Complete the DC circuit?

Re: Complete the DC circuit?

I've actually found one of my "bad" XLR cables that was mis-wired with the twisted pair on pins 1 and 2, and the shield on pin 3. When you turn on the phantom power, the shield is energized at 48 volts DC with respect to the mixer chassis ground. And, of course, anything grounded on stage such as a guitar amp will provide a shock path to a musician. This will hum a lot if the cable is long, or course. But a cable mis-wired like this won't show up even on a standard cable tester. I'll bet if he took a volt-meter and probed between the chassis ground of the mixer and the body of a mic, it would read 48 volts DC as he switched the phantom power on. A mis-wired cable is the only explanation I can think of.

And yes, DC voltage is much less dangerous to a human compared to AC voltage at 60 Hz. Yes, Edison was actually right about Westinghouse's AC power being a killer, and his DC power was actually much safer. Of course, AC power won out and the rest is history.

Mike Sokol
 
Mike, you may have the practical answer but the cable is mixed in with all the other band stuff and I am only likely to find it by accident.

I still am interested in what serves as the ground side of a dc circuit where the current source is a switch mode filter. Every sample schematic I found by searching just showed a generic ground with no mention if that ground needs to be isolated from the ac ground on the other side of the filter.
 
Re: Complete the DC circuit?

Jay Barracato said:
I still am interested in what serves as the ground side of a dc circuit where the current source is a switch mode filter. Every sample schematic I found by searching just showed a generic ground with no mention if that ground needs to be isolated from the ac ground on the other side of the filter.
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If you're referring to the separate ground planes found in my many audio devices with digitally switched supplies, then that's pretty simple. For instance, in many Sony professional video cameras there are 3 separated ground planes; one for the chassis (external) ground tied to the safety (earth) ground if you're plugged into a wall outlet, one for the analog audio (XLR pin 1), and one for the on-board switcher supplies that provide all sorts of voltage from the 12-volt DC battery pack. Both the audio and switcher power supply ground planes are tied to the chassis via a capacitor/inductor/resistor network. Essentially this form a high-pass shorting filter somewhere in the 1K Hz area if memory serves (via that series capacitor) with a small leakage current (series resistor) to keep the ground planes in the same general potential voltage. Some also include a small inductor to low-pass filter the highest frequencies of the switcher supply and keep sub-harmonics out of the audio ground (and sound)

if you accidentally plug in a male XLR mic cable with the shell tied to pin-1, you'll short audio audio ground plane to the chassis ground plane, and you'll hear the switcher supply sub-harmonics leaking into your audio (and how do I know this? :lol: )

So when troubleshooting these things, you can't think that all grounds are the same. Cross connecting them accidentally usually won't cause any physical circuit damage, but will introduce switching power supply noise into your audio, which can drive you CRAZY!!!

Is that what you're asking about?
 
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Re: Complete the DC circuit?

Jason Glass said:
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.
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I saw this static effect personally at a church a few years ago while troubleshooting their podium mic. Their sound tech complained there was a "short" in the gooseneck of the mic which would "pop" when anybody adjusted it. I watched it happen from back at the console, and was a little puzzled why it only seemed to "pop" one time, then would be pop free for a while. When I walked up and adjusted the mic myself I got a serious static shock which then made the big "pop" in the sound system. Looking down I noticed the carpet and asked them how long the mic had been misbehaving, which we found coincided with the new cleaning guy who was steam cleaning the carpets, but not spraying any anti-static compound after cleaning. Yikes!!!! 8O~8-O~:shock:

When troubleshooting wacky happenings in sound systems (or anything else), it's always best to try to experience the event yourself. That is, I try to observe the failure while watching everything else in the room. Once you see a correlation, it's pretty easy to determine cause and effect. So if the shock only occurs with the phantom power on, but not when off, then it's time to meter from the earth to the body of the offending mic. Understanding grounding is also a good thing for sound system troubleshooting since ground plane differential voltages are what causes electric shocks to humans. For those of you not familiar with using meters to measure voltage and and concepts of shocks on stage, here's a pretty good primer I wrote on the subject: Stage Electrical Safety
 
Re: Complete the DC circuit?

Mike Sokol said:
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if you accidentally plug in a male XLR mic cable with the shell tied to pin-1, you'll short audio audio ground plane to the chassis ground plane, and you'll hear the switcher supply sub-harmonics leaking into your audio (and how do I know this? :lol: )

So when troubleshooting these things, you can't think that all grounds are the same. Cross connecting them accidentally usually won't cause any physical circuit damage, but will introduce switching power supply noise into your audio, which can drive you CRAZY!!!
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That's because the unit has the XLR chassis connector's pin-1 tied to audio ground rather than directly to the chassis. The XLR pin-1 is just a shield, it's not audio ground, but lots of equipment still does it wrong! The AES standard has the shell unconnected.
 
Re: Complete the DC circuit?

Kevin Graf said:
That's because the unit has the XLR chassis connector's pin-1 tied to audio ground rather than directly to the chassis. The XLR pin-1 is just a shield, it's not audio ground, but lots of equipment still does it wrong! The AES standard has the shell unconnected.
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Yes, you're correct, the XLR shell is supposed to be isolated from the pin-1 shield connection. However, I've seen a lot of import XLR cables that have the shell bonded to pin-1 from the factory, which as you mention is against AES standards. Note that pin-1 connected to the shell can short across the metal body of a DI box that has a metal body XLR jack. That takes the ground-lift switch out of the circuit so it can't open up the XLR to Phone shield path as intended. So the next time you can't get rid of ground loop hum by flipping the ground/lift switch on your DI box, open up the XLR female cable connector and see if the shield is also bonded to the XLR shell tab. If so, cut it loose and you'll be lifting ground loops properly.
 
Re: Complete the DC circuit?

Mike Sokol said:
...[T]next time you can't get rid of ground loop hum by flipping the ground/lift switch on your DI box, open up the XLR female cable connector and see if the shield is also bonded to the XLR shell tab. If so, cut it loose and you'll be lifting ground loops properly.
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I can't tell you how many times I have done that...at my current job, I went through each cable individually and did that - no noise problems since (at least from that).
 
I returned to the scene of the crime Thursday night but without trying to use any of the condenser mics.

I bypassed the little cord that was powering everything but the safety ground is still missing on the power supply for the board and amps.

Strangely, the phantom power circuit stays on (at least according to the LED) for a good 5 minutes after the phantom is turned off OR after the entire board is shut off.
 
Re: Complete the DC circuit?

Jay Barracato said:
I bypassed the little cord that was powering everything but the safety ground is still missing on the power supply for the board and amps.
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If you don't have any safety ground from the mixer or amp rack, then your local "ground plane" really isn't earth grounded and could become dangerous at any time. I think you should consider making your own "grounding plug". I built something like this 30 years ago while playing in rat-hole clubs that had no grounded outlets in the wall. Instead of putting up with the shocks on stage, I built a simple grounding system which consisted of a pair of vice-grip pliers on a 250 ft spool of 10 gauge wire terminated on a lug connector. Since there were no outlet grounds to be found, I ran the "ground wire" vice-grips to a cold water pipe in the basement, then connected the ground lug under a screw on the metal body of the mixing board. Since the XLR shield connected the chassis of the mixer to pin-1 and the chassis on the power amps, everything was safety grounded. Now, this wasn't the best grounding system in the world, and not exactly to code, but it worked quite well.

For your situation, you should already have grounded outlets, so you either need to bring your own Edison-Euro power cable with an intact ground pin, or come up with a kludge cable that will get the safety ground of the wall outlet to the chassis of your mixing board. I'm guessing that the house "engineer" got tired of ground loop hum and simply cut off all of the power cord ground pins, which is not only illegal, but a serious safety issue for the performers on stage. I do know you don't want to make too many waves with club owners since they'll find another band who's willing to put up with the shocks, but that just perpetrates the assumption that getting shocked on stage is OK, when it's not really safe.

Contact me directly at [email protected] if you want some pointers on how to do this.
 
I mentioned the LED because the shock was just like a cap discharging rather than the tingle directly from a dc source.

The last show was a quick throw and go because everyone was late sue to a bad accident but in the future it will be simple to just unplug from the powersupply that has lost the ground and plug in one of my extension cords. Or simply chop off the molded plug and replace it with a good one.
 
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