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Author Topic: Hum in my speakers, how can it be corrected? Michael Moore Expert Film Handler Posts: 188 From: Dover, DE / USA Registered: Jun 2006 posted 03-05-2008 06:36 PM                        Hello all. I have not posted in a while, but sill come by to read and look at the latest post. However I have a problem and need a little help. I have a little bit of hum coming form my center channel. Before last week I never had this hum. I do however have a hum coming from my left and right channels if the Amps are on and the cinema processor is off. This is what I have for equipment. I have JBL speakers, QSC amps, and a USL JSD-80D processor. I also have a USL 35 something or other; monitor that sucks… We have a multi-use venue Live/Cinema We use Crown Switching processors to switch between all the formats. Now if I have the cinema processor on and the amps on I have no hum from the left/right/SR/ SUB however I do have hum from the Center. Now the amps for all the speakers are in one room, however the Center amp is in a separate amp rack that is on wheels. And it has no power conditioner Now this is what gets strange. If I have the Amps off to all the speakers and only have the Center Amp on with the processor off I have no hum, however once I turn on the processor I get just a little bit of hum…. Now if I turn on all the amps while the center amp is on and the processor is on the hum from the center gets even louder. If I disconnect the booth monitor from the processor some of the hum diminishes, but does not go away, in fact if I disconnect the monitor, the hum that I get from my left and right when the processor is off goes away. Everything is connected with XLR connections in Belden 1266A shielded cable. There is a Furnam power conditioner on everything but the center amp rolling rack. Could the addition of a power conditioner fix this hum problem? I never had this problem before until after this pas weekend, and I asked the techs who ran the live event on Saturday if they changed anything or noticed anything different and they said no. By the way there is no hum when we are in live mode… And the Hum goes away at my left and right if I pull the XLR’s at the booth return patch, and if I plug in an XLR cable into the left or right patch and the XLR is not connected to anything I get hum… This is very strange and hard to figure out. Help… Michael Moore  |  IP: Logged James Westbrook Phenomenal Film Handler Posts: 1133 From: Lubbock, Texas, Usa Registered: Mar 2006 posted 03-06-2008 01:22 AM                     I cast suspicion on the techs who ran the live event. "Oh no, we didn't do anything." If the hum started after they left, they likely did something.  |  IP: Logged Bill Enos Film God Posts: 2081 From: Richmond, Virginia, USA Registered: Apr 2000 posted 03-06-2008 11:58 AM                     That's the very reason we never allow techs from live events touch our cinema sound system, and never make any connections between our movie system and their system. Never take their word that they haven't interconnected either, check for yourself.  |  IP: Logged Ron Funderburg Jedi Master Film Handler Posts: 814 From: Chickasha, Oklahoma, USA Registered: Nov 2007 posted 03-06-2008 12:25 PM                        There are so many, many things that could cause this you have some work ahead of you. If it is something they did maybe you can track that down easier. Good luck and like the others I suspect it was the tech that tied in!  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 03-06-2008 12:56 PM                     Well, the first thing you want to check out is if this happens on all inputs on the JSD-80 and second... is that your "B-Chain" system gain is not turned up really high, especially if your JSD-80 is feeding another independent system. Having the amp gains up all the way is one of the most common mistaks I see. If the gains are cranked you will also hear the background noise...hiss and hum in the processor and A chain itself predominantly. You should be hitting the lines to the amps with about 750mv AC RMS average level. Good luck solving this.... Mark  |  IP: Logged Michael Moore Expert Film Handler Posts: 188 From: Dover, DE / USA Registered: Jun 2006 posted 03-10-2008 02:01 PM                        Thank you Mark. you know the front fill speakers, (L and R), amps are cranked all the way on the low end 3/4 of the way on the high and mid ends. Also my center amp gain was turned way down when I did the B-chain, went and checked it this past weekend and sure enough center gain was cranked all the way up! so I put it back to the original setting and sure enough hum disappeared at the center. Now L and R I turned them down and still had some hum... I disconnected the CM35 monitor and guess what all hum when away. I blame the left and right channel hum on the monitor, and the center hum on the techs. No how do I get rid of the hum on the monitor. the cm35 was connected to the JSD80 properly, although I was told it is not really compatible. It seems the negatives for all the speakers going into the monitor need to be tied together and then ran to one common ground on the JSD 80 and it was that ground that made the left and right hum. And even with the monitor off and unplugged I got hum so I just disconnected the speakers from the monitor, only thing is now I have no booth monitor. What is wrong with the monitor? who knows.... But any suggestion on a new replacement for the monitor would be great... Mike  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 03-10-2008 02:50 PM                        Any monitor that is tied across the processor output has the possibility of hum. Ground loops are the problem unless the monitor amp is isolated with a fully balanced, transformer input. That is exactly why I never do this. All monitors from Hadden are taken from the power amplifier output through a 10k resistor before then being amplified in the monitor amp. It is not possible for that system to cause hum since it is connected to the amp output. Louis  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 03-10-2008 04:44 PM                       I disagree with Louis. One of the functions of the monitor is for troubleshooting, as in where does the sound stop. If you have audio from the processor and not from the amplifier output, then we have divided the problem in half and possibly saved a service call in the bargain. There is the possibility of hum any time you connect any two pieces of audio equipment but properly connected equipment in the same rack won't hum.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 03-10-2008 05:59 PM                        Sam; you would be 100% correct except for: 1. amps are so reliable now that their failure is almost unique. 2. so few techs really understand ground loops, that the reality is that most monitors wired that way do hum in the auditorium all the time. 3. who, in a theatre, can troubleshoot? I am usually called to tell them what to write down for the home office. 25 years ago, I would have agreed with you. Louis  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-10-2008 06:29 PM                     I always connect the processor out to the monitor as I expect all of the projectionists in theatres we service to be able to use that function I suspect that there is a ground loop issue with the centre amp being in a different rack and possibly connected to a different power source (possibly even a different feed transformer that might even service the stage dimming system )  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 03-10-2008 09:01 PM                     I agree with Sam, especially if the JSD-80 has been utilized in true balanced mode.... hopefully it was all installed correctly from the onset. Even a failed power supply in a monitor itself should not cause hum in the system it is hooked to. The hum should only be in the monitor speaker itself... Any half decent tech will indeed understand ground loops Louis... He's working in one simple rack with perhaps a few dozen connections and he's done this many-many times. He's not wiring a 100 place patch panel with an equivelent number of distribution amps in a TV station where you do indeed have to sort out the grounding system... before you build the system! Mark  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 03-10-2008 09:26 PM                     I also prefer all points to be monitored Most of my service is interfacing multi-format, live, film sources and "hum busting" ground management is part of the job....It also helps in telephone walk thru with house sound and video / film projectionists to figure a problem prior to arrival ( or save a trip ).  |  IP: Logged Michael Moore Expert Film Handler Posts: 188 From: Dover, DE / USA Registered: Jun 2006 posted 03-17-2008 08:21 PM                        Ok. According to USL, there is a wire diagram that must be followed to the tee in order for there to be no hum at any end; at the monitor, or at the main speakers. I followed that to the tee, in fact they suggest all the negatives to be tied together, and then patched to the main ground on the back of the JSD-80. My situation is different from most since all my stuff is not in the same rack, our projection booth is on the third floor, the amplifiers are on the first floor stage left, in a room. The center amp is on stage right in its own rack. The booth rack is made of Oak and has not metal on it, and I think this is why I am having some problems. And I think the over 500 feet of wire running from the amps to the booth is enough to pick up cross talk, or 60 cycle noise too. I was told by USL that the CM-35 is not really the best monitor to hook up to the JSD 80 and that something like the CM-60 would be better but the CM-680 or a Dolby booth monitor would be best choice. Also they said that I should not connect the shield at the amps, however when the shield is not connect at the amps from the JSD80 I get major hum from all speakers. When the shield is connected to the amps hum goes away. I get no hum now that the booth monitor is gone.  |  IP: Logged

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