Higher Ground

Last time, we talked about AC power and its obvious importance and impact on your show. If you look at a North American 120 volt outlet, you see three connection points....known as hot, neutral, and ground. The hot and neutral both have to be there in order for power to be delivered to your gear. But it's that third connection, the ground, that can either save your life or play havoc with your show.

Ground loops...almost everyone has heard of 'em. But, do you actually understand what's happening and why it's causing that loud hum in your equipment? Those of you who do...go get yourself a beverage and meet us back here in a few minutes. Everyone else, get ready to roll us your sleeves and delve into this mystery.

Remember Road Tip #3 from last time?

Thou shalt have one and only one path to ground.

First, lets dispelled a very common misconception about ground. Ground is a reference. It's not a value. So, most folks assume ground is zero volts. Well, it may appear to be in a lot of instances. But if you measure the potential between two different ground points, you're going to see a voltage of some magnitude. This is because we live in an imperfect world.
 

OK, here's a really common place for a ground loop to occur.  If you have a sound board out

front and your power amps on stage (as you should), then electrically it looks like this:

The natural thing for your sound man to do is to plug the console into the nearest outlet, but this is what you wind up with.  With a current flowing on the ground, the 60 Hz (50 for some parts of the world) will wind up being introduced into your main signal.  The further away the console is from the amps, the worse it is.  So, how do you fix it?

You have to break the loop somewhere.  And this is where people get into trouble.  They put one of those 3-to-2 wire adapters on either the amp or the mixer (which should just be outlawed because I've never seen anyone use them correctly).  Everything gets quiet and they thing they've fixed the problem.  And while technically they have "fixed" it, they've introduced a very serious potential problem.  

 

The ground wire in an AC power cord is generally 16 gauge wire, which means it can handle 13 amps without failing.  The ground wire in the mic cable that's connecting the mixer and the amp, however, is probably more like 22 or 24 gauge (as the numbers get bigger, the wire gets smaller).  The max current that wire can take is about 2 amps.  At that point, it's probably going to burn up.

 

So, in the event of a short, your ground wire in the mic cable has a good chance of failing...leaving the chassis of mixer energized.  Then when you touch it, YOU become the path to ground.  Not pleasant at all.

 

So, how should this be dealt with?  One way is to plug the mixer into the same outlet as the amp.  This means running an extension cord along with your snake, but if you've prepared your cables correctly then it's no big deal.

 

The best way is to lift the ground on one end of the audio cable (the blue wire in the diagram).  This is what I will do in permanent installations.  Typically, I'll leave it grounded at the console (since everything has to run through that) and lift it at the amp.  That's really the best solution, since everything now has a solid AC ground connection and the loop is broken.

 

There are other solutions available that will accomplish this with varying results.  Using an isolation transformer in the audio link will break the ground loop as well.  This is how you would use a transformer to break the loop in an unbalanced audio line.

The transformer couples the audio (the process is called "mutual inductance" if you really want to get your geek on and Google it).  But sinces there is not direct connection between the two devices, there is no loop.

The only downside to this is that unless you use a high quality transformer, your audio quality will suffer.  By the way...if you've ever wondered what's in a passive DI box, you're looking at it.

There are also "ground filters" out there which use a variety of methods to treat ground loops.  That are IMHO at best band aids, and at worse tourniquets (and you know what happens if you leave a tourniquet on too long).  Let the ground connections do their job protecting you...fix the loop on the audio side.

And now, the question of "why does the mic shock me"?  If you have your mic plugged into a mixer and then into an outlet, then all of those components are at one ground potential.  Then, you plug your guitar into your amp and the amp into an outlet.  Now you've got all of those things at another ground potential (unless you plug them into the same outlet as the mixer).  

So when you have your hands on your strings (which are also grounded on most electric guitars) and walk up to the mic and touch it with your lips, you've just completed the ground loop.  That wonderful tingle you feel is the current flowing!

How do you fix that?  There are a few things you can do.  Some people advocate putting capacitors in series with the ground wire in your guitar going to the bridge (and ultimately the strings).  I won't go into the math behind this (email me if you want to see it), but it still goes against my mantra of not jacking with a ground wire.

I've seen some guys carry their own "uni-ground" power distro systems.  It amounts to a series of outlet boxes that plug into an outlet close by, but only use the hot and neutral.  The ground wire then snakes between the boxes and ultimately ties to one plug.  It works just fine on paper, but it's a cluster to carry around and deploy.

The one sure way I've found over the years to solve the "gettin' shocked by the mic" syndrome is to use a wireless device...either with the mic or on your guitar.  Then, you've absolutely broken the ground path between the guitar and the amp.  There are some really decent wireless guitar systems out there now for under $200 (unlike my first one I got in the mid 80's for $2000).  Plus, you've got one less cable to tangle up on stage.

Next week, let's get back into the audio world (because let's face it...it's a lot more exciting than talking about power and ground).  In fact, I see a few weeks in sound world coming down the pipe.

And I've received a couple of emails asking this, so I'll bring it up here.  If there's something specific you would like to see addressed, let me know.  I've got a list of topics I'm planning to write about that goes into mid 2013, but they're in no specific order.  Feel free to drop me a line any time if there's a problem or situation you're trying to work through.

Until next week, keep the meters out of the red.

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at kcarvertx@gmail.com