Shoebox Dimming ~ Ground Hum

[Lotos]

Alright, so here's the real "Uhhh....." moment of my week, perhaps the inquiring minds around here might be able to answer my riddles three... (Or one, really...)

We just opened (second preview tonight) our current show, and tech is pretty much nailed down at this point. In amongst the insane amount of lx that my crew(s) have managed to pull off over the past 4 days, are 240 MR-16 EXT Lamps, wired in series circuits of 10.
These circuits, for the most part, are working wonderfully, and are powered via several NSI DDS 5600 shoebox dimmers.

One of the channels (and only one) are causing an interesting ground hum through the main PA... This has happily stumped myself, our TD, and our Head of Audio...
Let me give you some details:

The sound console is located at the rear of our room, on clean conditioned power.
The lighting console is located nearby, on seperate power.
The shoebox dimmer in question is located under the stage, powered from a breaker panel backstage.
The lines from the circuit of 10 MR-16's runs directly to the dimmer, avoiding all audio cable on the way.

So naturally, when the hum showed up, we were confused... Especially when the audio operator stated that he had almost everything (including everything on deck) muted.
The problem went away when the main outputs of the sound console (Innovason SY80) were muted, so the problem wasn't beyond the console.
Finally the errant input was found, our SFX Computer's line in. (Also running on conditioned power)

The SFX computer is powered via a circuit in the balcony (currently closed, thus we have prime Stage Management seating right now).
When traced, this routes back to a breaker panel in the theatre's electrical room, running off a seperate transformer than the backstage breaker panel powering the shoebox dimmer.
The feeds would only cross at the pre-transformer point...

So I'm stumped... What on earth is causing this wonderful ground hum we're hearing?!?!?!
I mean, obviously it's that circuit of lights, and obviously the SFX computer's lines are producing it...

But how?!

 

[JD]

Only one channel in the pack? The rest of the pack is ok? Nothing special about how the wires are run to that one channel?

If so, shorted choke in the shoebox.

Confirm by swapping loads from that channel and the one next to it. If it stays with that physical channel, it is confirmed.
If it stays with the load, then you overlooked something in that MR bank. (Ground to neutral short on that string of lamps?)

 

[Lotos]

Rest of the pack is fine (4 Chan Pack), nothing special about the wiring.

Pack just returned from service for an (unrelated) DMX Control Problem... Suspect it is not the choke (Do the NSI DDS series even have chokes?), but when I get a chance to crawl under the deck and get at the pack, I will swap the load around to see.
Not a ground to neutral short, checked for and eliminated as a possibility.

Still perplexed how the hum is backing up so far, and getting to the SFX line.

 

[Aman121]

Could the lighting console itself be causing the issue somehow? Are the console and computer on the same power strip or circuit? It's about as equally far fetched as one highly isolated dimmer causing trouble but it might be something to investigate.
Also, while obviously not advocated for long term use, maybe try a ground lifter on the computer just to see if it makes a difference?

 

[SteveB]

Rest of the pack is fine (4 Chan Pack), nothing special about the wiring.

Pack just returned from service for an (unrelated) DMX Control Problem... Suspect it is not the choke (Do the NSI DDS series even have chokes?), but when I get a chance to crawl under the deck and get at the pack, I will swap the load around to see.
Not a ground to neutral short, checked for and eliminated as a possibility.

Still perplexed how the hum is backing up so far, and getting to the SFX line.

Dave Fox of ETC does/did a great class on troubleshooting. Essential premise, look for the simple solution first.

Do not assume that because the pack was just serviced that they were aware there might be a problem unrelated to the DMX issue and they only might have looked for and fixed that.

Do not assume the problem is anywhere else, console, control cables, whatever.

Do not assume the problem is anywhere else except the dimmer in question. Check that first my swapping the load to another dimmer as John D has suggested. If the problem migrates to another dimmer, then again, as John has suggested check the wiring in the circuit. Something made you narrow it down to "One of the channels (and only one) are causing an interesting ground hum through the main PA", so trouble shoot that. It does sound like induced voltage on the grounding system.

 

[teqniqal]

Use of "Ground lifters" (except for brief periods of trouble-shooting) are never a good idea from an electrical safety standpoint.

Is the power cable feeding the portable dimmer pack properly grounded? Is the power receptacle at the wall properly grounded? Swapped ground and neutral wiring problems are not easily detected (a 3-lamp power plug tester will NOT tell you if this is correct.)

DMX signals are just like microphone and line level audio signals (they are both shielded balanced signal transport systems) and they can suffer from 'pin-1' problems, too. Is it possible that one of your DMX cables or the dimmer pack has a shorted or open signal shield (drain, screen) wire at one end or the other? A 5-wire cable checker can verify this. The problem may be intermittent, so wiggling the cable ends and connectors while they are plugged into the tester is always prudent.

Since the dimmer pack was recently serviced, it is possible that the shielding connection for the date connector pin-1 was corrupted (could be a missing screw, or broken / fractured soldier trace).

An opto-isolator on your DMX line might solve the problem, too.

Alternatively, you might look at adding an isolation box (direct box) like the Radial Engineering JPC (Radial JPC? - Computer Direct Box) between the computer and the audio mixing board. Computer sound cards are notorious for transferring computer ground buss issues to an otherwise good sound system (laptops, tablets, and desktop computers can all do this, regardless of brand).

 

[FMEng]

There are times when the exact nature of ground loops, such as this, just cannot be easily explained. Chances are there is nothing really wrong with the lighting system. I would simply move the SFX computer to the same circuit powering the audio console. Separating the two serves no useful purpose and it would take a pretty big pile of audio equipment to overload the circuit. If that cannot be done, ensure that the audio interface between computer and console is fully balanced and drop the shield connections at one end. If neither of those quiet the hum, the ultimate weapon would be a good audio transformer between the two, such as a Jensen Isomax. Common mode currents cannot flow through a transformer with a Farady shield. What kind of sound card is the SFX computer using?

 

[JD]

There are times when the exact nature of ground loops, such as this, just cannot be easily explained. Chances are there is nothing really wrong with the lighting system. I would simply move the SFX computer to the same circuit powering the audio console. Separating the two serves no useful purpose and it would take a pretty big pile of audio equipment to overload the circuit. If that cannot be done, ensure that the audio interface between computer and console is fully balanced and drop the shield connections at one end. If neither of those quiet the hum, the ultimate weapon would be a good audio transformer between the two, such as a Jensen Isomax. Common mode currents cannot flow through a transformer with a Farady shield. What kind of sound card is the SFX computer using?

Keep in mind, if it is only the one physical channel on the dimmer pack, you may not be hearing a "ground loop" per say. What you may be hearing is a powerful radio frequency broadcast modulated at 60hz. This is why I bring the choke into question. It is the job of the choke to filter out all these higher harmonics. The one pack I once serviced was doing the same thing. In that case, a solder splash was bypassing the choke.

Time for a little exploration of why the choke is there:
The modified sawtooth waveform produced by forward phase dimmers is part sine wave, part square wave. Remember, a square wave is defined as a sine wave with every possible harmonic added. To make dimmers FCC compliant, as well as non-intrusive to other equipment, a choke is added to the circuit as a low pass filter. The choke takes the edge off the square wave portion of the waveform, limiting the harmonics. Cheap dimmers have small chokes that only chop off the RF. Better dimmers have bigger chokes that limit many more of the harmonics. The choke is also a compromise. Too big, and the dimmer starts dropping voltage (efficiency.) Ideally, the choke should remove RF harmonics and enough of the higher audio frequencies as to minimize "lamp sing", but small enough to allow for good efficiency.

As has been stated by others, time to divide and conquer this problem. Start by swapping loads. This will either confirm or eliminate the problem being with the pack.

 

[Lotos]

Alright, had some time today to spend crawling around under the deck, and work on this... And I'm still stumped.

SFX PC moved to different power, hum gone. Okay, so the problem is with that circuit.
That power circuit is still isolated from the dimmer pack all the way back to the transformer, but we now have a "workaround" solution... Though no idea of what's gone wrong.

SFX PC moved back to inital circuit for more testing.

Load moved to different dimmer, completely seperate shoebox pack... Problem moves with load.

Load has now been tested (just under half an hour to pull everything apart and check it), fixture by fixture, for any possible shorts, none found.
Ground temporarily lifted on the load, problem persists.

*scratches head*

 

[teqniqal]

Alex, it sounds like you are being methodical. That's good. When checking each load, particularly the one that moves the noise along with it, did you check to see if the line, neutral, and ground pins all connect to the correct parts of the fixture(s)? 'No shorts' is not the same thing as 'wrong connection'. The ground pin to the lighting instrument chassis is pretty easy to check The reversal of the neutral and ground would make the power coming into the lamp on the line (hot) return back through the ground conductor, and the chassis would be on the neutral conductor. In theory they are both connected together back at the transformer, but it could wreak havoc on your grounds from a noise standpoint.

A reversed neutral and line (hot) on a lamp socket isn't likely to cause a noise problem, however, it can make the fixture more dangerous should you be violating the rule about working on it while it's plugged-in. Should you reach-in to grab the lamp and touch the outer lamp base you could get zapped (or worse).

A reversed hot and ground usually shows up real fast as the line (hot) feed goes to the mounting pipe (which is usually well grounded) - and pop goes the breaker.

Thanks for the update and diligence!

 

[JD]

Problem moves with load.

There's your clue. You just cleared everything else on the stage of fault. Your answer is somewhere on that circuit run or fixture.

Next divide:
At the load end of the cable run, plug in a different fixture:

If the hum stays, then there is something about how that cable is run (or location) that is allowing it to broadcast RFI or induce hum into the audio system.

If the hum goes, something is going on inside that fixture or string.

Remember: When we think of "ground loop" we think of a voltage being induced into the ground path. This is only one of four ways that lighting can cause noise in the sound system.

#2 Direct induction- The current flowing through the hot or neutral is producing a magnetic induction in a nearby audio line or signal transformer (think Direct Box or Hi-z/Low-z adapter.)
#3 Supply noise- Noise from the dimmer packs is finding it's way into audio equipment through the power distribution source.
#4 Radio Frequency Broadcast- RF noise is "transmitting" from lighting cable or fixture and being received by an audio cable. (Usually not a problem if your filter chokes are working correctly.)

 

[Lotos]

Well, replacing the bulbs solved it.
(See: Inserting a new bulb in Fixture #1 on the strip, turn on, hear buzz, insert Fixture #1's bulb into Fixture #2, etc)

It was a bulb halfway down the strip causing it...

Still confused, ridiculously confused... Perhaps more-so than when I started...
But the buzz is gone...

 

[JD]

Take a meter and measure the lamp in both directions. Can't say I've seen this in MR lamps, but some lamps contain a diode to halve the waveform allowing the lamp to operate at a higher voltage then the design of the filament would allow. This could raise havoc on a dimmer that wasn't designed for it. In any case, something is unique about that lamp! Glad the problem is solved.