FAX: 815-874-8181
http://www.competitionelectronics.com

2011\05\17@111839 by Dwayne Reid

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At 07:00 AM 5/17/2011, Mark E. Skeels wrote:

>Yep, I knew about this....in part, this is what prompted this thread.
>
>Silly Baptists. They insist on standing in 3 feet of water while needing
>to be heard by hundreds of people.
>
>If only I were Presbyterian........
>
>But really, see why a wireless (with body pack and wearable mic) would
>not be really that good? Plus battery considerations, range,
>interference, and the need to hear more than just the person wearing it,
>and the inconvenience of somebody actually falling into the water, plus
>the fact that at least one person is going to be submerged..

Lectrosonics <http://www.lectrosonics.com/> makes water-resistant body pack transmitters.  These are what TV-show contestants and reality-show players are wearing when they get wet or soaked.

Be prepared for some serious sticker shock, though.  You will be spending several thousand dollars per channel (transmitter, receiver, mic capsule).

Do note: people in the business consider Lectrsonics to be low-cost.  You will understand what they mean by that when you start checking out the prices for the Sennheiser Professional SKM5200(hand-held mic mostly seen on music-type and awards shows (American Idol) or SK5212 body-pack transmitters (used EVERYWHERE) and the companion receivers.  Serious ouch factor there.

The advantage of using professional gear, though, is that it just works.  No muss, no fuss.  And its RELIABLE.

dwayne

-- Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\05\17@125815 by Dwayne Reid

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At 07:37 AM 5/16/2011, Mark E. Skeels wrote:
>All,
>
>I want to configure a microphone to eliminate the possibility of
>electric shock when it is used in (very) wet areas.
>
>The most important thing is that it not endanger the user via any
>possibility of electric shock.

You have two issues to deal with here.

1) Any passive microphone (dynamic, ribbon, etc) doesn't need any form of power to operate.  The acoustic signal is converted directly to an electric signal.

More sensitive or better-sounding mics usually have electronics associated with them.  These are typically Condenser Microphones, which come a variety of styles.

Classic (old-school) Condenser Mics require either an external power supply (tube-type mics) or 48V phantom power.  I'm talking about Neumann condenser mics here.

There is a related class of what I call classic condenser mics: those that are RF-based.  These have two RF oscillator inside them: one is stable, the other is frequency-modulated by the movement of the mic diaphragm.  The resulting difference is demodulated down to base-band audio.  The oldest of these that I've seen were transistor-based and required 48V phantom power.

Modern electronic microphones (condenser mics) cover a vast range.  These sort of divide into two different family types: electret or not.  Electret mics need only a tiny amount of power which can be obtained from an internal battery.  Condenser mics generally need phantom power but this can often be much less than 48V.  For example, AKG 451 microphone bodies will operate from about 10V through 54V phantom power.

Bottom line - choose a mic that doesn't need either an external power supply or 48V phantom power.

2) Most microphones are made of metal and use shielded cable.  This is to reduce / eliminate hum caused by external fields.  The shield and microphone are normally grounded via the audio console.

So - the mircophone is not normally the source of the problem.  Rather - it is a really good ground that someone is holding onto.  If that person contacts something that is live, it provides a ready path for current.  So: it would be nice if you could isolate that ground.


It is sometimes possible to use a dynamic mic without a shield.  You need to use high-quality twisted-pair cable feeding a good quality balancing transformer.

Whether this is possible or not depends entirely on the microphone used.  You are going to have to try it yourself and see.

However, I can tell you that I did this once in a trial many years ago (local Champ Car / Indy race car track).  The cable run was about 2000 feet of cat5 cable (4 pairs).  One pair was connected to pins 2&3 of the Sennheiser dynamic mic (don't recall the model number).  The other end of the cable went through one of my balancing transformers and into a Yamaha DM2000 console.  Pin 1 of the microphone was left floating.

About 500 feet of that cable run had the cat-5 cable sitting right on top of the 500 mcm and 1000 mcm cables running from the gen sets to the power distribution for the site's power.  This was the only available cable route.  The site was fully powered at the time of the test.

The audio operator at the console (many, many years of broadcast and recording-studio experience) said that the audio quality was superb.  He thought that it wasn't quite broadcast quality but was eminently suitable for PA system use.  He based this on listening to the air tone (ambient noise) as well as listening to race cars off in the distance.  He then listened to the on-air talent doing his usual talking (obviously much louder than ambient noise).  Residual hum was buried in the noise floor.

We then took the cat-5 cable and connected each of the pairs to the outputs of the Sennheiser receiver racks (+4dBu level) - the resulting audio WAS broadcast quality and was used for the entire (and all subsequent) events.

The reason for the initial test was to convince the Broadcast Technical Director that cat5 cable WAS a reasonable alternative to running thousands of feet of heavy, thick DT12 (12-pair) cable through rough terrain.  We did end up using multiple runs of cat5 cable - much, much quicker and easier - and it worked well.

Where I'm going with this is that it IS possible to galvanically-isolate a dynamic microphone and still get good results.  It really depends on the type and quality of the microphone chosen as well as the balancing transformer.

The transformers I used were un-shielded 600R-600R with 1500V rated insulation.  I have lots if you need any (~8 each in quantity, ~10 each singly).  You do have to be careful to keep them away from magnetic fields (power transformers, etc).

dwayne

-- Dwayne Reid   <EraseMEdwaynerspam_OUTspamTakeThisOuTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\05\18@001603 by IVP

face picon face
>> 'isolation' in this context refers to impedance isolation doesn't it ?
>
> Not sure quite what you mean here - if you mean impedance
> matching then it could do that too (e.g. the DI box would), but a
> 1:1 transformer would just be for galvanic isolation purposes

Not wishing to start anything, but a transformer can still pass a fault
current. eg a 1:1 mains isolating transformer does have 240VAC on
the secondary but is not referenced to ground, so the number of
situations where there's risk of shock is reduced but not to zero. It's
no bad thing having an isolation transformer, don't get me wron

2011\05\18@004702 by Oli Glaser

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face
On 18/05/2011 05:15, IVP wrote:
>>> 'isolation' in this context refers to impedance isolation doesn't it ?
>> Not sure quite what you mean here - if you mean impedance
>> matching then it could do that too (e.g. the DI box would), but a
>> 1:1 transformer would just be for galvanic isolation purposes
> Not wishing to start anything, but a transformer can still pass a fault
> current. eg a 1:1 mains isolating transformer does have 240VAC on
> the secondary but is not referenced to ground, so the number of
> situations where there's risk of shock is reduced but not to zero. It's
> no bad thing having an isolation transformer, don't get me wrong

Yes it can, now I see what you were getting at. I was just thinking along the lines of removing the ground reference. Just to be clear, I was talking about a 600 ohm 1:1 in between the mic and the equipment, not a mains isolation transformer. Anything with a route to mains voltage has this problem though.
I'm probably missing something here, but how were you thinking the SMPS would solve this?

2011\05\18@013254 by IVP

face picon face
> I'm probably missing something here, but how were you thinking the
> SMPS would solve this?

Aw, dirty pool, making me think on an empty stomach ;-))

And now that I do, it probably doesn't solve anything wrt to a mains
fault. I suggested it as Michael mentioned the possibility of a 48V
shock. Making a controlled 48V within the mic with an SMPS from
LV was just a thought. It is just as possible to have a fault on a LV
supply. But, specifically, if the current-limiting of a 48V supply was
somehow bypassed, it would be more likely to shock than unlimited
5V

For any situation to become hazardous it would take more than one
factor. eg if the chassis became live there and was no good ground,
RCD and/or fuse, the mic was not insulated, in the left hand etc

To be absolutely safe every potential (no pun intended) failure point
should be addresse

2011\05\18@020818 by Oli Glaser

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On 18/05/2011 06:32, IVP wrote:
>> I'm probably missing something here, but how were you thinking the
>> SMPS would solve this?
> Aw, dirty pool, making me think on an empty stomach ;-))

I do apologise  - on the other hand I've just this minute finished a very tasty special chow mein + fried rice.. :-)

> And now that I do, it probably doesn't solve anything wrt to a mains
> fault. I suggested it as Michael mentioned the possibility of a 48V
> shock. Making a controlled 48V within the mic with an SMPS from
> LV was just a thought. It is just as possible to have a fault on a LV
> supply. But, specifically, if the current-limiting of a 48V supply was
> somehow bypassed, it would be more likely to shock than unlimited
> 5V
>

Ah, I see - I thought that might have been what you were thinking of after more consideration, which makes sense as <12V is very unlikely to cause problems compared to 48V (unless you apply directly across the heart)
You can get also get condenser mics which work okay on these low voltages if you didn't want to roll your own.

> For any situation to become hazardous it would take more than one
> factor. eg if the chassis became live there and was no good ground,
> RCD and/or fuse, the mic was not insulated, in the left hand etc

Yes, unfortunately Murphy often conspires to find just the right circumstances for these things to occur.
I know from experience that it happens more than it should do, especially in the case of the music industry where you have a lot of (often badly maintained) old equipment still being used. There are so many links in the chain with something like a live/studio setup, it only takes one or two to be flaky and you have problems.

> To be absolutely safe every potential (no pun intended) failure point
> should be addressed

Yep - I think if it was me standing in 3 feet of water I would probably opt for something that ran on a couple of AAs :-)

2011\05\18@032738 by Michael Watterson

face picon face
On 18/05/2011 07:07, Oli Glaser wrote:
>> >  To be absolutely safe every potential (no pun intended) failure point
>> >  should be addressed
> Yep - I think if it was me standing in 3 feet of water I would probably
> opt for something that ran on a couple of AAs:-)

In an isolated rubber / plastic case, no metal work. There is a double concentric box symbol for such equipment that runs from mains I think.

2011\05\18@074647 by Olin Lathrop

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Oli Glaser wrote:
> Yep - I think if it was me standing in 3 feet of water I would
> probably opt for something that ran on a couple of AAs :-)

If I was standing in 3 feet of water, I'd not hold anything electrically
connected to elsewhere.  I don't care how much there are supposed to be
isolation transformers and the like in the path, I simply wouldn't trust it..

The only reasonable alternatives are a handheld mic that is battery powered
and sends the signal via RF, or a fixed mounted mic in the ceiling or
whatever where I wouldn't be touching it.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\05\18@080203 by Gerhard Fiedler

picon face
Oli Glaser wrote:

> Yep - I think if it was me standing in 3 feet of water I would
> probably opt for something that ran on a couple of AAs :-)

There is this situation that happens every now and then in the movies
where somebody dies in a swimming pool because somebody else throws a
device that is connected to mains into the pool. Is this possible?
I can't really imagine a mechanism for that, even though the contact
resistance between the water and the body is highly reduced. If the
water is conductive, it's unlikely that there is a high current through
the body. If it isn't that conductive, it's unlikely that there is a lot
of potential across the body... ??

Gerhar

2011\05\18@082858 by Michael Watterson

face picon face
On 18/05/2011 12:47, Olin Lathrop wrote:
> If I was standing in 3 feet of water, I'd not hold anything electrically
> connected to elsewhere.  I don't care how much there are supposed to be
> isolation transformers and the like in the path, I simply wouldn't trust it.
>

Was it you that experimented with dropping electric heater into paddling pool

2011\05\18@083357 by IVP

face picon face
> There is this situation that happens every now and then in the movies
> where somebody dies in a swimming pool because somebody else
> throws a device that is connected to mains into the pool. Is this
> possible?

Interesting and very watchable results for that when Mythbusters tried it
with a bath and poor old Buster

I think they did a fairly good job on this one. What amazed me in that
segment was a hairdryer (not the GFCI one) that kept going underwater !

http://en.wikipedia.org/wiki/MythBusters_(2004_season)#Appliances_in_the_Bath

Video

http://dsc.discovery.com/videos/mythbusters-appliances-in-the-bath-minimyth..html

2011\05\18@090230 by Mark E. Skeels

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face
[SNIP]

On 5/18/2011 6:47 AM, Olin Lathrop wrote:
> Oli Glaser wrote:
>> Yep - I think if it was me standing in 3 feet of water I would
>> probably opt for something that ran on a couple of AAs :-)
> If I was standing in 3 feet of water, I'd not hold anything electrically
> connected to elsewhere.  I don't care how much there are supposed to be
> isolation transformers and the like in the path, I simply wouldn't trust it.
>
> The only reasonable alternatives are a handheld mic that is battery powered
> and sends the signal via RF, or a fixed mounted mic in the ceiling or
> whatever where I wouldn't be touching it.
>
>

My intended solution, as of this moment is:

   * Sound deadening curtains on the hard walls within the room.
   * Wall mounted or boom mounted microphone out of reach of bodies.

We will not be depending on isolation.

Mar

2011\05\18@092123 by Gerhard Fiedler

picon face
IVP wrote:

>> There is this situation that happens every now and then in the movies
>> where somebody dies in a swimming pool because somebody else
>> throws a device that is connected to mains into the pool. Is this
>> possible?
>
> Interesting and very watchable results for that when Mythbusters tried it
> with a bath and poor old Buster
>
> I think they did a fairly good job on this one. What amazed me in that
> segment was a hairdryer (not the GFCI one) that kept going underwater !
>
> http://en.wikipedia.org/wiki/MythBusters_(2004_season)#Appliances_in_the_Bath
>
> Video
>
> http://dsc.discovery.com/videos/mythbusters-appliances-in-the-bath-minimyth.html

Thanks for the quick reply! :)

I can imagine the bathtub, but I still have problems imagining this in a
swimming pool.

Gerhar

2011\05\18@095851 by Oli Glaser

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face
On 18/05/2011 12:47, Olin Lathrop wrote:
> Oli Glaser wrote:
>> >  Yep - I think if it was me standing in 3 feet of water I would
>> >  probably opt for something that ran on a couple of AAs:-)
> If I was standing in 3 feet of water, I'd not hold anything electrically
> connected to elsewhere.  I don't care how much there are supposed to be
> isolation transformers and the like in the path, I simply wouldn't trust it.

Yes, that's what I meant also - a self contained battery powered unit.

2011\05\18@100931 by IVP

face picon face

> I can imagine the bathtub, but I still have problems imagining this in a
> swimming pool

True, a body in a bath is going to be a significant part of the volume
that the current could pass through

Scaling that up to the ocean, if I dropped a bar heater in the harbour
down the road, the chance of someone taking a dip at Jersey Shore
getting a shock is remote. More's the pity

In a swimming pool I guess it depends on where you are and the
state of the water. As Mythbusters noted, urine in the water helps
current flow. After a shock I'd venture there'd be a bit of E Coli in
the water too. If you happen to be directly between the source of
the current and the grounded drain or a ladder, that could be nasty.
If you were paddling down the shallow end, with the kids and the
tinkle, and something live hit the water up the other end, where
the drain is, you might not notice a thing

I think Mythbusters did have a go at different entry points for the
appliance in relation to the drain but I don't recall the result. Up
around the head was definitely fatal when toes were near the drai

2011\05\18@130950 by Al Shinn

picon face


Gerhard wrote:
>Thanks for the quick reply! :)
>I can imagine the bathtub, but I still have problems imagining this in a
>swimming pool.

see:
http://www.webmd.com/baby/news/20030523/pool-owners-warned-of-electrocution-risk

Looking forward,
Al Shin

2011\05\18@141040 by Dwayne Reid

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face
At 07:02 AM 5/18/2011, Mark E. Skeels wrote:

>My intended solution, as of this moment is:
>
>     * Sound deadening curtains on the hard walls within the room.
>     * Wall mounted or boom mounted microphone out of reach of bodies.

I think this is the least expensive and possibly the safest method available.  Again - I strongly suggest that you meet up with one or more of the professional location sound recordists in your area to learn how to make it sound good and what the different microphones bring to the table.  You should be able to figure out fairly quickly what's going to work for your particular environment.

And - yes - the professionals do have a selection of microphones that they choose from depending upon the particular requirements.  Their experience will help you a lot.

A microphone hanging above the people speaking really does work well - that's why that ancient technique is still in use today.

I don't know if any of you ever watch the early episodes of each season of American Idol, but the contestants in all of those auditions are recorded by a mic hanging from a boom pole.  You will occasionally catch glimpses of this when the cameras are moving around.  Same for the Canadian version of the show: Canadian Idol.

dwayne


-- Dwayne Reid   <dwaynerspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\05\18@142304 by Mark E. Skeels

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On 5/18/2011 1:10 PM, Dwayne Reid wrote:
> At 07:02 AM 5/18/2011, Mark E. Skeels wrote:
>
>> My intended solution, as of this moment is:
>>
>>      * Sound deadening curtains on the hard walls within the room.
>>      * Wall mounted or boom mounted microphone out of reach of bodies.
> I think this is the least expensive and possibly the safest method
> available.  Again - I strongly suggest that you meet up with one or
> more of the professional location sound recordists in your area to
> learn how to make it sound good and what the different microphones
> bring to the table.  You should be able to figure out fairly quickly
> what's going to work for your particular environment.
>
> And - yes - the professionals do have a selection of microphones that
> they choose from depending upon the particular requirements.  Their
> experience will help you a lot.
>
Noted: I have a guy in mind I have worked with before.

{Quote hidden}

Thanks, again to you and all for participating

2011\05\18@170045 by Gerhard Fiedler

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Al Shinn wrote:

> Gerhard wrote:
>>Thanks for the quick reply! :)
>>I can imagine the bathtub, but I still have problems imagining this in a
>>swimming pool.
>
> see:
> http://www.webmd.com/baby/news/20030523/pool-owners-warned-of-electrocution-risk

Thanks - interesting. For one, it confirms that it can be dangerous.
OTOH, ~10 incidents per year seems to be low, so I assume that it needs
a special setup for it to be deadly or "serious".

Gerhar

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