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  • OT - emf cancelling

    Some of us are into music and electronics. Maybe someone will know how to deal with a hum problem in the studio. My friend runs a recording studio, and any guitar without a hum-bucking pickup picks up a load of hum. If I orient the guitar just so, I can get nearly hum free. I wonder if anyone has dealt with this by trying to cancel the emf in the room via a coil of wire and feeding it a small ac current, phased to do the job. I would like to avoid using notch filters if I can. Any ideas?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Darryl I think this problem has been around for a while the sectret is to avoid facing the pick ups in line with any speakers in the vicinity.I am sure there are many people who have written about this.Why not ask at your local music sjop or see if anyone has written about this I know a chap from Holland who is an expert od speakers if you like I can put you in touch with him he is a decent chap and I am sure hre would try to help you.Problem is to do withmagnetic field in the pick ups and in the speakers I am sure.Alistair ps We are avid guitar people in my household with three guitar players including myself we have quite a collection.
    I intend to make a few guitars in the future when my new shop is complete will let you know how it comes out
    Please excuse my typing as I have a form of parkinsons disease

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    • #3
      I don't know if this is the same cause but when i Dj in old buildings the wiring can cause hums in the speakers. From my understanding is that sometimes the wiring is reversed. In other words the neutral line is switched rather then the power line. The new amps we use have their own filters and compesate for these problems. Maybe if you ran an extension from another room and tried that it might answer the question. This is just a suggestion and i am by no means an electrician

      Rob

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      • #4
        Rob You ate correct I have seen in some amplifiers hum caused by the transformer being incorrectly wired. However I don't think
        Darryl is referring to hum in that sense I think he's talking about feedback hum maybe he could tell us if its constant hum or directional feedback depending on where the pickups are facing Alistair
        Please excuse my typing as I have a form of parkinsons disease

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        • #5
          WEll this one is right up my alley....I work for Ampeg....

          Any single-coil will pick up hum from a local magnetic field at line frequency. After all, it is made to pick up small variations in the local magnetic field. The problem is the number of potential sources.

          To get a field, you have to have some form of a "loop" with current flowing around it. Old time wiring (knob and tube) often had a many foot separation between hot and neutral wires, and so old buildings can do that. How old is the building? and was it re-wired?

          Flourescent lights have ballasts, and old iron ballasts were made as cheaply as possible, so their cores "leak" magnetic fields. That is a typical source, use incandescent lights to get rid of it.

          Other equipment with transformers can also leak a field. Power amplifiers are notorious, partly because their current flow tends to be in short high current spikes synched to the line frequency (as the capacitors charge to peak voltage in the power supply). Move them away from the studio area.

          Tour own amplifier might be at fault. You might have to turn the amplifier, if it is in the room with you.

          It is possible for the ground system to have circulating currents, and those can cause a field also. It is more rare for that to be a problem. Usually it is a ballast or transformer that leaks flux, because the magnetic fields are strongest in those devices.

          I have seen good reults from the "balanced power" systems, where neutral isn't present (60-0-60 volts instead of 120V and ground). I have also seen them make no difference.

          A canceling field would work, but it is hard to set up, because the hum field may not be uniform. The canceling may work differently in spots only a few inches apart. The closer you are to the source, the more the field is "curved", so your canceling might be wrongly oriented in some spots versus others.

          The problem may also vry over time. A few minutes later it may be different.

          It is better to search for sources, and take care of them, in general. The canceling would work, at least theoretically, however.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            Thanks for the replies guys. To answer some of the questions, this is a fairly modern multiplex housing development. The wiring to the complex is underground, and from the oppisite end of the complex, about 200 ft away. We have tried using isolation transformers to power the gear, played with the polarity of the wiring, added grounded noise filters to feed power to the gear, and played with the orientation of the amps and other signal level gear. We have gone through the gamut of sources with this, and have eliminated lighting, power amps and anything else with transformers, tv's, etc. It's not feedback as Alistair has suggested as a possibility. I can only attribute it to the distribution lines that pass by on the street, and they are quite close to the studio. The hum is nearly constant in amplitude, and doesn't change when appliances or lights are turned on or off. This includes freezers, clock radios, the neighbor's electric appliances, etc. We have minimized higher frequency noises by installing a ground wire as close to the main panel as possible, but not affected the hum level at all, and not unexpectedly. What I have in mind is to set up a large coil, nearly the size of the end wall in the room, oriented at 90 degrees to the plane of the guitar when it's oriented for the least hum. This means the coil will have to lean away from the ceiling by about 1 ft or so, and away from one end of the wall by about 2 ft or so. I'll be able to feed an adjustable low voltage to it, and change the polarity. I may have my vector mechanics exactly wrong, meaning that the coil should maybe be circling the ceiling, and nearly parallel to it instead of on the end wall. I'm hoping the size of this proposed coil will give me a large enough 'sweet' spot to enable the musician to shift around a bit without leaving the 'nulled' area.
            There is of course one other alternative, that is to move the studio, and that would be best, but that can't happen for a couple of years yet. If I have to go with an inline notch filter, I'm expecting to have to notch 60 hz, 120 hz, and possibly 180 and 240 hz. I haven't researched the availability of such a filter yet, but I don't really want to add any devices between guitar and amp. If we're plugging in to the effects rack in the mixer room, we can use the computer's programs to do this filtering, but that doesn't help us in the isolation room.
            J Tiers, your comments about the curvature of the em field, and the possibility of correcting over only a small region are on target. We have determined that the interfering field is quite uniform, at least in this one room, by taking the guitar around the room, oriented for least hum pickup. This orientation stays the same wherever we carry the guitar in the room. This leads me to believe that the source is at some distance from the room, and again points to the passing power lines outside. I know that the current passing in these lines will be changing with the time of day, and the seasons, and the switching of large industrial loads, but for any period of a few hours it's relatively constant. I can certainly make a control adjustment to compensate, as long as my scheme works well enough to begin with. Thanks for any and all input.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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            • #7
              Hmm.

              Have you looked at your power with an oscilloscope? Any nonlinear harmonic distortion in the power caused by devices such as light dimmers? Be aware that 12 volt track lighting can cause serious emf from switching ballasts. If you need to use dimmers lose the solid state types and use a variac.

              Most important is to use a single point ground system. All electronics should be connected to the same ground line at the same distribution point to avoid ground loops.

              I'm not sure how effective the hum cancelling idea will be. You are dealing with a magnetic field in this case, not an electrostatic field (radio wave). Magnetic fields drop off as the inverse cube of distance so field strength changes dramatically with distance. It's likely that you will introduce more hum for nearly all locations in the studio with a very small sweet spot if one can be generated at all.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                Evan, we have ruled out light dimmers, high frequency fluorescents, track lighting, standard fluorescents, etc. I'm convinced we're dealing with an externally induced magnetic field, and I share your concerns that it may be nearly impossible to create a sweet spot at all, let alone a large enough one for a guitar player to move around a bit in. The problem is limited strictly to the guitars. I think that if we 'wallpaper' the room with steel sheet, that would do it, and if we use steel mesh, that probably wouldn't do it. Another concern I have is that if it's 3 phase power passing by outside, there won't be any way for me to counter the induced field from it, since I only have single phase in the building, and at best I would only be able to reduce the hum level, not nearly eliminate it. I've been trying to imagine what induced 3 phase hum would sound like. Would that be more like 180 hz hum- or 360 hz ?
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9

                  Evan, we have ruled out light dimmers, high frequency fluorescents, track lighting, standard fluorescents, etc. I'm convinced we're dealing with an externally induced magnetic field, and I share your concerns that it may be nearly impossible to create a sweet spot at all, let alone a large enough one for a guitar player to move around a bit in. The problem is limited strictly to the guitars. I think that if we 'wallpaper' the room with steel sheet, that would do it, and if we use steel mesh, that probably wouldn't do it. Another concern I have is that if it's 3 phase power passing by outside, there won't be any way for me to counter the induced field from it, since I only have single phase in the building, and at best I would only be able to reduce the hum level, not nearly eliminate it. I've been trying to imagine what induced 3 phase hum would sound like. Would that be more like 180 hz hum- or 360 hz ?




                  [This message has been edited by darryl (edited 02-17-2004).]
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                  • #10
                    I'm assuming that you're running single ended circuits from the guitars (ground/shield and signal). Studio mics would normally be balanced and run into balanced inputs on mixers, etc.

                    In a balanced circuit, especially if going into good quality transformers (anyone remember transformers?), common mode rejection is high and the resulting noise is very low.

                    One idea I had would be for you to create a balanced operation.

                    Get someone to build a single ended to differential line driver powered by a couple of AA batteries. Stick one behind the guitar (or down the performers briefs, panties, or whatever). At the input ends, another box for each input would convert balanced to single ended.

                    IMPORTANT: The input end must be designed with high common mode rejection.

                    This should kill anything originating outside the connectors. If the hum is getting picked up internally to the amp / mixer, those would need shielding or replacement with a better design

                    added : almost forgot, you need balanced cables with this new arrangment and mic cables should be fine.

                    Den

                    [This message has been edited by nheng (edited 02-17-2004).]

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                    • #11
                      I would take a guitar pickup and attach it to a small battery powered amp and some headphones. Then start wandering around including outside and listen to the hum to see where it gets louder. That will at least tell you where it originates. To shield the studio would be very difficult. This isn't a faraday cage were talking about. You would have to cover all surfaces with mu-metal. Ordinary steel won't do as it doesn't have high enough permeability.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Evan has hit it on the nose. Use the guitar pickup as a directional mic and prowl around listening until you come across (hopefully) whatever it is that is causing the problem.
                        At work I occasionally get requests to track down RI which the customer suspects is coming from the power lines and it is no easy matter, basically just a process of elimination mixed with a smattering of knowledge and a touch of luck sometimes thrown in. If the problem is coming in via the 3 phase power line, the cause could be blocks away, but more than likely it`s in the immediate neighbourhood.. and anything can be a source of RI, a loose nut and washer, a bad line insulator, a lightening arestor, a capacitor, connections anywhere, transformers.. you name it and it has been a culprit at sometime someplace. Lots of times I have found it to be other customers faulty equipment causing it, sometimes it`s a piece of equipment owned by the customer. Could be caused by telecomunication circuits. If they are on the power poles, that would acount for the hum seeming to originate from the line. You have my sympathy, you have a tough hoe to row tracking this problem down!
                        Location: Saskatoon, Saskatchewan, Canada

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                        • #13
                          darryl: I read your original post again and I guess you can scratch my balanced lines idea. If your hum bucking pickups have no problem then you're actually getting real signal generated by the pickup and the balanced system will only deliver it better

                          Arcane made a few good points and another major source of serious noise is large ac drives. They are used commercially and institutionally (nursing homes, hospitals, etc.) in clothers dryers and air moving systems. A proper filter for them is the size of a large breadbox.

                          If you decide to try notch filters, watch out for the noise levels. Computer/sound card based filters have their own headaches, some of the worst being noise. A USB interfaced sound system where the analog electronics are separated from the PC might work out if there is software support for filtering.

                          Den

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                          • #14
                            Den, good thought about the balanced inputs and common mode noise rejection. In this case, it's strictly hum coming into the pickup, so it's part of the output from the pickup. The only way to kill this is to prevent it entering the pickup, or electronically remove it from the signal, which means using a notch filter.
                            We have noticed that the problem is not quite as severe, though nearly so, in the control room, which is about 30 ft further away from the power lines. It's quite possible, as Evan said, that by nulling the magnetic field in the one room, we'll be changing it, and quite possibly increasing it in another area.
                            So far I have relied on my ears to determine the nature of the hum. I think I'll try 'scopeing the area with a wire loop to see how complex the field is. At the least, that will show me how to orient the corrective loop, if we do try that.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              darryl:

                              Would it be possible somehow to "sum" the output of one pickup with the (inverse polarity) output of another?

                              One would pick up legitimate string energy while both would pick up the magnetic field and cancel. Even a series connection of the two should work if they are feeding a low impedance input.

                              Again, maybe the 2nd pickup could be silly-puttied out of sight onto the back of the guitar

                              Den

                              [This message has been edited by nheng (edited 02-17-2004).]

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