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Author Topic: Faraday cage  (Read 6100 times)

Fenris Wulf

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Faraday cage
« on: June 28, 2010, 02:49:16 am »

Please delete if it's the wrong forum. Has anyone ever built a Faraday cage?

I'm getting a lot of buzz when I record guitars with single-coil pickups. Replacing the pickups or eliminating the source is not an option. I'm going to have to build a portable Faraday cage, just large enough for the guitarist to sit or stand in.

Is one layer of steel chicken wire, with 1/2" spaces, going to block 60 Hz mains hum and its harmonics? Can I join the sides with solder, and connect the ground to the ground plug on the electrical mains?

For a door, I'll probably make a wood frame, a thin sheet of fiberboard to support the chicken wire, and metal strips around the door to make good contact.
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bruno putzeys

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Re: Faraday cage
« Reply #1 on: June 28, 2010, 03:26:08 am »

I think it's one for the tech forum. Faraday cages are electrostatic shields. You seem to be having a magnetic problem. To verify, see if you can find an orientation of the guitar where the problem goes away. If you can, it's magnetic and a faraday cage won't do the trick. So first check this and report back.
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Fenris Wulf

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Re: Faraday cage
« Reply #2 on: June 28, 2010, 03:39:41 am »

It's typical mains buzz, it's reduced but not eliminated by rotating the guitar. But a Faraday cage also blocks EMI according to Wikipedia.
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bruno putzeys

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Re: Faraday cage
« Reply #3 on: June 28, 2010, 07:36:44 am »

For a faraday cage to stop magnetic interference alone it has to have very low resistance. The lower the frequency you want to stop, the more solid it has to be. Alternatively you can use magnetic materials to shield magnetic fields but that is technically not a faraday cage.

Unless your problem is capacitive coupling, chicken wire won't stop it. And even then.
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syntheticwave

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Re: Faraday cage
« Reply #4 on: June 28, 2010, 04:37:01 pm »

..as Bruno says, you wouldn`t need faraday cage but magnetic cage from highly permably material. The problem is, your Fans cannot see you anymore at the stage.

I would try different pic- ups. Or you can place additional coin near the picup, but not in magnetic range of the strings. Wired in opposite direction this coun will only grasp the hum. Is the amplitude the same and the phase reverse because of the alter wire direction, the hum of the pick- up and the hum of the coil will subtract each other.
But I think many good pickups including such compensation coins.

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compasspnt

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Re: Faraday cage
« Reply #5 on: June 28, 2010, 07:26:31 pm »

Some people have removed the pickups, then lined the pickup cavities with magnetic shielding (I seem to remember some sort of shiny tape stuff), and then replaced the pickups. Ring any bells?

EDIT:  Here are a couple of links, I don't personally know these, but could be worth a look...

http://www.fretnotguitarrepair.com/ElectronicsRepair.htm

http://www.guitarnuts.com/wiring/shielding/shield3.php

Good luck!
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martindale

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Re: Faraday cage
« Reply #6 on: June 28, 2010, 07:57:30 pm »

Fran and I designed a very effective Faraday shield at Skyline Studios in NY--we were just a couple blocks away from Empire State Building, where all the FM stations broadcast from--very high RF field. COMPLETELY surrounded rooms with copper mesh during shell construction, sewed every seam (with more copper wire); grounded this shield. What a pain!
The size and spacing of the holes in the copper mesh correspond to frequency of RF you will block; but since there may be a range of frequencies you want to block, to an extent you have to "broad band" this. You might want to rent an RF signal strength meter and find out intensity and frequencies most troublesome and choose mesh hole size and spacing accordingly. I have since used this method in other studios, all to good effect. Be careful on implementation; you could waste a lot of time and money on metal mesh. Note that a typical, massive studio shell will also provide a definite attenuation of RF, so if you're starting a buildout from scratch take this into consideration.   PM me if you'd like more info
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Dominick

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Re: Faraday cage
« Reply #7 on: June 28, 2010, 10:34:53 pm »

We built one at Right Track. A knock-downable 7' cube of 2x4's and copper screening (like window screen). It was effective.
After that we lined the walls, ceiling & floor of the small iso room in studio A.
that worked too,
Producer Ben Grosse and guitarist Carl Bell, working on Fuel's Something Like Human", wanted to do guitar overdubs with Carl playing in the control room with his amp heads in the control room and cabinets in the live room
Carl mainly used telecasters through hi gain amps and the rig picked up RF & EMI.
We lined the entire 20' x 15' x 9' control room over a weekend to make this happen.
They were happy.
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Dominick Costanzo

Fenris Wulf

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Re: Faraday cage
« Reply #8 on: June 28, 2010, 10:41:00 pm »

It's not RF, it's electrical hum/buzz. I record many different guitarists and don't have the opportunity to do work on their guitars. Some kind of wire/screen that allows sight & ventilation is the only practical solution, if that won't block EMI I'm out of luck.
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Fenris Wulf

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Re: Faraday cage
« Reply #9 on: June 28, 2010, 10:43:57 pm »

Dominick wrote on Tue, 29 June 2010 03:34

We built one at Right Track. A knock-downable 7' cube of 2x4's and copper screening (like window screen).
...
Carl mainly used telecasters through hi gain amps and the rig picked up RF & EMI.


Did the 7' cube block both RF and EMI? If so, that's great news.

*edit* Apparently RF and mains hum are both forms of EMI, but at different frequencies. I really need to brush up on my science.
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compasspnt

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Re: Faraday cage
« Reply #10 on: June 29, 2010, 01:45:26 am »

TRY SHIELDING A GUITAR ITSELF AND SEE WHAT HAPPENS.

(See links above.)

Do you get the same "mains buzz" with a humbucking pickup as with a single coil?

I would guess not.
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Fenris Wulf

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Re: Faraday cage
« Reply #11 on: June 29, 2010, 01:57:44 am »

No buzz with humbuckers.

Shielding the cavity will probably work, BUT, I don't have the opportunity to do that to each and every guitar that comes in here. I do quick demos for a large number of bands, and they bring in a wide variety of poorly maintained guitars which I have to record as-is.

I'll have to try a few things and see what happens.
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Dominick

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Re: Faraday cage
« Reply #12 on: June 29, 2010, 08:07:38 am »

Our problem at Right Track was mostly EMI (hum). We weren't picking up WCBS or gypsy cabs.

Fenris Wulf wrote on Mon, 28 June 2010 22:43

Dominick wrote on Tue, 29 June 2010 03:34

We built one at Right Track. A knock-downable 7' cube of 2x4's and copper screening (like window screen).
...
Carl mainly used telecasters through hi gain amps and the rig picked up RF & EMI.


Did the 7' cube block both RF and EMI? If so, that's great news.

*edit* Apparently RF and mains hum are both forms of EMI, but at different frequencies. I really need to brush up on my science.

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Dominick Costanzo

Fenris Wulf

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Re: Faraday cage
« Reply #13 on: June 30, 2010, 03:58:16 am »

syntheticwave wrote on Mon, 28 June 2010 21:37

 Or you can place additional coin near the picup, but not in magnetic range of the strings. Wired in opposite direction this coun will only grasp the hum. Is the amplitude the same and the phase reverse because of the alter wire direction, the hum of the pick- up and the hum of the coil will subtract each other.
But I think many good pickups including such compensation coins.


Thanks for the idea! Apparently you can make a compensation coil/dummy coil that's buffered by an op-amp so it doesn't change the pickup impedance, and it cancels the buzz.

All I have to do is build one that clips onto any guitar and connects to the output jack with a Y cable. Ho ho.

*edit* I finally found the source, it's coming from 2 elevator shafts on either side of the room. Duh! Moving the guitarist 15 feet down the hall eliminates most of the buzz.
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