Complete the DC circuit?

Jay Barracato

Jay Barracato

Graduate Student
Jan 11, 2011
1,528
5
38
Solomons MD
I had one of those throw up your hands at someone elses poor decisions shows the other night.

At a local restaurant/bar, they have a minimal system that was anything but professionally installed. I went in there about 3 weeks ago with a band and at least got everything they had in a reasonable working configuration. All the power was being drawn from a single edison on the wall through a "power conditioner" on the amp rack. This is one of those restaurants that converts into a bar right in front of your eyes and they don't expect anything to come out of the system until the first song of the bands performance, so I was a little limited to how much tweaking I could do. I did fix a couple of things like subgroups are not necessarily for subs, and it is not necessary to patch everything into everything on the board (there was more effects in the monitors than signal).

I went back with the same band last week and as soon as we setup, the singer tells me his mic is shocking him. I shutdown and start checking it out. It turns out someone had replaced the extension cord feeding the whole rig with one of those two wire lamp extension cords, and since the three prong from the rack wouldn't fit into it, they had pulled out the safety ground prong.

Wouldn't you know that this was the one night I didn't have a single of my good extension cords in the car?

A little checking showed the problem was the global phantom power on the mixer. Which led me to start thinking about interfacing AC and DC equipment. I know that the phantom is DC fed along pin 2 and pin 3 with the ground on pin 1. What I am not sure of is the connection inside the mixer between the ground of the XLR jacks and the safety ground of the entire mixer. Why would breaking the ground connection on the AC feed open the DC phantom power circuit? It seems strange to turn the safety ground into a current carrying wire, even if it is only DC. If I were to measure between the safety ground wire of the cable feeding the rack and the ground of the wall jack, should I be able to see that 48 V DC? It seems to me that this DC current could end up all sorts of places you wouldn't want it.
 
Re: Complete the DC circuit?

Phantom power is local to the mixer and has nothing to do with mains power ground.

I would look elsewhere for your shock... The mic is generally grounded to the console, but if that is floating who knows.

A simple VOM should help you troubleshoot.

JR
 
Re: Complete the DC circuit?

What kind of console is it?
only thing I can think of is that the phantom on/off switch actually turns the phantom supply on/off instead of connecting an already energized supply. that supply may be faulty and causing ground currents.

I think your answer may lie in finding out what problem they were trying to solve by ungrounding the system..

Jason
 
Re: Complete the DC circuit?

Du to personal experience, I can testify that pin 1 on a Crest X-vca is tied to the safety ground. I found this out by using a mic cable with the shield connected to pin 1 and touching a cable ramp in the roof that was hot. 115 volts through your fingers is not fun...
 
It is a behringer analog board with global phantom.

Next time there I will re-meter the wall outlet but there was no previous problem at the outlet.

I have no idea why they would have pulled the ground other than to use a 2 wire extension cord.
 
Re: Complete the DC circuit?

48v DC could probably be felt if your hands are very sweaty but generally it is not what we typically are getting shocked by. You can feel 9V on your tongue (but please don't).

JR
 
Re: Complete the DC circuit?

Hi Jay,

Just a shot in the dark; is the stage carpeted? A few set carpenters that I know keep a stash of Static Guard spray in their work boxes for this very reason. I learned about it the hard way with several blasts to the lips while ringing wedges back when I was a puppy. The carpenters rolled with laughter as one of them grabbed a can and started spraying the floor all around my feet. All I could say was, "Really?"
 
Re: Complete the DC circuit?

XLR chassis connector pin #1 should always be connected directly to the chassis at the connector. The AC power safety ground must be connected directly to the chassis in some way that won't loosen or unsolder it's self. These two chassis connections should not be at the same point on the chassis.

The idea of the safety ground is to (are you ready?) connect your equipment to ground!

Musician Safety | No~Shock~Zone
 
Re: Complete the DC circuit?

Kevin Graf said:
XLR chassis connector pin #1 should always be connected directly to the chassis at the connector. The AC power safety ground must be connected directly to the chassis in some way that won't loosen or unsolder it's self. These two chassis connections should not be at the same point on the chassis.

The idea of the safety ground is to (are you ready?) connect your equipment to ground!

Musician Safety | No~Shock~Zone
Click to expand...

Forgive my naivete, but if both the XLR Pin 1 goes to chassis ground, and the AC goes to chassis ground, why would making the connection at the same point on the chassis be any different than two separate connections to the chassis?

thanks
frank
 
Re: Complete the DC circuit?

Jay Barracato said:
I had one of those throw up your hands at someone elses poor decisions shows the other night.

At a local restaurant/bar, they have a minimal system that was anything but professionally installed. I went in there about 3 weeks ago with a band and at least got everything they had in a reasonable working configuration. All the power was being drawn from a single edison on the wall through a "power conditioner" on the amp rack. This is one of those restaurants that converts into a bar right in front of your eyes and they don't expect anything to come out of the system until the first song of the bands performance, so I was a little limited to how much tweaking I could do. I did fix a couple of things like subgroups are not necessarily for subs, and it is not necessary to patch everything into everything on the board (there was more effects in the monitors than signal).

I went back with the same band last week and as soon as we setup, the singer tells me his mic is shocking him. I shutdown and start checking it out. It turns out someone had replaced the extension cord feeding the whole rig with one of those two wire lamp extension cords, and since the three prong from the rack wouldn't fit into it, they had pulled out the safety ground prong.

Wouldn't you know that this was the one night I didn't have a single of my good extension cords in the car?

A little checking showed the problem was the global phantom power on the mixer. Which led me to start thinking about interfacing AC and DC equipment. I know that the phantom is DC fed along pin 2 and pin 3 with the ground on pin 1. What I am not sure of is the connection inside the mixer between the ground of the XLR jacks and the safety ground of the entire mixer. Why would breaking the ground connection on the AC feed open the DC phantom power circuit? It seems strange to turn the safety ground into a current carrying wire, even if it is only DC. If I were to measure between the safety ground wire of the cable feeding the rack and the ground of the wall jack, should I be able to see that 48 V DC? It seems to me that this DC current could end up all sorts of places you wouldn't want it.
Click to expand...

My knowledge of wiring in the US is limited.

So FWIW …. If you use a 3 wire extension cord and the earth is disconnected (broken or cut off) at the plug end, what happens is that the active and neutral act like they are connected by two series capacitors with earth wire connect in the middle dividing the voltage in about half. Because the earth wire is connected at the plug end, and hence the metal part of your equipment you will get a shock.

The current will be limited by the amount of capacitive coupling … and you will need an earth return such as the strings on your guitar. (the strings are connected to earth via the amp which is plugged into an earthed socket at the stage end)

I would suggest you measure the voltage with an AC meter connected from the mic body to an earth somewhere. If you see about half of your normal AC voltage, chances are something like this is happening.


Peter
 
Last edited:
Re: Complete the DC circuit?

frank kayser said:
Forgive my naivete, but if both the XLR Pin 1 goes to chassis ground, and the AC goes to chassis ground, why would making the connection at the same point on the chassis be any different than two separate connections to the chassis?
thanks
frank
Click to expand...

Maybe my statement was too strong.
The XLR Pin 1 should be connected to the chassis right at the connector.
The AC power safety ground should be connected to the chassis near the power supply primary circuit.
The power supply primary circuit and the XLR connectors probably should be separated.

Dave Davenport has a nice paper on the subject:

Audio Component Grounding and Interconnection
David Davenport - 21st April 2010

Audio Component Grounding and Interconnection - diyAudio
 
Re: Complete the DC circuit?

Kevin Graf said:
Maybe my statement was too strong.
The XLR Pin 1 should be connected to the chassis right at the connector.
The AC power safety ground should be connected to the chassis near the power supply primary circuit.
The power supply primary circuit and the XLR connectors probably should be separated.

Dave Davenport has a nice paper on the subject:

Audio Component Grounding and Interconnection
David Davenport - 21st April 2010

Audio Component Grounding and Interconnection - diyAudio
Click to expand...

Merci!
 
Re: Complete the DC circuit?

While pin-1 on the XLR inputs of a console are bonded to the board ground, it's possible it create a phantom voltage on the body of a condenser mic if you accidentally use a pin-1 lifted XLR cable. However the mic won't work. The 48 volts DC on phantom power is current limited, so not really dangerous. But it is really scary to a guitar player putting his lips up to it. I've also seen a bunch of import XLR cables that were simply wired wrong, so I could see how a miswired XLR could place the 48 volt DC on the body of a dynamic mic and it would still operate.

A really cheap gadget to keep in your pocket for gigs is a Fluke VoltAlert or Amprobe NCVT (Non Contact Voltage Tester) like this: VP-600SB
Even though these are rated for 90 to 1,000 volts, it will beep like crazy with as little as 40 volts if you hold it anywhere near a mixing board that's been energized by an ungrounded extension cord or even a RPBG (Reverse Polarity Bootleg Ground) outlet. See No Shock Zone or Hot Skin RV proximity test full scale - YouTube for a video of me testing an electrified RV with a NCVT.

Email me directly if you have any questions at [email protected]
 
Mike,

I have the fluke.

For all,

My question is really more about what is the dc return path in a switch mode power supply.

None of the explanations that treat the mic body and ground as the return path (assuming the guitar rig was the source of the potential difference) explains why the effect comes and goes with the global phantom power and was present even when the singer was not touching the guitar.

The shock was also more like a discharge then a continuous flow.
 
Re: Complete the DC circuit?

Jay Barracato said:
Mike,

I have the fluke.

For all,

My question is really more about what is the dc return path in a switch mode power supply.

None of the explanations that treat the mic body and ground as the return path (assuming the guitar rig was the source of the potential difference) explains why the effect comes and goes with the global phantom power and was present even when the singer was not touching the guitar.

The shock was also more like a discharge then a continuous flow.
Click to expand...

Chances are, it’s not DC, read JR posts. The effects of DC and AC on the body are different. AC is more likely to give you a shock especially at the voltages we are talking about. I think you would be lucky (but you could) to notice phantom power.
“The minimum current a human can feel depends on the current type (AC or DC)and frequency.A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC”
http://en.wikipedia.org/wiki/Electric_shock
 
I did read his post. The effect described to me was very much like licking a 9 volt battery but stronger.

When you lick a battery it gives you the initial zap and then stops as if a buildup of charge has been discharged.

And if it is an AC source why would turning the phantom on and off start and stop the effect? The mic that was being powered by the worked correctly.

On the other hand, if something in the mixer is dumping enough AC current to ground to energize one of the mics wouldn't the other metal parts of the system sharing the same bond also be energized?
 
Top Bottom