Film-Tech Forum ARCHIVE: Hum in sound, looking for small fm transmitter

This is topic Hum in sound, looking for small fm transmitter in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Dave Ganoe (Member # 53) on 11-27-2003, 03:00 PM:

Ive got my outdoor screen setup, and the booth complete with platter. Ran some stuff and looks really good. However I do have a hum in the sound system. The setup is a cp50 with a smart showman monitor. I have a cable running to my deck outside and listen to sound via headphones(dont want to disturb the neighbors). Problem is this hum especially noticable during the quiet parts of a film. Id rather have a small fm transmitter instead of using wires but havent found an affordable solution. I dont need much power as the deck Im watching from is right outside the booth window. Cheapest thing Ive seen in a ramsey fm25 for nearly $200. Probably much more power than I need. Any ideas on the hum and/or transmitter? Thanks for your help.

Posted by Dennis M Dow Jr (Member # 1970) on 11-27-2003, 03:19 PM:

Is your sound rack all on the same phase? or else check your ground

Posted by Josh Jones (Member # 447) on 11-27-2003, 10:58 PM:

I agree, an FM transmitter is not going to fix your problem. More then likely, you have a ground loop happening between the processor and projector. Make sure that the shields to the cable are only connected at the processor end, and that nothing is thouching them. Also, make sure you are not running them parallel to any AC lines running the machine.

JJ

Posted by Ken Layton (Member # 133) on 11-27-2003, 11:44 PM:

Remember that the shield wire of the cable going to the solar cell is NOT connected at the soundhead end of the cable.

Posted by Jeff Taylor (Member # 443) on 11-28-2003, 09:26 AM:

You don't say when the hum problem emerged..has it always been there, or just since you added one component or another. One embarrasingly elementary question--are you running incandescent exciters and if so are they on well filtered DC?

Posted by Mark Gulbrandsen (Member # 72) on 11-28-2003, 12:16 PM:

Almost all of the D.I.'s I've been to in this area use Ramsey transmitters but mainly the deluxe model. The others in the mountain states are using FM exciters. All are broadcasting in stereo and most go through either USL or Smart processors. All of these locations have top notch sound quality.

Mark @ CLACO

Posted by Paul G. Thompson (Member # 655) on 11-28-2003, 04:36 PM:

I agree with the ground loop theory. It could be caused by something as simple as Ken mentioned.

A situation I was once confronted with was the audio shielded cable was run through EMT that was physically attached to the sound head as well as the amplifier equipment rack. The shield of the audio cable was not grounded in the soundhead but it still hummed until the EMT was separated from the sound head.

BTW, are you using an AC exciter supply or a DC exciter supply?

Posted by Jean-Michel Grin (Member # 1360) on 11-28-2003, 05:26 PM:

Hi !

Check out the "technical note 101" from Rane, they give a lot of tips to cancel or reduce the ground loop and hum problems.
For me the Smart Showman 4 booth monitor is not the best I ever seen: I dislike the idea to pass the center processor audio signal and the center speaker wires trought the monitor. (For thoses they din't know this monitor, it offer the possibility to use the internal amplifier as an emergency amplifier to "feed" the center stage speaker with the center audio signal from the audio processor, in case of all power amplifier failure) It seems to me that this monitor could bring somes troubles to You.
Are the amplifier audio shield connected to the audio shield of the processor ? that could be a sourcce of ground loop.
Remember that the shield should be connected at one point. (In case the processor has unbalanced audio outputs and the power amplifier use balanced audio inputs, the shield of the audio cable must be connected only at hte amplifier side.

Past Year we has a special setup for a film festival: All the power amplifiers was behind the screen: We runned an multipairs cable from the projection booth to the screen and all outputs channels from the cinema processor was balanced and ground-lifted with six audio transformers. and the result was noise an hum free !

Good luck and sorry for my weak English [Smile]

Bye,

Posted by Peter Hall (Member # 695) on 11-29-2003, 07:43 AM:

Does it hum without film running ? If no it's an alignment thing..

SWitch to mag r non sync - does it still hum ?

Yes - forget the cell, exciter lamp psu etc.
Remove the cell wiring from the CP50 - if the hum goes it's a cell grounding or cabling thing. If the hum persists then it's a grounding or bal / un thing between the CP50 and the output source - we use balancing tx on our hire CP50s and running at 300mv we can happily send these signals to an amp stack on stage.

No - pull the exciter lamp or supply - if the hum stops it's the PSU. If hum persists, try turning all the lights off !

Good luck !

Posted by Dave Ganoe (Member # 53) on 11-29-2003, 10:57 AM:

Im looking for the fm transmitter so that I wont have to run headphone wires out to my deck to listen to the movie, Id rather just use the fm trans and a set of radio headphones so I dont bother those living next door with the sound. I use a kelmar supply for the incand. exciter bulb. The hum is in the non sysc as well. I was only using the booth monitor because it had the headphone output jack so that I could use the headphones. I tried lifting the ground from the solar cell at the processor end and the hum still persists. Would it make a diff if I lifted the ground at the cell instead of the processor? The problem may be the monitor itself since I just started using it when I got my outdoor setup going. When I was using my setup inside I just used speakers and did not have the booth monitor as part of the setup. The hum is there the moment I turn everything on. Anyone know of any smaller fm transmitters that could work for my setup instead of spending the $200 on the ramsey fm25? Thanks for all the info and help so far. Everyone here is greatly appreciated by me.