Faraday Fabric for Shielding Amp Noise?

Stan Townsend · Posted 20 Apr 2022 4:24 pm

I live across the street from a cell tower that recently went 5G. My Webb 614E amp has been so noisy with radio interference that I can't hardly use it. When I take the amp out of the area, as I did to a repair shop, it's as quiet as can be. This is the only thing I can think that it could be. I've checked house outlet grounding. I've taken it down the street to another house (but still in the cell tower vicinity) with the same noise. I've had the capacitors changed and pots cleaned. Still makes the radio noise.
Would this new copper or Cu/Ni Faraday fabric help if I made an amp cover or half cover out of it? Any thoughts or other solutions?
I know there are some brilliant electronics guys out there who might have a solution. Thanks.

Georg Sørtun · From: Mandal, Agder, Norway

You may try Ferrite beads…
https://resources.altium.com/p/how-do-ferrite-beads-work-and-how-do-you-choose-right-one

Stan Townsend · Posted 20 Apr 2022 4:51 pm

I don't know how all that ferrite bead works, or where I'd get one, but I've determined it's not from the outlet line plug AC. All outlets are properly grounded and my other solid state amps don't do this. I've tried hum blocking devices on the AC line which do nothing.
But thanks for the idea!

Georg Sørtun · From: Mandal, Agder, Norway

https://www.amazon.com/ferrite-beads/s?k=ferrite+beads

You probably need to filter/block HF from entering the low-level signal connections and amp-stages in the amp. I don't know the amp, so cannot guide you in where exactly to fit them. What I do know is that no degree of shielding of the cabinet will prevent the HF signal from following the cabling into the amp.

Mike Auman · From: North Texas, USA

Could you describe "radio interference" a bit more? Do you hear people talking, or music playing? Or is it more like digital data with bursts of noise, either random or in a pattern?
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Stan Townsend · Posted 20 Apr 2022 7:02 pm

Here's a link...

https://linksharing.samsungcloud.com/oPmY9oXWWDsA

Mike Auman · From: North Texas, USA

Yep, that sure sounds like digital RFI. You can get bursts of that from a cell phone laid on top of an amp, but your noise is continuous and depends on orientation, so I suspect you're right about the cell tower. As Georg mentioned it's probably the low-signal parts of your amp (input jack to preamp) that are most susceptible, and shielding those would be a good start. One way is to remove the amp head from the cabinet, and line the inside top & bottom of the head section of the cabinet with a thin layer of conductive metal. You can use aluminum foil (requires glue), aluminum tape (hardware store) or copper tape (StewMac.) Make sure you ground the liner to the metal chassis somewhere, either with a screw or by soldering a wire. https://www.stewmac.com/electronics/shielding/conductive-copper-tape/
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Bill Fisher · From: Oklahoma, USA

Call the company with the damn cell tower, and have them fix it.

Bill

John Ducsai · From: New Hampshire, USA

As Bill mentioned, call the cell tower operating company. I would let them know that you're copying your state's public utilities commission as well. You have a very legitimate complaint.

Dave Grafe · From: Hudson River Valley NY

Before taking the amp apart best determine that the noise is not coming through the guitar. Just saying.

If the amp remains noisy when nothing is plugged into it the noise could be introduced by faulty ground wiring in your home acting as an antenna.

Most commercially produced amps already have RF shielding installed, as anyone who has ever sliced a finger while reaching for the standby switch on an old Fender amp can attest. There is a limit to what shielding can do, but essentially the internals of any electronics should be surrounded by a grounded conductor. Aluminum foil will do if it is securely attached and grounded, I use it to line the bottom boards of my reverb tanks.

That's as far as I am going to take it, so rather than furthering hearsay advice here's a good source for vetted, credible information on this subject:

http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/shielding.html

As close as you say you are to this tower there may be no real fix, but there is no reason 5G deployment should generate any more noise than the 4G transmissions, it's all RF.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Noise...Hum? Buzz? Squeal? Howl? Hash? What, exactly?

I'm not an electrical engineer, but here are my thoughts.
If nothing else in your house is affected, and if your neighbors aren't having similar problems with their stuff, the F.C.C. likely wouldn't respond. Whether or not your outlets are properly grounded also probably has nothing to do with the problem. (Since you said your amp also has the same issues at your neighbor's house.) The offending interference could still be coming in on the power lines. (I'm not familiar with Webb amps; are they fully shielded, with a plate or metal screen in the cabinet or on the amp chassis bottom?)

Other questions came into my mind, but once you said...my other solid state amps don't do this, I realized that the problem is definitely in your Webb amp. (That presumes that the amp doesn't have a bad component causing the problem.) If there is no problem with other amps or AM radios in your house, there is something in that Webb amp that's responsible for the issue. Equipment is supposed to be designed with these things in mind.

A good tech could install a power line filter in the amp, and add chokes or caps to the amp inputs that may eliminate the problem. The added parts for RFI/EMI suppression could total $20-$40, while labor would likely be $150-$250.

ajm · From: Los Angeles

Stan: Strange that you should post that video/audio.

Lately I've been getting a similar high whiny noise mixed in with my amp also.
I have not tried another amp.
I live near a bunch of cell towers/antennas, and it's tough to keep up with who is doing what, so I wouldn't know the correct party to address a complaint with.

In your video you didn't say where the volume control(s) is set.
If you turn them all of the way OFF, does the noise go away?

The amp that I'm using is not an old Fender, but I got the idea from seeing one.
I added an aluminum shield under the top of the amp like the old Fenders have.
I think that I bought a roll of it years ago at Home Depot.
It is way thicker than foil, and I stapled it to the underside of the top of the amp.
When you install the chassis and screws on the top of the amp, the chassis is pulled up and it makes contact between the shield and the amp chassis.
I think it's a good idea to do, but like I said, it may not be the solution to your problem.

Some Carvin amps used to show on the schematics a ferrite bead at the input.
The Vintage 33 and Vintage 50 were two of them.
Note that the ferrite bead would be in SERIES with the signal whereas a small cap (10-100 pf?) would be from the signal line to ground.

Stan Townsend · Posted 21 Apr 2022 7:44 am

ajm,
Yes, the noise stops with the volume off. The volume in the video was around 9, just so the noise was clearly heard, but it is present and annoying when volume is at 4 or 5.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Stan, I can't open the link you posted, so I don't know what the sound is. Does the amp make the noise with no instrument plugged in? What else are you using at the input; type of instrument, stomp boxes, preamps, type of volume pedal, cables, etc. Have you tried different, or shorter cables?

Robert B Murphy · From: Mountain View, Arkansas, USA

Stan, I am an eletrical engineer, retired, and that sure sounds like digital modulated RF to me. Interference on AC power lines gets filtered out pretty effectively by transformers, and in the case of solid state amplifiers, by the voltage regulators on the DC power supply. Because your demontration showed the noise being both directional and without anything plugged in, I think your amp needs more shielding. Everything everyone has said here about foil or conductive fabric is right, any metallic faraday cage around the amp will work but any unshielded antenna getting into the circuit board, say the tone and volume wires to the pots will be a path for unwanted signals.

At one time I used a conductive paint on gov/mil circuits. It had really small shavings of copper that made a conductive surface when dried but it was pretty pricey. Whatever you cover it in has to be grounded. Fender's foil made contact to the chassis and the edge contact was enough, no dedicated bond wire needed.

I'm not familiar with the 5G modulation format but I'm guessing that it is pretty similar to the CDMA scheme: direct digital modulation using orthogonal codes to make each channel look like noise to the other channels. Every phone that the tower sees gets at least one channel and each call or data-link gets more channels opened up as needed. What this means is you are in the worst possible place where the tower is blasting away at max power to the farthest users and varying power levels that drop as each user gets closer. The channels are not specific frequency bands, everyone shares the same giant wide RF band. You get the same sounding intereference all the time, right?

Good luck complaining to the FCC. I don't mean to be Eeyore here but, as an FCC General Class Commercial License holder, I can tell you that it ain't the same agency that I knew.

A pretty simple conclusive test would be to turn your amp on and wrap it in tin foil. Those guys with tin foil hats aren't completely wrong about shielding.

Ferrite is an iron impregnated ceramic that acts like a series connected choke (coil). Sometimes they are little donuts, sometimes tubes, sometimes plastic shrouded sections that clamp over a wire. They are pretty good at choking off high frequencies and keeping them out of the high-gain amplification parts of your amp. Sometimes an incoming wire will be wrapped several times through the core to boost it's high frequency rejection.
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Donny Hinson · From: Glen Burnie, Md. U.S.A.

Try moving just the line cord (not the amp), or coiling it up to see if the noise intensity changes. If so, it's getting in via the line cord.

Stan Townsend · Posted 21 Apr 2022 4:28 pm

Donny, the coiling of line cord or anything to do with the AC power cord makes no difference in the noise. Turning the volume knob to zero stops it.

Stan Townsend · Posted 21 Apr 2022 4:39 pm

Robert B, thanks. That sounds like what most people are telling me. But there is a high, very HIGH end audiophile shop across the street, in the building upon which the cell tower antennas are mounted. He has had no problems with interference in his audio equipment. He believes it is radio, RF, not cell tower. I took the amp across the street to his shop and plugged it in...same noise and intensity as my house. Seems like shielding or grounding may be the sol'n. But how to do it simply without tearing the amp apart. Whoa!

Robert B Murphy · From: Mountain View, Arkansas, USA

Stan, cell phone towers radiate RF. Admittedly most of the transmissions are in the gHz spectrum and up but in the U.S. they can start as low as 600 mHz. Since audio frequencies are -20Hz 20kHz, you might ask: How come I can hear anything that high? The digital signal that modulates the RF carrier can create side bands that can go, as the saying went; 'from DC to daylight.' I used to live across the street from an ATC radar and I could hear the audio pulse rate of the gHz RF signal as the dish swept past on my TV. It sounded like the roadrunner cartoon-meep...meep.

Your audiophile neighbor's equipment I assume is pretty well shielded? Gold plated coax connectors? Full metal boxes housing the amplifiers? The sound I heard in your demo was pretty complex, a very high pitched whine with a lot of grating noise. I'm used to hearing simpler digital modulation like v.22 and v.23 but 5G QPSK is way more complex too. You can get annoying noise like that from arcing brushes on motors but not across the street from a different power pole transformer too.

If it is RF digital sidebands from the cell tower, the amplitude is going to drop off fast as you move away from the tower. Keep in mind that the tower with all the antenna elements, those long white vertical panels that slope down a little, will have a pattern with a null underneath and maximum gain toward the horizon. Even so the signal will decrease rapidly as you move away from the tower. If you take the amp a block away in any direction and the noise decreases, and 2 blocks away it drops yet again, you have proved the source of the interference.

Try the tinfoil, you saw yourself how the orientation of the amp changed the noise level. You probably won't need to completely cover it, just take a sheet and move it around but you might need to touch the input jack to ground out the sheet.
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Bob, small o.

Michael Brebes · From: Northridge CA

Looking at a schematic of the preamp section, it looks like there is a Sensitivity control as well as tone controls and a 3 position switch before the Volume control. I would check how the noise changes with those controls. That might give you more of a clue as to which part of the circuit is the problem.
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Bob Sykes · From: North Carolina

Stan, You say the noise stops with the volume down. Do you have the noise when the volume is turned up but nothing plugged into the input jack?

This will determine if the noise is being picked up by the guitar, cabling (and whatever else is hanging off the input), or if the noise is being picked by the low level (sensitive) electronics inside the amp.

If it's the amp, you may want to take it to someone experienced in fixing noise problems. As mentioned, Ferrites and shielding can help, but must be properly applied to be effective. You pretty much need to know which internal part, cable etc. is playing receive antenna. I can give you some pointers on troubleshooting this if you are comfortable working on live amps.

Calling the cell company or the FCC would be a waste of time. Cell carriers share towers and co-locate their antennas. Which one ya gonna call? Doesn't matter. As long as their transmissions are in compliance with the FCC Rules, they are not doing anything wrong.

The FCC (gov't agency) offers no interference protection to guitar amps Smile

They only provide interference protection to licensed transmitters. All other electronics must accept any interference.

Now if the cell tower was interfering with your T.V. set rabbit ears reception, you might get some relief from the FCC.
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Jerry Erickson · From: Atlanta,IL 61723

If you have a microphone stand, try wrapping the cable going to the amp around the mic stand 5 or 6 times and see if that helps. If not, try wrapping your amp power cord around the mic stand before plugging it in and see if that helps.

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