line to line noise coupling

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garyspence@charter.net

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Have hum in audio and video projectors!

Wiring tray, stuffed tightly full, has switching dimmer power and load power mixed with AV wires!

Dimmer power can be in the 20 to 30 amp range.

Runs are 4 to 5 feet in parallel! Is there likely to be sufficient coupling into the AV leads to cause hum in the AV systems?

Thanks for any thoughts or info.

Gary
 
Gary, start with where you are, geographically. (Did you ever live in Richmond? I knew a Gary Spence.)

Yes, you're clearly on the right track. Analog audio and video do have their weaknesses. A few experiments:

Does the hum vary with operation of the dimmers? What if you de-energize the lighting power supply?

Is your AV cabling in good shape? Are you using balanced audio lines? Can you move the cabling around?

Troubleshooting is all about a logical process of elimination.
 
081114-0812 EST

Twisted pair wiring will reduce magnetic coupling. Conductive shielding will reduce capacitive coupling. Conductive magnetic shielding will reduce both magnetic and capacitive coupling.

Then there is ground path noise resulting from a difference in the common (ground or chassis) voltage between the two ends of the communication path. This is solved by electrical isolation or good differential line drivers and receivers. Here transformer isolation, shielded twisted pair, and no connection of either signal line to common is a good solution for audio. Connection of one signal line at one end would be OK.

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line to line coupling

line to line coupling

Am in Mankato Minnesota, 80 miles south of Minneapolis.

Also I am an electrical/electronic engineer (approx 50 yrs) with some experience in noise problems but not enough experinece to estimate how bad some of the things in the system may be.

This is a new church building and all things were not done properly!

Am aware of the grounding and various type of inputs. Some lines are balanced and some are not! Have solved much of the audio problems by removing/isolating stray grounds but still have hum bars in video projectors. They are being driven by VGA signals which are not isolated from ground. Noise is definately coming in on the signal lines. If we use only the internal menus in the projectors there is no noise!

There are two sets of dimmers. One set is 4 Leprecon 12 channel running the house and lobby incandescent lights. The other set consists of 7 ETC 6 channel dimmers mounted on the catwalks above the stage running the incandescent spots etc. All dimmers are controlled by DMX.

We have definately determined that the noise is from both sets of the dimmers. All-on or all-off produces no noise but in dimming range there is 60/120 cycle noise.

The electrical equipment room, back stage, has three large breaker panels side by side. One panel powers the Leprecon dimmers and one panel powers the ETC dimmers. The other panel powers the sound/video equipment on the stage and sound booth. This panel has all isoground connections including the grounds and neutrals. We are still double checking all the equipment for stray grounds to the building ground.

We discovered that the electrical connections for the three panels and the Leprcons all have leads running tightly bundeled together in a cable tray above the three panels and the Leprecons! The outputs and input power to the Leprecons, AV wires to iso grounded outlets, and power (208 three phase with neutral) to the ETC units. This is a tray approx 6" by 6" of approx 6 to 7feet long.

It appears that the tray is not wired to code as the tray is STUFFED full. It is likely that this wiring will have to be changed.

My thought is that this tightly bundled group of seperate wires may be coupling noise from the dimmers into the AV lines.

It most likely is but I don't have any way of telling how bad the coupling may be and if the wiring were to be seperated will this help our noise problem?

Don't have any experience to be able to even guess how much coupling there could be.

Thanks for any thoughts or info.

Gary
 
Theatrical light dimmers have always been a problem for the Audio/Video department (I have heard the newest dimmer designs are much better).
Seems like you have two questions:
a) Is the AC power wiring both safe and legal.
b) How to keep the AC power from interfering with the A/V.
For the A/V situation distance is your friend. The farther you can move the signal cables from the power wires the better. Some audio equipment is more sensitive to EMI than other units (depends on just how the input and output circuits were designed). As for the AC power wires, I would group all the associated wires together (i.e. all dimmer wires together, all HVAC wires together).
 
Line to Line ground noise

Line to Line ground noise

We have determined that there is coupling. We measured one ground lead with it disconnected but still in the cable tray with the dimmer power leads.

It measured 8 volts peak to peak with dimmer noise distorting the sine wave.

After we pulled the ground lead, of approx 8 feet in length, from the tray there was no noise! Just goes to show that theory still works!

Thanks for all the thoughts.

Now we have to figure out how to solve the problem and who is going to pay!

Gary
 
081122-21720 EST

garyspence@charter.net:

I do not believed you have proved anything.

If what you mean by
"We measured one ground lead with it disconnected but still in the cable tray with the dimmer power leads."
is that both ends of a wire or shield were disconnected from anything, then you have simply created one plate of a capacitor that couples to the other wires in the tray. Your result is to be expected.

If you have a shielded cable in this tray, ground the shield to a ground terminal on the scope. The other end of the shield should be floating. Connect a wire that is inside the shield and is unconnected at both ends to the scope input terminal, then I expect you will see a much lower signal and you may not even see any signal or noise. You might need some shunt resistance and capacitance at the scope input to reduce broadcast signals.

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line to line coupling

line to line coupling

The ground wire we pulled out is simply a single conductor.

However your suggestion for a shielded wire is a good thought. But there are not any shielded wires in the tray. Could possibly string a lenght of shielded wire into the tray and look for circulating current with both ends of the wire grounded. This would indicate any magnetic connection. Have a small toroid with a bunch of turns which I could run the wire thru and check with the scope.

Have a meeting Tuesday with the engineering firm that designed the system. Will wait and see what they have to say.

Happy Thanksgiving

Thanks

Gary
 
081123-1016 EST

garyspence@charter.net:

A coaxial cable will somwhat cancel magnetic coupled noise but a shielded twisted pair may do a better job of magnetic cancelation. For electric field coupling (capacitive) a double shielded wire will probably be better than a single shield, and a foil shield (Beldfoil) is probably better than a braided shield.

A separte major problem is is using unbalanced signal lines, In other words a system where one side of the signal path is ground or a ground conductor. Ground may or may not mean earth, and earth is a very poor conductor. Many ground rods to earth may be greater than 10 ohms.

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garyspence@charter.net said:
Have hum in audio and video projectors!

Wiring tray, stuffed tightly full, has switching dimmer power and load power mixed with AV wires!

Dimmer power can be in the 20 to 30 amp range.

Runs are 4 to 5 feet in parallel! Is there likely to be sufficient coupling into the AV leads to cause hum in the AV systems?

Thanks for any thoughts or info.

Gary
Click to expand...

I suggest not running youre cable in this tray, it is already missused. Fix the tray or pick another route.
 
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