Shielding Paint, Neo Mags, and Guitar Circuits

[verhoevenc]

Hey folks. So I'm trying two new things on a build and am wondering if their interaction is going to cause me problems?
1) Shielding the guitar's control cavity with Stew Mac's shielding paint.
2) Holding the cavity cover on the back on with magnets instead of screws.
However, the shielding paint is touching the magnets... is this going to cause me issues? Having magnets essentially "in" the circuit? Something in my head tells me this may lead to ground loops? And something in my head tells me this is not an issue at all?
Any thoughts either direction?

Side question: for those that use shielding paint often, is it robust enough after 3 coats to use AS my grounding medium? IE: if I attach a lead to the paint and the paint touches all my components I shouldn't have to ground each component's housing? Or more importantly, maybe I SHOULDN'T wire-ground each component because it could cause ground loops if already grounded by the paint?

Thanks,
Chris

[Steve A.]

Hey folks. So I'm trying two new things on a build and am wondering if their interaction is going to cause me problems?
1) Shielding the guitar's control cavity with Stew Mac's shielding paint.
2) Holding the cavity cover on the back on with magnets instead of screws.
However, the shielding paint is touching the magnets... is this going to cause me issues? Having magnets essentially "in" the circuit? Something in my head tells me this may lead to ground loops? And something in my head tells me this is not an issue at all?
Any thoughts either direction?

I don't see why it would matter with the magnets touching the shielding paint. I don't understand the point of using magnets instead of screws. Ground loops have to do with electrical currents and not magnetic fields.

Side question: for those that use shielding paint often, is it robust enough after 3 coats to use AS my grounding medium? IE: if I attach a lead to the paint and the paint touches all my components I shouldn't have to ground each component's housing? Or more importantly, maybe I SHOULDN'T wire-ground each component because it could cause ground loops if already grounded by the paint?

No, I would not trust the shielding paint to do that. To connect the shielding paint to ground I will secure the ground wire to the body with a screw and then paint over it. If you keep the signal returns from the pickups and pots separated from the ground you shouldn't have problems with ground loops. Join all of the signal returns together and connect them to "chassis" ground at one point. (That is the star grounding that John Atcheley from GuitarNuts recommended over 15 years ago- I just found a printout I had made of his first website before going commercial. )

I've had trouble with shielding paint flaking off so after shielding a control cavity I may put a coat on top of it as an insulator and to help keep it from flaking off. My first choice for shielding is copper foil tape and I solder all of the joints. And usually put a coating of solder on all of the copper. For this I use my 80 watt iron with 60/40 solder that has no flux. I'll put Scotch 33+ anywhere that a wire might touch the copper.

Just my opinions...

Steve Ahola

[Enzo]

I don't care about your magnets, but if the cover shielding does not make electrical contact with the body shielding, it won't act as a shield, so those magnets either need to be conductive or a wire needs to go to the cover.

No, don;t rely on the paint as a circuit element. Use wire for all ground connections within the guitar, then run one wire from that ground to the painted surface.

[diagrammatiks]

Is it a good idea to use magnets?

My gut says no.

It just seems unnatural to stick unnecessary magnets around a circuit that is already depending on so many other magnets.

[Steve Conner]

Magnets themselves are unnatural, so whatever.

Earlier this year, however, the pair further elaborated on their worldview with the single Miracles, a quasi-mystical number about all the wonders of the natural universe that Insane Clown Posse don't – but more importantly, don't want to – understand. The widely circulated highlight finds Shaggy 2 Dope pondering "Fn' magnets – how do they work?" before announcing his question was effectively rhetorical, because all scientists are "lying motherfers" (Saturday Night Live took this anti-science angle up and ran with it, in style).

[jrannanpaa]

I'd rather use copper tape (as Steve already mentioned) instead of conductive paint. Just remember to solder each tape together ("tick" all the way with constant grid) and remember that if iron is too hot, tape will loose its hold. Tape is not as "neat" as paint, but if you really want to shield things, it's much more secure. It might be a good idea to paint the cavity before tape if its bumpy plain wood.

Anyways, I'd keep two things in my mind:
1) sketch the whole system to paper before doing anything
2) remember that shielding control cavity does not help if pickups and incorrectly grounded bridge catch the interference; it's already in the system

By the first I mean, make sure that all things are star-grounded to jack connector GND, the shield @ control cavity as well (only one point). This is how you avoid ground-loops.
Before doing anything, you should check that all parts such as bridge, pickup covers etc. are also star grounded (separate cables). Cut all cables as short as possible and use twisted pair when applicable.

Per your original question, it seems that you already know the problem since you're planning to start quite a job to shield control cavity. Are you sure that resolves the problem?

Jarno

PS. There should be no problems using constant field magnets, they're not at least causing any interference.

[soundguruman]

This is funny because the shielding paint will not do squat, and neither will stick on foil.
The noise you are trying to get rid of is coming from the pickup! Read my lips: from the pickup!
And you can shield all you want, adding all kinds of parallel capacitance to the circuit, it will only kill the high frequencies more and more.
But what it won't do, is block the 60 cycle fields from entering the pickup, then being amplified as always.
Wake up and smell the hum.
I have worked on these things for 37 years. I have seen "shielding paint" systems and "shielding foil" systems come and go, one after the other. And guess what? Nobody ever came up with a system that really worked.

[km6xz]

What are you trying to accomplish? Note that noise is a many sourced problem so the solution requires attacking it in a number of ways. Part of the noise is magnetically induced but that is a minor portion of the noise because the M fields diminish quickly when going out from its source. Low frequency hum is induced into pickup but as you know, that is not the buzz that is so irratating. Not many guitar pickups or amps are plagued with 60/50 Hz hum due to the strong magnetic fields close by magnetic field generators like motors, unbalanced power lines, and transformers. The field can be strong, for a few inches, getting further away takes care of the magnetic fields.
The most troublesome noise is E fields which can be effective for great distances(radio signals E fields dominate anything more than a few feet from a transmitter yet both E and M fields are generated by the antenna system. The M field is hard to measure a dozen feet away, yet the E field can easily span the ocean.
If you are hearing and wanting to reduce buzz, the raspy high harmonic content E field electrical shielding does well. Paint can help but not as much as copper flashing tape, or a metal box covering the components inside the guitar well. Star grounding works fine in circuits were there is need to reduce ground current, or or precisely, the imbalance in ground currents through parallel paths. But in guitars, ground currents are not the issue, it is E field pickup, like a radio antenna. The star ground is a nice idea....for systems with ground current flow like studios, mic pres, guitar amps etc...but sort of useless in guitars.
A good bond to the shield( and in this case, an unbalanced signal path, the ground and shield are the same connection) to whatever electrical shielding you have, say copper flashing tape, with a solder bead running along the junction of overlapping layers can be cheap and very effective and will not flake or age like the fancy paints. You can forget all that and buy a small metal box just large enough to cover the controls of the guitar from underneath the plate and mount the pots and switched to the box with their shafts extending through the mounting control plate.
There are also active noise suppression systems for E fields, where a sense antenna is used to sample the field, and cancel the noise that is introduced to both the signal path and the sense antenna(the output of the sense antenna is inverted) and cancels the common mode noise leaving the differential signal.
The way to attack noise is usually to use reasonable precautions then measure the nature of the remaining noise and its sources and apply logical remedies. General purpose internet folklore can be counterproductive. Noise is an issue with all signal systems and can be dealt with using logic and sound engineering a lot better than by high priced miracle cures. Listening to the noise or measuring its spectrum will tell you what you are up against and suggests how to fix it. I suspect it is NOT magnetic or else you would be getting a clean low frequency hum, not raspy buzz.

[Steve Conner]

You got it! Sometimes the problem is E-field, sometimes it's H-field (some people call it M-field or B-field but whatever! ) and sometimes it's both.

You need to figure out which is causing the problem and take the appropriate steps. Copper tape and so on will only stop the E-field. In particular, it won't stop single coil pickups from buzzing like hell by picking up H-field from transformers and so on. This can be quite a dirty buzz, because the stray field from transformers is very distorted and rich in harmonics, and guitar amps emphasize the harmonics even more with treble boost.

To deal with H-field you need humbucking pickups or a dummy coil system.

[Steve A.]

Star grounding works fine in circuits were there is need to reduce ground current, or or precisely, the imbalance in ground currents through parallel paths. But in guitars, ground currents are not the issue, it is E field pickup, like a radio antenna. The star ground is a nice idea....for systems with ground current flow like studios, mic pres, guitar amps etc...but sort of useless in guitars.

I've been wiring up custom harnesses for my guitars for 25+ years and until I ran across John Atchley's site in 1997 (this is before GuitarNuts) I always treated grounds and signal returns as being completely interchangeable. In fact I thought it was better to wire them up that way since it kept the leads much shorter and presumably less likely to pick up noise. When I started wiring the signal returns and the grounds separately the noise level of my harnesses was reduced considerably. I do agree with you that it is not necessary to have just one star ground point as you would have in a guitar amp. (For a few of my guitars I used John's initial recommendation of separating the signal returns from ground with a large value poly cap- I was able to fit a 2.2uF/250v cap inside one of my teles! I thought it made things quieter but I guess its main purpose would be to prevent shocks.)

I have long considered wiring up my guitars with a stereo jacks and use 2 conductor shielded cable (like a balanced mike) but terminate it at the amp with a mono plug connecting the signal return to the ground. Or better yet use a stereo jack on the amp and run the signal return to the grounded cathode ground of the initial gain stage.

In Les Paul's you are supposed to ground the pots with 3 ground wires (not 4) so that you don't have a little loop antenna inside the control cavity. Does that really make a difference?

Thanks for your very informative post about guitar noise! I'm saving it as a text file and pinning it to my taskbar.

Steve Ahola

P.S. I just rewired the controls of my 1960 10 string console steel guitar so that the signal return from the single coil pickup goes to the ring terminal of a stereo jack. This is to connect to a floor-mounted box with a dummy coil wired up similar to the Suhr system. Plugging in a mono cable connects the signal return to ground so it can work with normal stomp boxes and amps. I wired up the dummy coil by hand wrapping 38GA magnet wire around an aluminum soft drink can (570 wraps worked out to 330 ohms) and dipping it in potting wax. That project is on hold right now as I am in the middle of sorting out crap I've been collecting for 20+ years. I still need to determine whether a plastic or metal enclosure would work better. A P-90 with a metal cover and baseplate still has single coil noise so I figure that the dummy coil should work to some extent in a metal box. I would like to use a metal box because I want to use it with the dummy coil switched off as volume and tone controls for steel guitars that don't have them. But if plastic works better for the dummy coil I will skip the volume and tone controls.

[Steve A.]

Nothing is more embarrassing than trying to figure out why a guitar or amp you are working on developed a nasty buzz and then realizing that you had grabbed a speaker cable by mistake! Especially if you started tearing everything apart to find the problem. D-oh!

[km6xz]

If the source of noise that is getting in....note that most does not since we are in a sea of RF and magnetic noise constantly...is identified and steps taken to address it directly a lot can be done with unbalanced systems but guitars have not really changed in many decades. During that time, ambient noise levels have increased a great deal. Putting a short antenna on a broadband spectrum analyzer will confirm that instantly, particularly if one is old enough to have made the same measurement 40 years ago. Since living here in St Petersburg I have watched how gradually the base noise floor has increased by 30db in the city center in 8 years. A good deal of that is the increasing use of high efficiency bulbs, leakage from DSL, cable TV leakage, cell phone towers everywhere, personal electronics, computers. The only thing that has gotten electrically quieter is computer displays and TVs.
So noise is something the industry needs to deal with if electric guitar, with the super high gain commonly used, is going to be viable in the future. If the industry adopted a standard for levels, connector and termination z, a universal balanced system could make life easier all the way around. Pickup signals are not weak, but all concerned with the path are worse case S/N ratio conditions. Very high Z cables/amp inputs make the difficulty in keeping noise at bay harder. By shifting to terminated low z links, and reworking pickup<>tone circuits to low z driving capability, the tone dependency of guitar setups with short low capacitance cables would end. A simple balanced line driver<>line receiver arrangement could lower noise and make tone more consistent in one step. As builders of home made gear, a system style approach would be much easier than a whole industry adopt to a new method of generating and transmitting pickup signals. But I am not seeing much effort in that regard, most home builders have ignored engineering and seek out magic and mythology like high end hi-fi has in its drive off the deep end of insanity.
Building a front end that handles the high Z network in current guitars would not be much change as a first step, a fet buffer would maintain the hi-z of current guitar tone and pickups. That could be used to drive a balanced line driver that for $1 would compete with the best matching transformers in common mode performance and could drive almost any capacitance balanced line. Using a companion line receiver at the amp end would do a few things, end the shock hazzard for one, cut down on RF and electrical noise sensitivity by 60db and make high end guitar cable companies go bankrupt since any twisted pair would be as good as a tweaky golden-ears annointed "interconnect" and its $3000 a meter costs. That would be the link. The pickup however, and tone/volume circuit in the guitar, if left as it is, is still bathed in a noisy hostile environment. Instead of the correct way...designing an inherently balanced tone and volume circuit which is not that hard considering the PU starts life as a balanced device....would be to add a sense antenna near the PU for a electric signal cancellation. Humbucking uses that principle for a limited bandwidth of magnetic noise but is pretty limited in effectiveness because is only impacts a portion of the noise energy and impacts the tone. Most of the noise is not cancelled by humbucking, because at further distances from the source of noise, more and more of the noise picked up is picking up the electric field, not magnetic.
The use of external bucking coils like described in your post are limited in effectiveness because the sense coil needs to be in the same orientation, and phase as the magnetic signal that the pickup is subjected to. Notice how sensitive the magnetic noise is to orientation when you can rotate the guitar and cancel it out to a degree? There are ways to "steer" phase however for the remote sense coil or E field antenna but it does make the circuit a lot more complex. It works in a number of fields as in RF communications, Wi-Fi and instrumentation. Sort of the electronic equivalent to the electronic noise cancelling headphones worn by travelers and pilots, or Noise Blankers used for 50 years in two way radio...back when spark plug radiation could ruin communications and even AM radio broadcast reception.
Noise can be dealt with, all other areas of electronics has attempted to deal with it except musician gear. If someone was serious about it, home builders would be the logical place for that to happen since there would be less need to seamlessly integrate into existing systems. The whole home made system could be optimized for sound AND quiet.

[Markizz]

thx for help

dude you very help me

[J M Fahey]

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Always nice helping you spambots.
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