Buzzing fixtures

Something to try: (You said it was getting worse.) With only one fixture on at 33%, try rotating the axis of the fixture a bit. The filament and supports may have sagged a bit and you may find some positions are quieter than others.
 
For what it's worth, I used the Ushio lamps almost exclusively in the Source 4's my college had. Now, either I was oblivious and occupied by other things, or we really didn't have much problem with lamp sing. This was in a 250 seat house with rigs over 200 instruments. They were all 575's, so maybe that made some difference.
 
...With only one fixture on at 33%, try rotating the axis of the fixture a bit. ...
You mean along the long (longitudinal) axis of the unit? Like we (are supposed to) do with Xenon Super Trouper globes every 300 hours? PAR64's "spin the bottle"? How would one do that on an S4 PAR or ERS? I can see flipping the lamp in the socket 180°, but one can't do that with a 750W due to that blasted extra locator pin. And I don't think it would be as effective on the segmented short filaments of the HPL, as say the saggy single coiled-coil of an FEL/EHG/EHD.
 
You mean along the long (longitudinal) axis of the unit? Like we (are supposed to) do with Xenon Super Trouper globes every 300 hours? PAR64's "spin the bottle"? How would one do that on an S4 PAR or ERS? I can see flipping the lamp in the socket 180°, but one can't do that with a 750W due to that blasted extra locator pin. And I don't think it would be as effective on the segmented short filaments of the HPL, as say the saggy single coiled-coil of an FEL/EHG/EHD.

No, actually rotate the whole fixture! It's worth a try since the option is free. If it doesn't work, then that's that. If it does, then it's time for some creative aiming. In practice, we like to keep fixtures upright but it may be a solution. Flipping the lamp 180 might also work, although by the time you got it back in there, you may have a hard time comparing to the prior buzz level unless you have a fixture right next to it with about the same amount of buzz. And, as you mentioned, that locator pin stops it on the 750.
 
Hi Folks,

Derek dragged me in as he often does. :confused: We have gotten varied reports on lamp hum over the years on everything from Metal Halide lamps to HPLs/FELs, but have never been able to replicate it in the factory with any consistency to determine a singular cause. The most recent claim we had were on a few CYX lamps (2kw), but when we got them back and tested them in-house we could not notice any noise. As a result the belief is that it could be related to a variety of things as many posters here have theorized-- Angle of the lamp, orientation of the lamp inside the fixture, the rise time and dimmers used, etc. To discover actual cause you would almost need to put a lamp in a fixture with a certain dimmer rise time, have the fixture orientated into a variety of positions, change the dimmer or rise time, repeat for every angle and intensity, etc., etc. And well.. Who has time to do that? Any volunteers? :)

The ceramic base is an interesting theory, and seems like it would make sense-- except our latest instance as I mentioned was on a CYX-- and those are ceramic base lamps as well. I'm not saying the ceramic base doesn't have the potential to reduce the issue, but it certainly isn't the root cause and be-all-end-all. (And I'm not just saying that because our HPLs have metal bases!)

Cheers,

Mark
 
What is Lamp Sing:

Whenever electricity flows through a wire, it produces a magnetic field around it. Coil the wire, and you end up with more wire in the same space. A lamp filament is just that, a coiled wire. As a result, the alternating magnetic field causes to filament to vibrate as it interacts with itself, the metal of the fixture, and any other stray magnetism. With a pure 60 Hz waveform, this low a frequency would be almost inaudible. Unfortunately, phase chop dimmers produce part of another waveform- a square wave. (Both are combined into something called a saw-tooth waveform.) The definition of a square-wave is a sign-wave with every possible harmonic added! On a 60 Hz waveform, these harmonics extend all the way up to radio frequencies. (Actually, in theory to infinity.) To quiet things down a bit, a low pass filter is part of the dimmer. (The choke.) The problem is, the lower the frequency you want to set the pass at, the bigger the choke has to be. In practice, a compromise must be made. The result is that Most audio frequency noise gets through. The "Lamp Sing" usually constitutes 60 Hz, plus about 8 octaves of "sing" above it. That is why such a small thing can be heard so well.

If you look at the waveform of a dimmer, the greatest amount of "square" is found at the mid-point settings. That is why "Lamp Sing" is loudest at middle settings and almost disappears when the lamp is on full.
 
I'm probably a little late in posting this..

I actually work for a performing arts center in southwest Michigan- we have noticed the same thing. We have mounted, for audience lighting, around 100 ETC Source Four ParNels. Only when dimming the lights or raising them do I ever notice the lamp sing you're talking about. Whenever they're at 100%, the sing fades away. I do notice the sing coming back when at anything lower than 90-100%, but it is honestly not a problem for us. No one has ever said anything about it except for us sound techs. Try to keep the bulk of the lights off if possible- we use a number of Source Four ParNels, Zooms, 14 Degree, and 36 Degree lights for stage illumination. However, with your US, I don't know if this would work.
 
Hello again everyone,

Thanks for everyone's input on my buzzing fixtures. Even with 800us dimmers the problem is most definately the dimmers. At 100% intensity they are nice and quiet. I think the ND suggestion is the most viable one for me at the current time. So I'm going to order in some ND and some heat shield and give that a go.

To give everyone a feel for the back lighting issues here, attached is a photo of a concert last weekend. This was taken with 30 instruments (12 S4 PARs and 18 S4 Lekos) lamped at 750 running at 100%. For this show we were mostly gelled with R114 and CTB 1/4.

photo.JPG
 
Similar issue here. I'm working in a very small black box with my lights hanging 3 or 4 feet over the audience. I recently received some Lekos that are lamped with EHG 750 lamps and are Way too loud to use. We are using chauvet dimmer 4packs and running the fixtures at around 25%.
I'm hoping to relamp, but I don't know how low to go. The fixtures light the whole stage so the throw is from 5 - 20 feet. Next show is Friday and my time comitment to this project is pretty limited. Lamp suggestions?
 

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Any chance I can lamp down even more? I've never worked with stage lights lower than 500W

As a matter of fact, it looks like there is a 375w version. I think its code is GLG.


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The chokes are really designed to get rid of the electrical interference a phase-chop dimmer produces. To get down into the audio frequencies, the choke would have to be huge and therefore inefficient. I once had a problem in a very quiet location with a single dimmer (think museum) that I was able to mitigate by installing a 10mfd power-factor cap across the feed to the fixture. It worked but I would not recommend doing that. (All sorts of safety and code concerns as the circuit could hold a charge if the lamp was open and the power was disconnected. Bleeder resistor topic for another day.) Anyway, there are only two dimmer types which will not produce lamp sing. 1) old tech- Autotransformer, dimmer, autotransformer. 2) new tech- Pure sign dimmer, dimmer, sine wave ($$$).
So, now the question about the fast fix. Radically dropping the lamp wattage so that your dimmer is close to 100% would work. Dimmers produce the greatest amount of lamp sing at the lower settings. You also could use a neutral density filter on the fixture to allow the dimmer to be used at a higher setting. I am tempted to suggest borrowing some ETC dimmer-double adapters and make up some cables so they can be used on your lights, but I suspect that would involve too much work. Again, the idea there is the DD adapters work by passing only one half of the waveform, thus letting you use a much higher and quieter dimmer setting. As to what the Chauvet pack would do with a half-wave load, I don't know.
 

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