This is topic Slide projector hum in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Michael Rourke (Member # 1142) on 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.
Posted by Aaron Sisemore (Member # 145) on 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
Posted by Craig Hanham (Member # 1010) on 04-14-2002, 04:04 AM:
Sounds a lot like earth hum.
Simple test, not recommended for permanent fix or if you are not an electrician, anyway lift earth wire from projector power plug. If hum gone all good, see below.
Like Aaron said above get slide off same phase as sound gear.
If thats not easy invest in an isolating transformer. No doubt sodding accountant will moan, so go for the 'run new power outlet to slid pro' option (coz have an extension lead running hither and yon aint sfe and leads to all sorts of cock ups) What with what sparkies charge ...... 
Posted by John Anastasio (Member # 607) on 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.
Posted by Bruce Hansen (Member # 281) on 04-14-2002, 07:23 PM:
Is your slide projector sitting ontop of any of your audio equipment? If so, the motor coil, or a transformer could be inducing hum into the audio equipment. Or did someone wire a neutral together with a ground on your power to the slide projector? That can cause there to be voltage on the ground wire when the slide projector is running.
Posted by Frank Angel (Member # 248) on 04-15-2002, 11:35 AM:
Are your showing slide ads with this slide projector? That may be the culprit. This is just Mother Physics telling you never to use slide projectors to project ads in a movie theatre. 