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Author Topic: How to get rid of an audio hum Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 08-26-2011 11:47 PM                        Among other things, I am the webmaster for CHXL, a unique little radio station that's owned and operated by the Okanese First Nation on their reserve in Saskatchewan. "Fifty Thousand Watts of Pow Wow Power!" I have a streaming audio server in my theatre that streams their signal on the Internet. You can find it through their webpage if you're interested. Unfortunately, there is a slight hum in the Internet stream that I have never been able to get rid of. This is the setup: I have an external antenna on a tower that hooks to a tuner with standard coax, the tuner is plugged into a small Behringer mixer board, and the mixer is plugged into the "mic in" on a computer. The power for everything listed here is hooked up to a single UPS so everything is plugged into the same electrical circuit, and the antenna ground is hooked to the same circuit as well, so everything has a common ground. The stations sounds perfect if I put some speakers directly onto the tuner. But through the computer, the stream has a slight hum. I connected a wire from a screw on the tuner case to a screw on the computer case and that cuts the hum by a significant amount, but it's still there. I tried hooking a wire from the mixer case to the computer case as well, but that increased the hum by a lot. So, my question is: Does anyone have any ideas that I haven't yet tried to get rid of this hum?  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 08-27-2011 01:29 AM                        First, why are you putting line level output from the tuner into a mic input on the computer? Line level out should always go into line level in. Secondly, you could double check everything to see if there is a hidden ground loop in your setup, but they can be peksy and if you want to try a down and dirty fix, you can get an isolation transformer that will break any ground loop. You can get them at many electronic outlets like this: here: ART hum blocker There are cheaper ones on the market, but be careful -- they are all just transformers in boxes....some are expensive xformes, some cheap. Cheap transformers can muddy and even distort the bass. You want a unit that has a wide frequency response and no phase shifting in the bottom end. I have used these ART products and have been very happy with them. Again, why are you using the MIC input? Is there a level problem from the tuner...too low for the line IN on the sound card?  |  IP: Logged Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 08-27-2011 01:59 AM                        The server doesn't see the line level input, even though it is physically present on the server. Therefore, when I set thing thing up the path of least resistance (no pun intended) was to simply use the mic in, which it does see. That's actually the purpose of the mixer board -- to cut the input signal from the tuner down for the mic in. And it works fine for that purpose. If I plug a CD player into the mixer board and stream that instead of the radio station, there is no hum. Which appears to eliminate the mixer board as the source of the hum. I really don't see where the hum can be getting introduced, even though it obviously is sneaking in somewhere. Everything has a common ground and is being fed from a single power supply and I thought that would be sufficient. Perhaps relevant: The tuner has a two-prong (ungrounded) power cord. The mixer board is powered by a wall-wart adapter, so it also has no separate ground of its own. The computer has a standard three-prong (grounded) power cord. I find it interesting that the hum is greatly reduced by connecting a wire from a screw in the tuner case to a screw in the computer case. Without that wire, the hum is pretty bad. I've been living with this hum for a couple of years, but it sure would be nice to get rid of it if I can.  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 08-27-2011 03:09 AM                     quote: Frank Cox The tuner has a two-prong (ungrounded) power cord. Is the plug polarized - one hot and one neutral tang, or a common type of non polarized plug. If you look on the wire (if it's the standard 18g flat zip cord type), there will be two different leads to the plug: one smooth and one ribbed. The smooth one is hot and the ribbed is neutral. Mark your plug so the one blade representing hot goes in the smaller receptacle of the socket. One way to reduce ground looping with that wire trick. Grounds don't like each other too well and they can argue real bad. Gonna have to attenuate that input from the AUX to MIC then. Maybe a 50k trimpot can attenuate that high line current down to match that mic input current.  |  IP: Logged Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 08-27-2011 03:13 AM                        Again, that's what the mixer board is doing. It attenuates the output from the tuner so it will match the mic input. And it works: There is no hum when I plug a CD player "line out" into the same input on the mixer board. The hum shows up only when the tuner "line out" is plugged into the mixer board.  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 08-27-2011 03:19 AM                     Yep, got some real bad "ground looping" going on. There are 'ground loop isolators' out there that one can purchase to eliminate this problem then. Do a google search on "ground loop isolators" I have a bunch of them that I can shoot off to you. PM me if interested. -thx Monte  |  IP: Logged Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 08-27-2011 03:27 AM                        The tuner is an old one that was laying around at the station when we decided to undertake this project, and it has an old-style non-polarized plug on it. I just checked how I have it plugged in and I see that it was, purely by chance, plugged in with the smooth side of the cord into the smaller slot in the receptacle. So I guess that lets that out as the source of the hum.  |  IP: Logged Leo Enticknap Film God Posts: 7474 From: Loma Linda, CA Registered: Jul 2000 posted 08-27-2011 10:20 AM                        quote: Frank Cox The server doesn't see the line level input, even though it is physically present on the server. Therefore, when I set thing thing up the path of least resistance (no pun intended) was to simply use the mic in, which it does see. Sounds to me like a problem with the driver software for the sound card. Is it an old-ish card running a much newer version of Windows, and the driver running forced compatibility mode? Might replacing the sound card for one for which a driver is available that is fully compatible with your operating system be worth trying?  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 08-27-2011 11:26 AM                        Remove all existing grounds like the third wire on the AC plug. Then wire all chassis together with a wire of some substance like a #12 with lugs. Sometimes a smaller wire adds hum and a larger wire stops it. I have used battery jumper cables to search for a good ground. THAT'S a big conductor. Louis  |  IP: Logged Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 08-27-2011 12:54 PM                        Leo: It's a relatively new computer running Centos 5, not Windows. (I'm like the maid: I don't do Windows.) I don't think that's the source of the problem, anyway -- the hum is not present when I plug a CD player into the same input on the mixer board. Louis: The whole works is plugged into the same circuit (and therefore the same ground) right now. Doesn't that amount to the same thing? As it sits right now, wiring the tuner and the computer together reduce the hum, but extending that wire to the case of the mixer board causes the hum to increase. Monte: I sent you a PM that includes my email address (though you can find it pretty easily with Google, too.)  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 08-27-2011 03:30 PM                     quote: Louis Bornwasser I have used battery jumper cables to search for a good ground. THAT'S a big conductor. (Course, this is DC current, but similar..:)Why some auto hobbyists, esp when doing major mods with the engine, will add many grounding connections with the same gauge wire (and with higher count strand wire) used from the battery to various parts under the hood - like shock towers, transmissions, intake manifolds, power steering brackets, et.al. - to really distribute the current from the negative post for a lot better circuit path than just the two from the battery-which is the firewall and the one engine ground point. These owners really want the current to flow freely for better operational power.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 08-27-2011 03:36 PM                        Frank, Leo is right -- wire size is a key issue for the potential (no pun intended) where hum can originate. The old RCA method of ground hum elimination still works -- you solder a ground wire to a piece of equipment, if the hum lessens, you leave it on; it the hum increases, you snip it off. Trial and error. The jumper cables are great for this. Try them on each piece of equipment to a GOOD ground point. It is not as unlikely as it my seem that even ground pins in an outlet can have higher potential to earth than zero. I once read a voltage potential at the ground pins between two outlets in the same room at 7 volts differential! 7 volts of 60Hz is a LOT of hum. Short of driving a copper stake 5 feet into the ground, you can't really trust grounding points in a building, especially an older one. Try various places and various jumps between equipment with those battery cable clamps to see if you can kill the hum. Let me ask you this -- can you get a clean signal out of the tuner if you send its output directly to the computer -- well, not directly to the mic input, you will need a Direct Box (commonly found in any sound engineer's toolbox) to get it down to mic level? See if it is clean going directly and bypassing the mixer. If it is, they it has to be the interconnection and the mixer that is the culprit. If none of these steps solves the hum, then I wouldn't hesitate grabbing an audio isolation transformer and put it between the tuner and the mixer (you say you can play other source thru the mixer and the audio out of the computer is fine. yes?). That leaves you with a tuner that interacts badly with the mixer and an isobox between them should resolve that. On the other hand, I would strongly suggest that you replace the sound card and get one that accepts a line level 1v/p-to-p input. Needing to have the mixer drop the line level signal down to mic level only to then require the sound card to amplify it back up to line level is an unnecessary, potentially noise inducing step; at the very least it has the potential to degrade the signal. The flimsy cable and the consumer 1/8 mini plug between the mixer and the computer itself can be a source of noise, which is why on pro equipment they use XLRs and hefty, well shielded cable. Why boost a signal 100 times (and whatever noise is present) when you already start off with a 1v line-level signal? I'm just sayin....  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 08-27-2011 04:16 PM                     Here's one that I done recently: At a drive in that I take care of on occasion, they use an Mic amp to play their CD player into along with their PA announcement through the FM transmitter. The Cd player went kaput and they got another CD player to put in line, but the output of that new CD player was a lot stronger than the old one which caused major distortion on playback. I took a 100k stereo pot and put it inline in the RCA cables so the output can be attenuated down to match the mic amp, but did I get the hum like a bad open. Took some 12g strand, soldered one end of the wire to the common side of the pot where the ground leads from the RCA cable met, and the other end attached it to a near by "J-box" case which was heading to the earth. Hum suddenly disappeared. CD playback now works excellent with no distortion.  |  IP: Logged Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 08-27-2011 04:17 PM                        I can't plug the tuner directly into the computer because I don't have anything other than the mixer board that will attenuate the signal. (If I did, I probably wouldn't have the mixer board.) And I don't live in a place where I can just walk down the street and pick up that sort of thing. The next time I go to "the city" I will go to the music shop where I purchased the mixer board and see if they have a ground loop isolator -- I'm pretty sure I can get one there since they have all sorts of doodads. I did pop in at the hardware store here this morning on the off-chance that they might have one of those things since they carry some telephone and television hookup stuff, but they didn't have it and it wasn't on their list of things that they can order, either. Meanwhile, I've lived with this hum for quite a while so I guess I can live with it for a bit longer. I guess it bugs me more than it bugs anyone else; I've never heard anything mentioned about it by any of the listeners or the folks who work at the radio station -- everyone is just happy that the station is available online (and they have a surprising number of listeners). The audio server runs on Centos 5. I suspect that Centos 6 (released a couple of months ago) would recognize the line level input, but setting that up would involve reformatting the server and setting the whole thing up from scratch again, which is a lot of work that I would be just as happy to not do. I think I had to do a custom compile of Icecast to get it going on Centos which, while not hard to do, is once again something that it's nice to not have to do, as well. When the point comes that the server quits, I'll set the next one up with the latest and greatest, but until that point arrives, well... If all I need to get rid of the hum is a ground loop isolator, that's not hard to do so I'll just put one of those on my shopping list.  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 08-27-2011 04:30 PM                     First thing I would try is to un-ground the antenna. Then try turning off the tuner, then try unpluging it. Does the hum go away? Try turning the AC plug around in the socket. If at all possible find a way to use the line in to the computer. Taking line level and dropping it to mike level, then having the computer bring it back up to line level is a very bad practice. There should be a program for the sound board somewhere. You may have to find it, in order to turn the line input on (and the mike in off). Caution: most audio transformers are LINE level. Do not try to use them with a mike level signal. You will have lots of problems.  |  IP: Logged

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