Electricity, ground loops, hum

More on electricity: powering your stuff, reducing noise, etc.

Be aware that 120V AC CAN kill you extremely dead.  If you’re not comfortable working with it, or you’re not sure how to go about it, have an expert work with you!  Even things that “should be” safe can be deadly under certain conditions.  These are by no means “The Final Word” on electricity, grounding, or safety.  They are simply methods I have used successfully over the years, along with some of the reasoning why I did it that way.

Pure AC (Alternating Current) is a sine wave (60Hz in the US).  A sine wave is a nice smooth waveform, with no harmonics.  A smooth waveform tends to not radiate noise nearly as much as a waveform that has any sharp edges.  However, a waveform that has sharp edges will radiate noise, and can introduce that noise into other cables that are run too close.  An example of this is the infamous buzz that a lighting system causes. A  light dimmer works by cutting off parts of the AC waveform, until it is no longer a sine wave.  This has the effect of reducing the RMS value of the applied voltage, thereby reducing the heating of the light filament, and hence, the brightness of the light.  The resulting waveform has lots of sharp spikes, and these spikes cause an electrical field around the lighting cable that collapses rapidly, which can induce this noise into nearby cables.  Those same spikes also cause RF noise which is picked up by all sorts of things, including single-coil guitar pickups, poorly shielded cables everywhere, and bad solder joints in anything.

How can you avoid this?  The easiest way to avoid a lot of it is to make sure that there are no audio cables that run close to, and parallel to, any lighting cables.  Also, make sure that all of your audio cables are well shielded, and that your grounding scheme is solid.  The shielding of the audio cables will “intercept” this noise, and send it through the shield to ground.

You might think that the “best” way to get a good grounding scheme is to simply ground everything, but that’s not necessarily the case.  Remember that wire (any wire) has a certain resistance per foot, and that causes a “voltage drop” across the wire.  This means that the voltage at one end of the wire is not exactly the same as the voltage at the other end!  The longer the wire, the smaller the diameter of the wire, or the more current flowing through the wire, the more voltage drop there is across that wire.

If two audio devices are both grounded through their AC cords, and both ends of the audio wire’s shield are connected, that means that as soon as ANY current flows through that shield, the two ends of the shield are at different voltages.  It can be a tiny difference, but it’s enough to cause a very irritating hum (called a ground loop).

There are many opinions on the best grounding scheme. Some work better than others, and some that seem like they should work, don’t.  Certain equipment works better with different methods than other equipment, even though they have nearly identical specs.  Sometimes it seems like some variety of black magic, since you’ll try something that “can’t possibly work,” and it will fix a problem completely.

An electrical ground serves two purposes: it provides a path so noise can be shunted to ground, and not be introduced into the audio path.  Secondly, it provides a path to ground for current in case a piece of equipment develops a fault.  Obviously, you need a good, low resistance path to ground to accomplish both of these.  But, you can’t just ground everything, and have all the shields of all the audio cables connected at both ends, or you end up with the previously mentioned ground loop.

Some pieces (usually power amps) have a removable link on them which isolates the audio ground from the chassis ground. If your equipment has these, try isolating the grounds using these.  In a lot of cases, this will work, and it’s certainly a lot easier.  In other cases, you’ll need to be a little more creative, and plan it all out.

In a typical show setup, you can have an audio path that is going to have several pieces of gear on stage that plug into the AC (bass amps, keyboards, etc) that will also be directly connected to the PA using direct boxes, or outputs in the amps or keyboards themselves.  They will go through a mic cable to a splitter onstage which will “split” the signal, sending it to both a monitor console, and the house console through 150′-200′ of snake.  There, the signal goes into the mic preamp of the house console.  There may be outboard devices used as insert effects on this channel, and are also certainly outboard effects plugged into AUX sends of the board, returning into other channels or effects returns.  The signal then goes out of the console, into a graphic EQ, and then back through 150′-200′ of snake to a crossover, which divides the signal into various frequency bands which it then sends to 3, 4, or 5 different amps to power different components of a full-range cabinet.

The monitor system looks a lot like this, multiplied tenfold.  The consoles look much the same, except instead of each channel sending its signal to one bus, it can have 8, 10, 12, or more “matrix’ bus outputs, which are sent to 8, 10, or 12 totally individual EQ/crossover/amp combinations, each feeding a stage monitor or side-fill.

Obviously, if every piece of equipment was grounded, and the shield (ground) of every cable was connected at both ends, there would be multiple ground loops, and the entire system would be unusable.

You need to decide how to separate the grounds, while providing safety, and still have a good path to ground to shunt induced noise to.

You have to take into consideration several things: how much current a device draws from the wall (and potentially how much it could dump onto the shield of an audio cable), how long the cables are that feed the audio to it, how susceptible it is to noise, and how central it is to other, nearby pieces of equipment.

For example, you would want a bass guitar amp to be grounded through it’s AC cord.  It is a high current device, guitar/bass pickups tend to generate a lot of noise, and it will be feeding 20′-50′ of mic cable before it ‘hits’ anything else (the splitter).  The direct box you plug the bass into will have a ground lift switch which disconnect pin 1 (the shield/ground) of the XLR which feeds the mic cable.

The splitter (the very next thing the signal sees) should have ground lift switches which can isolate the monitor console from the house console.  The signal goes through the snake to the house console, which has a dozen or so effects units connected to it, and then goes straight into a graphic EQ.  Since the console is at the heart of this grouping, and is also the highest current draw at the mix position, it should be grounded through its AC cord.

Now you have a choice: you can either ground all of the other pieces of gear, and “lift” the grounds by disconnecting pin 1 on the XLR connector ON ONE END of each cable, or you can connect all of the pin 1s and lift the AC ground of all the effect units (and the EQ).  Since the cable lengths are short, and effects units draw very little current, there is little safety risk in doing this, as either way provides a good ground to all of the devices while preventing ground loops.  It can be somewhat of a trial and error process to find out what works best with your equipment.  Just be sure that every cable has the shield connected at at least one end, and that it is connected at the end where a grounded piece of gear is.

Pin1 should DEFINITELY be connected on the snake channel which then runs from the output of your EQ back up to the amp racks.  Disconnect pin 1 at the amp rack end.

Since the power amps most likely have the ground link, I strongly suggest you try using it to isolate the grounds, and then ground every amp through its AC cord.  They are EXTREMELY high current devices, and require a very good path to ground for safety reasons.  The crossover can be grounded, since it is isolated from the house console ground by the disconnection of the XLR Pin 1, and from the amps’ grounds by the ground link on the amps themselves.

The same grounding technique holds true for the monitor system, even though you are feeding 8 or 10 EQs into 8 or 10 crossover/amp setups.

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