Film-Tech Forum ARCHIVE: Grounding 35mm Sound

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Posted by James Michael Smith (Member # 4372) on 09-04-2007, 01:22 PM:

Hey guys,
I'm running and nonprofit cinema in Columbia, South Carolina. There is a hum that i'm thinking might be the result of the sound head not being grounded. Where do I ground into in the sound head and then in the projection booth. If anyone has a suggestions, let me know. Thanks!

James Smith
Director of Theater Operations
Nickelodeon Theatre

Posted by Rick Raskin (Member # 1561) on 09-04-2007, 04:50 PM:

Welcome to this forum. You may want to give a little more information. What type of sound heads are you using? Are they using exciter lamps or led readers? An old problem with exciter lamps was that the emergency lamp supply sometimes was fed directly from low voltage AC. In that case 60HZ hum would be heard in the sound until the supply was fixed. Another issue could be the exciter supply itself. I also saw a problem once where the feeds to the lamphouse rectifiers were run in the same trough with the sound feed. That also was causing hum. A little more info and the folks here will probably nail your problem.

Posted by James Michael Smith (Member # 4372) on 09-04-2007, 08:07 PM:

Hey,
I am using Red LED lights. Its a 40 Series Component Engineering reader

Thanks!

Posted by Louis Bornwasser (Member # 3063) on 09-04-2007, 09:14 PM:

Follow the instructions with your LED units. Usually you don't want to connect to your soundhead frame, but leave the LED kit floating as per its instructions. Louis

Posted by Henry O'Connell (Member # 2238) on 09-04-2007, 10:11 PM:

it also could be a ground problem from your rack. check grounding of any other equipment you are using. don't forget the evil ground loop too. did this just happen or is this an ongoing problem?

Posted by Joel N. Weber II (Member # 3530) on 09-04-2007, 10:34 PM:

If you switch the processor to the non-sync input (or some other input that doesn't use the optical sound readers), does the problem go away?

Posted by Gerard S. Cohen (Member # 994) on 09-05-2007, 01:29 PM:

A frequent cause of 35mm sound hum is light from a booth fixture, either AC incandescent or fluorescent, shining into the sound head. Shielding the photocell or simply closing the soundhead door can often eliminate this.

Posted by Brant Veilleux (Member # 2877) on 09-05-2007, 02:54 PM:

Check to see if you might have a transformer or "wall wart" plugged in on the same circuit as a audio amp or other audio equipment. I have run in to this problem with A.L.D. power supplies being the cause.

Posted by Demetris Thoupis (Member # 833) on 09-05-2007, 03:08 PM:

That's the beauty of having the BACP babies!! When you have humming you can see it from the LED's on the reader thus you can make several tests until they dissapear (or blow the reader!!!! [Big Grin]

) You need to provide with more info like projector model, what power supply you use for the Red LED Reader, what sound equipment you have (processor, amps e.t.c).

First suggestion would be to have the projector and the reverse reader completely switched off. Do you still hear the hum in the speakers if you select film (SR or A Mode on your processor?) If yes then the problem most probably derives from your rack.

If not, then I would suggest grounding the 0V of your DC power supply directly on the projector chassis. Make sure your projector is properly grounded and the rack also.
Demetris

Posted by Bill Enos (Member # 440) on 09-05-2007, 03:16 PM:

Having some parts of the system on different phases of a 3 phase power system can result in hum. We battled that problem for a long time.

Posted by Ken Lackner (Member # 1002) on 09-06-2007, 05:30 PM:

One of the theaters I now service had major hums when in any optical format when I first took over. The soundheads all had BACP readers installed several years ago. However, the original 3-conducter solar cell cable was never reaplaced. Running two Individually shielded pairs took care of the problem.