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Topic: Emi |
Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 5 Feb 2001 8:13 pm |
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| After the Steel Show in Branson, I brought my equipment back to my shop and set it up. I had a hum! I unhooked my pedal and ran a straight guitar cord to the guitar. The hum was still there, without my pedal even hooked up. By chance, I moved the two output cords coming out of my Peavey ProFex II. The hum changed in sound. Then I noticed these two cords were laying right on top of two wall wart adapters that were plugged into a power strip. One adapter was the unit that powered the Peavey ProFex II, and the other was the adapter that powers my rack mount Sabine tuner. Once I moved these shielded cords off the top of these little transformers, the hum went away. I knew these adapters give off a magnetic field. They cause problems when you bring them real close to your pickup. I always "assumed" that the shielding on a hookup cord would take care of EMI--Electro Magnetic Interference. One of two things took place. Either the shielding on my cords is not working, or the "Field" given off by these transformers will cause problems even with good shielding. I don't know why I had never had this happen before! It makes me feel really stupid, even though I know what is going on. |
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Jack Stoner
From:
Kansas City, MO
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Posted 6 Feb 2001 3:37 am
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Keith, check your cables. I have my Transtube Fex and a MosValve 500 power amp in a rack case with a surge protector type power strip mounted in the bottom of the rack case. I've got the cables that connect the Transtube Fex to the MosValve, the (unshielded) speaker cables and two shielded two conductor cables for the 600 ohm XLR outputs - all on the bottom and I'm not getting any hum.
Also those Korg rack tuners have been known to cause ground hums if not isolated properly. |
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Bobby Lee
From:
Cloverdale, California, USA
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Posted 6 Feb 2001 8:25 am
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| All cables are not created equal. |
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Larry Bell
From:
Englewood, Florida
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Posted 6 Feb 2001 10:58 am
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Keith,
(love that pedal, by the way  )
I have seen the same problem, both in my rack systems and even using my Session 400 with a stomp box or two. I solved it by using cable ties secured to the side of the rack to route signal-carrying cables on one side of my rack and power on the other so that the wall warts and power cables do not cross the cables carrying a signal. The problem went away.
Even when I don't use the rack, I've noticed that if my speaker cable or patch cords cross the power supply for your GREAT volume pedal or the one for my Boss delay the same symptoms can appear.
You probably already knew this, but my ElecEngin buddy tells me that the magnetic field encircles the cable and the worst case scenario is when your signal cable runs along parallel to the power supply cables (or power to the amp). If it crosses at right angles it is much less of a problem. For what it's worth.
Hope this helps.
LTB |
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Steve Feldman
From:
Central MA USA
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Posted 6 Feb 2001 2:27 pm
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| And yet another crazy fix: I took the power cables off my preamp, FX unit, and amp and I replaced them with new cables that I got from Radio Slop, cut down to ~2 feet in length. All fits nicely in the rack, and it changed what was already a very quiet system into a virtually silent one - even with the Korg tuner. |
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Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 6 Feb 2001 2:44 pm
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| I like all you guy's solutions. Thanks! New, short,quality,cords make a lot of sense! The electronic field being parallel or at right angles also makes a lot of sense. |
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Bill Terry
From:
Bastrop, TX
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Posted 6 Feb 2001 3:29 pm
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For inquiring minds, I found this web site with some pretty good emag info. This guy puts it in the context of antennas and radio waves, but it's all the same concept. I guess I'm a real geek but Maxwell's equations are truly cool... How'd he figure that stuff out????
Oops... forgot the link: http://hawkins.pair.com/eRadiation.html
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bterry.home.netcom.com
[This message was edited by Bill Terry on 06 February 2001 at 03:30 PM.] |
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Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 6 Feb 2001 9:39 pm
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| Bill, I went to those web sites---COOL! [This message was edited by Keith Hilton on 07 February 2001 at 05:49 AM.] |
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Terry Downs
From:
Wylie, TX US
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Posted 9 Feb 2001 9:23 pm
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Hi Keith,
The normal copper shield is only very effective on electric fields. Large magnetic fields can only be attenuated by shields whose properties have permeability. This would require a steel or iron shield. Aluminum or copper will have attenuation at a certian skin depth, but it will usually need to be thick. The easiest solution is avoidance. Remember the amplitude of a susceptable cable from a magnetic field has a log relationship to its distance from the cable. After you get so far away, it is not so much different than miles away. If you are interested in the math relationships of skin depth of shielding materials let me know and I will send them or post them.
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Terry Downs
http://nightshift.net
terry@nightshift.net
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Blake Hawkins
From:
Florida
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Posted 10 Feb 2001 6:46 am
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As has been said above: "All cables are not created equally.' (or equal as the case may be)
Most audio cables will have a shield effectivity of from 65 to 95%.
The ones with 100% use aluminum foil and that deteriorates when you flex it a lot.
Premimum cables from Mogami, Belden and Canare use tightly woven shields and in some cases may have two shields.
Of course, using balanced audio and "star quad" cables is even better but that is not done in steel guitar world.
The George L cables that Jack uses work pretty well.
Separation is always the best bet. Keep amp input lines away from transformers and speaker leads.
Blake
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 10 Feb 2001 6:55 am
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Cables are rated (but not always "publicly") on the percent of shielding in the outer wrap. "Poor" cable runs from 70% to about 85%. "Good" cable from 90% to 95%, and "excellent" ranges above 95%. But I don't think it's EVER 100%.
Yes, you DO have to be aware of what causes hum when it "suddenly" shows up. Electrical "things" near your cords, pickups, and amps are a major source of problems. |
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Terry Downs
From:
Wylie, TX US
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Posted 14 Feb 2001 9:44 pm
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If the cable could be shielded 100%, it still would not be effective on magnetic fields unless the shield has permeability. It would need to contain an adequate thickness of steel, iron, or cobalt. If your audio line was differential (i.e. balanced mic line), it would have some common mode rejection to the field. Since the guitar cable setup is single ended, there is no rejection of the magnetic field by virtue of a twisted pair.
The best solution as Donny mentioned is to keep it away.
Regards,
Terry[This message was edited by Terry Downs on 14 February 2001 at 09:45 PM.] |
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