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Sheilding

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(@ricochet)
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Joined: 21 years ago
Posts: 7833
 

Shielding's not about to stop single coil pickups from humming around computer monitors and TVs with CRTs in them. The pickups are doing what they're supposed to do, generating an electrical signal from a fluctuating magnetic field. The only way to reduce that is to switch to humbuckers of one sort or another.

"A cheerful heart is good medicine."


   
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(@slejhamer)
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Shielding's not about to stop single coil pickups from humming around computer monitors and TVs with CRTs in them

True, not entirely, but it definitely helped in my situation. Also, my basement practice room has fluorescent overhead lights that cause the '51 to hum, and now the guitar seems relatively quiet. YMMV.

"Everybody got to elevate from the norm."


   
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(@ricochet)
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Joined: 21 years ago
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The shielding's helped hum that was being picked up by the wiring. If you could shield hum out of the pickup, you'd prevent it from working.

"A cheerful heart is good medicine."


   
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(@mcdouggy)
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Joined: 20 years ago
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The shielding's helped hum that was being picked up by the wiring. If you could shield hum out of the pickup, you'd prevent it from working.

Hmm..as I understand it, you can reduce the amount of hum absorbed by the pickup by shielding the inside of the cover, over the coil of wire. According to what it says in the shielding kit I bought, you insulate the coil of wire around the side of the bobbin, and where the pickup wire leaves the coil on the base. Then you take normal foil, and put this on the inside of the pickup cover, and connect this to ground or the rest of the shielding network with a wire. You will never be able to completely isolate hum from the circuit, but you should be able to reduce it to a non-audible level. This is all theory though, eventually I'll try it out for real.

Doug

Visit my band's website!


   
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(@slejhamer)
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The shielding's helped hum that was being picked up by the wiring.

That's true. :)

"Everybody got to elevate from the norm."


   
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(@ricochet)
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Joined: 21 years ago
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Doug, I think it was Gnease that correctly explained in an earlier post that shielding helps with electrostatic interference, but very little with magnetic interference.

Pickups are very sensitive to varying magnetic fields. Their own magnetic field's disturbed a little by the strings vibrating in it, and they make a voltage from that. Any external varying field also creates a voltage in them. Humbuckers are built with reverse polarity magnet poles and reverse wound coils, so that the vibrating strings produce reinforcing voltages in the two coils, but an external field produces opposing voltages in them that cancel out.

"A cheerful heart is good medicine."


   
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(@mcdouggy)
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The shielding's helped hum that was being picked up by the wiring. If you could shield hum out of the pickup, you'd prevent it from working.

Sorry! But can you explain what you meant when you wrote that then?

I'm a bit slow! :D

Doug

Visit my band's website!


   
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(@ricochet)
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Joined: 21 years ago
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See my last post directly above. The pickup works by responding to a variable magnetic field. Most hum comes from varying magnetic fields. If you could shield them out, the pickup couldn't work. You can partially shield out varying electrical fields, and humbuckers can cancel the effects of an external magnetic field, but you can't shield it out.

Transformers, fluorescent light ballasts, light dimmer switches and CRT deflection coils are common sources of noisy magnetic fields. If you play around any of these and are bothered by hum, use humbuckers.

Many guitars with single coil pickups have a switch position that puts two reverse-wound, reverse-polarized pickups together to act like a big humbucker, and this helps a lot, but it's not as quiet as a humbucker because the pickups are far enough apart that the magnetic field can be sufficiently different between them that it doesn't get cancelled out.

"A cheerful heart is good medicine."


   
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(@mcdouggy)
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Ahh, so the metallic shielding wouldn't have any effect on the magnetic field interference? I thought I remember reading on http://www.guitarnuts.com that CRT interference and the like was practically annihilated

Doug

Visit my band's website!


   
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(@ricochet)
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Joined: 21 years ago
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No, they're dreaming. CRTs use electromagnets whose field is varied to deflect the electron beam across and up and down the screen. The hum we hear's mainly from the slower vertical deflection coils. You can hear the horizontal scanning frequency from a U.S. TV screen (15750 HZ, a really high whistle), but most guitar amps won't reproduce it well, and most computer monitor settings nowadays put the horizontal frequency into the ultrasonic range.

Magnetic fields are really difficult to shield. Remember the early '90s, when there was a lot of furor about the supposed health hazards of magnetic fields? Many monitors were redesigned to reduce the exposure of users to their magnetic fields. Heavy metal shielding around the coils helped, but it took in many cases redesigning the system so as not to produce as much magnetic field in the first place. You're not going to get very far using metal foil around your pickups and guitar cavity to keep a monitor's magnetic field out.

What DOES reduce the effect of magnetic fields is to completely surround the object to be shielded with an electrical conductor, the lower its resistance the better. It doesn't need to be ferromagnetic, just conductive. The varying magnetic field produces "eddy currents" in the metallic shield that generate their own magnetic field, and it opposes the induicing field. That's partly how the shields around your "preamp" tubes work, along with being grounded to ground out electrical fields. It takes thicker metal to do this effectively than you're going to put in your guitar (even metal pickup covers obviously don't shield them too much from magnetic fields), and like I said you can't do it all around your pickups.

"A cheerful heart is good medicine."


   
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(@mcdouggy)
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Joined: 20 years ago
Posts: 57
 

No, they're dreaming. CRTs use electromagnets whose field is varied to deflect the electron beam across and up and down the screen. The hum we hear's mainly from the slower vertical deflection coils. You can hear the horizontal scanning frequency from a U.S. TV screen (15750 HZ, a really high whistle), but most guitar amps won't reproduce it well, and most computer monitor settings nowadays put the horizontal frequency into the ultrasonic range.

Magnetic fields are really difficult to shield. Remember the early '90s, when there was a lot of furor about the supposed health hazards of magnetic fields? Many monitors were redesigned to reduce the exposure of users to their magnetic fields. Heavy metal shielding around the coils helped, but it took in many cases redesigning the system so as not to produce as much magnetic field in the first place. You're not going to get very far using metal foil around your pickups and guitar cavity to keep a monitor's magnetic field out.

What DOES reduce the effect of magnetic fields is to completely surround the object to be shielded with an electrical conductor, the lower its resistance the better. It doesn't need to be ferromagnetic, just conductive. The varying magnetic field produces "eddy currents" in the metallic shield that generate their own magnetic field, and it opposes the induicing field. That's partly how the shields around your "preamp" tubes work, along with being grounded to ground out electrical fields. It takes thicker metal to do this effectively than you're going to put in your guitar (even metal pickup covers obviously don't shield them too much from magnetic fields), and like I said you can't do it all around your pickups.

All is now understood, thankyou for sharing that, I think the guy at guitarnuts.com should let people know this on his website!

Doug

Visit my band's website!


   
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(@kramer-smc)
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Joined: 18 years ago
Posts: 9
 

This is COOL. I have learned alot about the issue and now have direction clarified. I will swap out to a HB PU on the Neck and Mid positions like the Fender noiseless.

Thanks to all for the help. I love this forum !!! :wink: :o :lol: :D


   
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