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Topic: Slide projector hum
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Michael Rourke
Expert Film Handler
Posts: 159
From: San Luis Obispo, Central Coast of CA
Registered: Feb 2002
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posted 04-14-2002 01:01 AM
The guys who installed my slide projectors from NCN (national cinema nimrods) hooked up to the SLC convenience outlet and the motor so when the motor runs the slide projector turns off. Good thinking.However, when the slide projector is on it creates an audible hum in the speakers. When the SDDS player is on the same circuit I have had people come out and complain about the hum DURING INTERMISSION! Once the motor comes on and the slide projector goes off everything is right with the world again. I assume others have had this problem, and again I assume you have corrected it because you are very smart. Please share your knowledge with me. Thanks. BTW, it's not the 25 pin SDDS cables, like I said if the slide projector is off there are no problems, except that NCN will want it back on.
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Aaron Sisemore
Flaming Ribs beat Reeses Peanut Butter Cups any day!
Posts: 3061
From: Rockwall TX USA
Registered: Sep 1999
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posted 04-14-2002 03:20 AM
Some places with slide projectors have it so bad (humming in the theatres in all formats, humming in the frigging TELEPHONES!!!) that the only way to eliminate the interference is to say goodbye to the slide projectors themselves.First thing to check: Are the slide projectors and *ANY* portion of the audio chain sharing a power phase? Getting the slide machines off the audio phase is the first thing to do, it alone may eliminate the humming. Second thing: Are you using a music distribution system to send nonsync to the various theatres? If not, the daisy-chain nonsync wiring may be the culprit. Separate CD player for each theatre? Once again, look at the signal wiring. -Aaron
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John Anastasio
Master Film Handler
Posts: 325
From: Trenton, NJ, USA
Registered: Sep 2000
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posted 04-14-2002 07:54 AM
I think Craig is right here. What you have is called a GROUND LOOP. It's a real bugaboo in recording studies. In TV stations, it shows up as nasty hum bars in the picture. We solve it by designing the studio so that all circuits in the building are connected to a common ground, eliminating the difference in ground potential between them. Try running the machine on a cord with no earth ground on it. If that solves your problem, then the loop was the cause. Unfortunately, this causes a safety hazard, since that ground is there to protect you in the case of a short in the equipment. Having an isolation transformer would be the correct fix in that case, since it both eliminates the direct ground connection and isolates the circuit so you won't get zapped if there's a short.(Tell your bean counters that it's a safety issue...that usually gets them moving.) The problem is, of course...that the transformer may cause the same problem as the slide projector, but I doubt it.
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