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Author Topic: Chasing hum bars? Paul Mayer Oh get out of it Melvin, before it pulls you under! Posts: 3836 From: Albuquerque, NM Registered: Feb 2000 posted 05-30-2009 12:16 PM                        I think balancing phases only matters when you're dealing with really big loads. When we used to install temporary TV lighting in hotel showrooms or convention areas (like for the Jerry Lewis national Telethon) we'd get loads around 600A per phase with everything lit. We'd try to distribute the dimmer racks to keep the phases within 50A of each other. The sound guys used to insist on a separate "clean" drop (i.e. no noisy reactive or switching loads) just for them, usually around 200A capacity.  |  IP: Logged Fred Tucker Film Handler Posts: 90 From: Sugar Land, TX Registered: Sep 2007 posted 05-30-2009 07:04 PM                     The only time I would ever attempt down sizing a neutral is when wiring a house & then downsize it according to the 220V loads. For instance a 200A service would get a 1/0 neutral instead of 2/0 since things 220V appliances like water heaters, furnaces, a/c & stoves use very little 120V power and have little use for a neutral. I've upsized neutrals in a office building where the electrical contractor tried to save a buck & hum was present on the computer monitors. Only the service wires everything else was #12.  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 05-30-2009 08:12 PM                     Also, make sure that someone has not connected the neutral and the ground together somewhere (other than at the entrance panel. I have had a number of calls form people who have done the wrong thing of putting everything on the same leg, and have bad hum problems. I tell them to try to get close to balancing the legs, and bingo, the hum goes away.  |  IP: Logged Austin Pelnar Film Handler Posts: 10 From: Tallahassee FL Registered: Dec 2008 posted 05-31-2009 08:19 PM                     Given its an auditorium My guess would be dimmer/SCR noise getting into the video cabling or possibly a bad power supply somewhere I say this because... I spent half a day looking for a bad switching power supply. It was blowing thru the 1694A/RG-6 SD that we use and was out of band you couldnt see it on a waveform monitor or a video monitor but once it was converted to serial digital it was a mess. Looked long and hard, space heaters, printers, lights, dimmers, sniffed around the building with a spectrum analyzer there was powerful noise everywhere starting at ~6Mhz. Gave up at 5 PM stuck a low pass filter on it and called it a night. 2 hours later was called back in for a dead camera, bad power supply replaced it and for the heck of checked and the noise was gone. Myself I wouldnt think ground loop lest somebody has done something horribly wrong. The shields of analog video cables are supposed to be isolated from the chassis. Patchbays are made from phenolic, feed thrus and equipment mounted connectors should have plastic bushings on them. NOWHERE read NOWHERE should the shield be tied to equipment/safety ground. Grounding to clear up the problem is dangerous at best I once found a video cable with 7 amps of ground loop on it... the hard way. Austin  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 05-31-2009 09:51 PM                     quote: Austin Pelnar The shields of analog video cables are supposed to be isolated from the chassis. Where did you read this fantasy? I can't think of the last piece of equipment I've seen where the BNC connector wasn't right on the metal chassis. Once that goes into an equipment rack that is on ground...whola...you've grounded, somewhat poorly, the sheild of the cable. Anyway...you all believe what voodoo you want on what does and does not cause hum and hum bars. Steve  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 06-01-2009 08:55 AM                     Steve is right!  |  IP: Logged Austin Pelnar Film Handler Posts: 10 From: Tallahassee FL Registered: Dec 2008 posted 06-01-2009 08:55 AM                     Yes it can be grounded on the source and terminating end altho isolated is preferred. Grounding is a very complicated subject with some basic rules and is way beyond a fourm discussion. Do you ground the shield on both ends of a ballanced audio line or just the terminating end. In what situation would you ground on both ends, what situation would you not ground it at all. Check the connectors on equipment from broadcast equipment mfgrs LEI, Grass Valley, Evertz, Leitch... The connectors are isolated. Patch bays isolated, proper feed thrus isolated, DA's isolated, analog side of a converter isolated. Isolated for this very reason if you got noise and the shields are grounded everywhere lots of luck to you.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 06-01-2009 09:07 AM                     You'd have quite a bit of problem if they were not grounded as proper RF shielding requires grounding at BOTH ENDS. As for audio grounding...a PROPER system actually does ground at both ends. The problem is, most are not proper and grounding at one end avoid one problem while causing another...the lesser of two evils. Steve  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 06-01-2009 08:58 PM                     Sorry to disagree, but for balanced audio, the shield should only be grounded on one end. BTW, the shield does not go through a "normaled" audio patch, unless you "make" it go through. This can be a problem if someone has wired mike inputs to an audio board through a patch panel. Phantom power will not make it to the mikes, you will need to put patches into the patch pannel to have the shields connect. I have seen a number of pieces of video gear where the BNC input can be treated as sort-of a balanced input, where the center pin goes to the + input of an op-amp, and the shield goes to the - side of the op-amp. There is usually a jumper on the board you can use to ground the shield, if you want to.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 06-01-2009 11:03 PM                     You can disagree all you want...the fact remains, the proper way is to bond the shield to the case at both ends...if it causes a problem, the cover-up is to lift one end. The solution is to correct the potential difference. For some fun reading: http://www.rane.com/note151.html As for patch bays...it all depends on how one sets them up and what patch bay they use. I use "Sleeve Normaled" type such that all three conductors are normaled or disconnected if a patch is inserted. It is a little more costly but it is a better/safer way to go. Again, do what you want. Steve  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 06-01-2009 11:24 PM                     I agree with steve on the issue of shields a properly designed and grounded system will have better hum and sn with the shield connected at both ends ALTEC even had data sheets long before the quoted rane one on the issues of grounding and shields one needs to get back to basics first  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 06-02-2009 02:27 AM                     What is the drain wire and what function does this wire do? I've seen connections where one side has 4 runs and the other side only has the three w/drain that isn't connected to anything... thx-Monte  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 06-02-2009 07:46 AM                     OKAY GUYS: Can we please take the general discussion of audio hum problems and proper grounding practices to another thread? Both because those are useful to people who don't care about video and spend their whole life in the FHF, and also because I'd like this thread to be useful to people debugging hum problems in the future. In case you're wondering, I've passed some of the very helpful comments here (thank you!) on to the folks responsible for the auditorium in question, and they're going to try to see what to do... --jhawk  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 06-02-2009 10:11 PM                     Actually troubleshooting audio and video hum is identical and normally caused by the same mistakes/problems. Steve  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 06-03-2009 10:01 AM                     Yeah, I hear you; but few are going to find the good suggestions here because they'll be searching in the Film Handler's Forum rather than the Straight to Video forum. On the other hand, no one is recommending disconnecting the shields from BNC video connectors at one end, right? --jhawk  |  IP: Logged

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