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Author Topic: Subwoofer is humming...help! Aaron Garman Phenomenal Film Handler Posts: 1470 From: Toledo, OH USA Registered: Mar 2003 posted 07-25-2003 03:11 AM                     Hello all. Well I investigated a few things tonight, and here is what I got. The humming remains with all amps on and the processor on. However, when I adjust the volume on the processor, the hum remains. Also, the hum disappears when all amps are on, but the processor is off. The hum also disappeared slightly when the processor was on, and the LCR and Surround amps were off. I just read your post Manny, and I totally understand your advice. My main goal here was not to tinker with anything, just figure out what the hum was coming from. I would not change anything without approval from my superiors. The trouble is, most people around our theatre including our tech are very apathetic to recent booth problems, which just makes things frustrating. I know two wrongs don't make a right; I was writing that post with too much emotion and wasn't thinking. Thanks for bringing me back down to Earth. As far as following the chain of command, I totally have. I noticed the problem months ago and researched it for the past couple weeks, and nothing has been done about it. I've told at least 3 managers, as well as the tech himself. He said he would look into it, and he did a month and half after I brought it to his attention. Then, he claimed he fixed it. I really do feel ashamed that I have to get upset, but the whole thing just gets to me. Again, the apathy I have seen is extremely discouraging. Anyhow, thanks for all the help guys, and I'll see what I can do. AJG  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 07-25-2003 09:30 AM                     quote: Also, the hum disappears when all amps are on, but the processor is off. This rules out your amplifiers themselves but the trouble could still be with the connection into the amps from the processor. That's what I would check next with one amp on at a time. quote: when I adjust the volume on the processor, the hum remains. Just curious if this means that the hum gets louder and softer when you change the fader setting? If you turn the fader all the way down, does the hum go away?  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 07-25-2003 10:09 AM                       Okay, that sounds like a ground loop, then. Or a bad system ground in your rack. All the chassis have to have a good electrical bond to the rack (the mounting screws may even be loose). Sometimes, the paint has to be scraped or filed off under the ground and/or mounting screws. The easiest way, though, is to just connect all of the chassis together with a single grounding wire, and then attach the other end of it to bare metal somewhere inside the rack.  |  IP: Logged Aaron Garman Phenomenal Film Handler Posts: 1470 From: Toledo, OH USA Registered: Mar 2003 posted 07-25-2003 10:26 AM                     When I adjust the fader, whether it be 0 or 7, the hum remains the same volume. AJG  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 07-25-2003 11:18 AM                     Then I'd say the fault lies in the connection of the processor to the amps. It's probably those 1/4" plugs. A simple test would be to turn on the processor and just one amplifier (of the channel that hums the most) and then unplug the 1/4" plugs (remember which one goes where if there are 2). The hum will probably go away. If so, then you have successfully isolated the cause of your hum. Next item on the agenda would be to go ahead and report your findings to your tech and your theatre manager (a copy to the manager can force a lazy or apathetic tech to deal with the issue).  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 07-25-2003 11:20 AM                        quote: Maybe it's humming because it doesn't know the words. I had the same idea, but I resisted it. I had a hum problem recently with an UltraStereo processor, and it turned out that the problem was caused by a faulty power supply.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 07-25-2003 08:12 PM                     quote: Michael S: I had a hum problem recently with an UltraStereo processor, and it turned out that the problem was caused by a faulty power supply. Good idea! Here's a spin-off on that idea: The power supply goes bad and somebody plugs in the backup supply to get the processor running in "emergency mode". In order to do that you have to press the "Bypass" button on the format module. All other functions of the processor are shut down and it runs off the backup optical preamp. First, it's noisy as hell. It's only meant to "save the show", not to run long-term. Second, people often run the booth with one or more overhead lights turned on full time. Because there's little to no noise reduction, any 60 Hz hum from the lights can be heard in the system. (Less of a problem if you have IRED optical sound.) Sometimes that button just gets pressed in by some bone head but nobody ever notices. 1) Check to see if the button is in. 2) Check the power supply by swapping it with a known-good house. (You can do this without pissing off management if you do it carefully... Turn off all power first, etc.) If these are the "Super Mega" racks, et. al. that were wired up by CFS like I think they are, there are wiring F-ups all over in there! Unbalanced lines going right near and parallel to power mains cords. Shields are grounded on both ends. There are poor connections and cold solder joints all through them! I have spend hours fixing those beasts!  |  IP: Logged Aaron Garman Phenomenal Film Handler Posts: 1470 From: Toledo, OH USA Registered: Mar 2003 posted 07-26-2003 02:35 AM                     This is all starting to make more sense to me now guys. I think my next chance to check this out is on Sunday night, so I'll try to report back then. I think I'm going to print parts of this thread to show our manager that night. You all mention groundloop hum...what exactly does that mean? I've always heard of it, but never knew exactly what it really is or how it can happen. Also, any links on how wiring these racks works and what has to go to what. I know audio pretty well, but only on the homefront. I'm still a newbie to commercial stuff. Thanks for the help again. AJG  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 07-26-2003 07:10 AM                        quote: Good idea! It wasn`t my idea. I called the technical support at UltraStereo.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 07-27-2003 09:09 AM                     Yeah, yeah... but you though of posting it!  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 07-27-2003 09:33 AM                        Years ago one of the techs who was trained by the RCA Theatre Sound Service told me that they taught the RCA Cut-and-Solder method of troubleshooting those elusive groundloops. What you do is, keep the system on and listening to the hum, you cut the shield. If the hum decreases, you leave the shield off. If the hum increases, you solder it back on. Repeat until you run into overtime. Pretty scientific, eh? Eventhough you may think you understand the theory about how 60Hz hum is introduced in sound system wiring, when you come into facility, especially older ones, you are faced with years of different engineers and techs following the beat of their own private drummer, many times resulting in such a hodgepodge of techniques and no discernable, consistant protocol. It then becomes a guessing game since you have no idea what the guys before them did or how the mains electrical system has been modified or added to. When I first came into the recording studio attached to our theatre, I was able to put a meter lead on a ground pin in an electrical outlet on one side of the room, the other lead on the chassis of the main equipment rack -- I read +17v! The three phase legs of the service had become so unbalanced over the years that no matter what you did in the room, you were fighting huge amperage discrepancies across the legs. Any idea what kind of hum +17v above ground represents if it gets into the audio circuit? Sensuround, my friends -- then speaker and amp failure! Try blocking THAT much unbalanced energy with an electronic balanced input. Frank  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 07-27-2003 02:49 PM                     Transformers....lots of transformers Grounding methods are not understood as well as they need to be my most technicians. One suggestion I see many times is the equipment to rack bond...if putting "star-washers" or other schemes "fixes" the ground loop hum...then you really haven't fixed it...the problem lies elsewhere and you just minimized it by using the rack as your common "wire." Don't get me wrong, the rack MUST be grounded for safety and RF reasons...not to mention that it must not contribute to the ground loop. Grounding is actually a simple concept at heart but it can be rather elusive. The simple rule is...every piece of equipment MUST be grounded at the exact same ground potential such that ground between each piece of equipment has a zero resistance. Believe it or not, lifting the shield at one end is also a cover up...in a perfectly grounded system, both shields should be tied to minimize RF interference. The problem is often that perfect grounding concept. It is difficult to impossible to achieve in a typical theatre environment. A good ground has to be designed in by a competent engineer. If you have unbalanced equipment (older Dolby, CP-65 and prior, USL JS series...etc.) then you have to be even more deligent in your grounding. For unbalanced equipment uses the "return" often at ground potential. If ANY piece of unbalanced equipment grounds their "low" or return side then EVERY piece of equipment must also ground their return side to the EXACT same ground. Otherwise you will get current flow on the returns and a ground loop is formed. Troubleshooting Hums and groundloops are like any other troubleshooting...be methodical and logical. You hear the hum in the speaker work your way back...starting with the wire...believe it or not a speaker can make noise without the amplifer...it just needs that wire to get some induced noise. From there...the amplifiers and then on back to the processor and eventually the soundhead(s). Once you isolate at which section the hum starts (be it in the form of just a noisy piece of equipment or electrical ground loop type hums) then you can delve into what is the cause. Steve  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 07-27-2003 02:52 PM                     There is a strange notion that you should put everything on the same phase (leg) to keep hum out of the system. This is actualy the way to make sure you DO have hum. Balanceing the legs is the best thing to do.  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 07-27-2003 03:10 PM                        quote: There is a strange notion that you should put everything on the same phase (leg) to keep hum out of the system. This is actualy the way to make sure you DO have hum. Balanceing the legs is the best thing to do. Some of that may have come from installations that had feed lines of different lengths. We once had a studio in a radio station that had horrendous RF problems. All of the normal fixes failed rather miserably. It wasn't until someone discovered that two of the outlets (within 4 feet of each other) had separate feeds to the panel, at the other end of the building, taking different routes to get there. Needless to say, the ground potential between the two lines was NOT zero!  |  IP: Logged Jim Alexander Film Handler Posts: 71 From: Greenwood, Nova Scotia, Canada Registered: Nov 1999 posted 07-27-2003 06:38 PM                        Quote Aaron: You all mention groundloop hum...what exactly does that mean? I've always heard of it, but never knew exactly what it really is or how it can happen. Also, any links on how wiring these racks works and what has to go to what. I know audio pretty well, but only on the homefront. I'm still a newbie to commercial stuff. In essance, a ground loop is an antenna. Since the most common type of RF in most buildings is 60Hz, which is radiated from every electical wire and system, it is the one that is most likely to be picked up. By grounding a shield at both ends, the ground becomes a ground plane and the shield becomes the antenna, which in turn induces whatever it picks up onto the audio lines. Low level audio is more suseptable to this than high level, as well a unbalanced over balanced, but once in the system, it can be a real pain. Proper shield grounding should look like a tree, and idealy each shield is grounded to a fresh ground (not one derived from component to component). Having said all this, the ground must be capable of handling the grounding load...a 12 gauge wire will not handle the grounding requirements for a complete booth. Understanding this and elimintaing it will go a long way to "quieting" your audio. Another consideration is the placement of your audio lines. Don't run low level with high level...the sheilds are good, but not always that good. And never...NEVER EVER run any audio lines with power cables...60hz and 120/240 will over power audio at 30mV or 1.5V. Having said all this, from listening to your posts as well as others, your problem may or may not be a shield problem. From your comments, when the processor if off, the hum is gone. If it was a sheild problem, between the processor and the amp, this would not have eliminated it...it could be induced on the optical/digital pickup side, but not between the processor and amp. Sounds to me like a "active" hum which could be ripple in the power supply or a bad output card...but I'd say the problem is in the processor/power supply....  |  IP: Logged

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