Shielding a single coil

[r13211]

Really appreciate the ongoing thread on shielding. After reading through the recommendations, I've settled on the spray on nickle paint. I have various guitars that I plan to do including strats and LP's (one with P90's). Re: pickup cavity shielding with P90's, is the conventional wisdom to coat the PUP cavities and run a wire from there to the control cavity (also coated) and star connect there along with a wire from the pickup switch cavity? i.e., that won't be an issue with tone sucking eddy currents? Thanks for any advice based on results with P90's.

[Bruce Johnson]

Personally, I think that Eddy Currents and the Tone Suckers is an excellent name for a rock band.

My basses are shielded with a combination of:
1.) MG Super Shield spray in all of the cavities
2.) Aluminum plates covering the tops of the cavities and mounting the components.
3.) Brass shells around the outside of the pickup coils, with leads connecting to ground. Only the tops of the coils are exposed.
4.) The coils are wound and wired in alternating pairs, as true humbuckers.

To make an instrument really quiet, you have to cover all of the different problem areas. There isn't one single solution.

[jetcode]

Since I started using Lawrence and Barden strat pickups the need to shield has been relegated to shielded wiring and some protection around the tone and volume controls. Even more amazing is the tone of these pickups. While they are not quite vintage single coil I would be hard pressed to know the difference unless compared side by side. Most certainly I do not miss the hum.

[Steve A.]

I've had very good luck with the nickel spray paint on the top and back of the PU as it grounds all the magnets at once. I just wrap a piece of copper tape around the coil and insulate the ends so they don't form a loop. Ideally you'd avoid loops through the magnets and only connect them to the top or the back. Just look out for shorts to the magnets on your inside coil connection.

Good point! I was going to mention checking for any continuity from the slugs to the coil. Back when I used to do that kinda stuff the copper foil I had did not use a conductive adhesive; what I did have was a conductive felt tip marker with silver paint inside for repairing or prototyping printed circuit boards. It was very pricey 15 years ago and is probably through the roof at this time.

So I would paint around the slugs that had continuity to the coil but I never did think of using the conductive paint on the top bobbins- D'oh!

Steve Ahola

[Steve A.]

There is a website somewhere that is mostly about making strats quiet, they use something called star grounding I think it is. I did some strat pickups for an up and coming "star" and his tech used star grounding and some other technique he wouldn't tell me about and got the guitar dead silent without screwing with the pickups. I found it:
GuitarNuts.com - The Internet Home for Wayward Pickers

I wonder what the secret technique was... The basic Guitar Nuts idea of star grounding is really simple: the first step is recognizing the difference between the signal returns (the black wires on strat pickups) and shield grounds. Yes, they all end up in the same place but the path there does make a difference. To keep it simple I will run the ground from the jack to the "cold" terminal of the volume pot and connect all of the signal returns there. At that point you have two choices: you can then run a lead from the "cold" pot terminal to the case of the pot and connect all of the shield grounds to that point. The second choice offers a bit more isolation between the grounds and the signal returns; I would connect the two with a huge mylar capacitor- something like 1.0uF at 100-250 volts. I thought it made things quieter but later incarnations of the Guitar Nuts site mentioned that it was there mainly to prevent electrocution if there is 120vac between the PA ground and your guitar amp ground. Those caps cost a buck or two and are hard to fit into some control cavities so I haven't used them for awhile.

In any case I owe much to Mr. Guitar Nut. I have been designing and building custom control harnesses for almost 20 years and I was always treating the ground leads and signal returns identically. For one thing it would reduce the length of the leads inside, making things neater. But cleanliness is not always next to godliness and when I started following the basic Guitar Nut guidelines my harnesses were less prone to noise.

The explanation I have for this observation is that by grouping all of the ground shields together it kinda lets them hash out their differences. I think that noise from some of the shields might cancel each other a bit. If these ground shields were connected directly to the signal returns then the audio signal could pick up more the crap.

Well, that's my story and I'm sticking to it!

Thanks!

Steve Ahola

P.S. Electric guitars would have been much quieter had they used a cable with two center conductors from Day One. It would not be a balanced out, but just an unbalanced signal going to the amp with a shield over both of the couplers. Only at the amp would the "cold" center conductor be connected to chassis ground.

You could easily rewire your guitar and amp to work with a TRS-TRS cable instituting this design, but if you wanted to use any stomp boxes you would have to rewire them as well. This is a untested theory but I am sure that it would reduce RF noise but not how much, and whether it would be worth going to all of that trouble.

[Steve A.]

There is more to it than just a coil though, it also has some accompanying passive circuitry... This is listed as a "simplified schematic."

David:

The simplified schematic is pretty simple:

R18 is like a volume control but it bypasses the dummy coil so you can control the % of the dummy coil that is in the circuit. R20 is like a tone control wired up to R18.

With those two controls you can adjust how the dummy coil will work with the sensing pickup (which is wired in series). Too much dummy coil will compress the sound- I would back it off until you got the desired amount of hum reduction. At that point you can dial back R20 to balance the brightness and fullness. Perhaps the Suhr System has something else in there but I think with the two trimmers it should work just fine.

Here is a link to the Suhr patent application, while I will also try attaching to this post:

http://www.blueguitar.org/new/misc/p...chiliachki.pdf

I am not a patent lawyer- nor do I impersonate one on the internet anymore- but I don't believe that you will run into problems if you use the design as an inspiration for your own non-commercial use. I doubt that you will have a problem if you sell these guitars on a onesy-twosy basis. But if you plan to market more than that you might want to keep quiet about the hum reduction...

Thanks!

Steve Ahola

P.S. Fry's Electronics sells packages of 200' of 30-32GA magnet wire (I forget which). I made a coil out of one of them wrapping it around a CD spindle cover, which was slightly tapered so it slips off easily. I then wrapped it securely with Scotch 35 Gray electrical tape and that was that. Here it is 8 years later and I never did try putting that into a guitar. My bad!

Actually I am getting ready to test it out on various single coil lap steels just taping it to the back with that blue masking tape. I just need to make up an adaptor that plugs into the guitar and allows the dummy coil to be connected in series.

[jimboyogi]

Personally, I think that Eddy Currents and the Tone Suckers is an excellent name for a rock band.
My basses are shielded with a combination of:
1.) MG Super Shield spray in all of the cavities
2.) Aluminum plates covering the tops of the cavities and mounting the components.
3.) Brass shells around the outside of the pickup coils, with leads connecting to ground. Only the tops of the coils are exposed.
4.) The coils are wound and wired in alternating pairs, as true humbuckers.

To make an instrument really quiet, you have to cover all of the different problem areas. There isn't one single solution.

There's a band here in Melbourne called "Eddy Current Supression Ring"

[Steve A.]

Quite welcome. One thing to be aware of is that depending on how and how well one sprays the shield paint, one can end up with the paint on the bottom of the routed cavity insulated from the paint on the sidewalls, reducing shielding effectiveness.

Just wondering if this spray will adhere to control cavities which were sprayed with the same stuff used on the body? I had trouble getting the Stew-Mac conductive paint to stick to the cavity in one guitar so I used the copper foil tape which I cover completely with 60-40 solder so that Superman can't look inside and steal my wiring ideas!

Does anyone remember the old MEK-based conductive paint used 30+ years ago? It was my 1975 Burgundy LP Standard and I thought I better shield the control cavities not realizing that the metal plate and cover offered plenty of shielding. The instructions said to put masking tape on the pot holes so that the paint would not damage the finish. I didn't have any masking tape but I had some Scotch Magic Tape and used that instead. I thought everything was just fine until I removed the tape and saw that the MEK had mottled the finish around the pots. Ouch! Of course I had to add insult to injury and I later chiseled out the top to mount a Kahler tremelo... That guitar sits on my wall to remind me that some of my ideas really aren't that good.

Steve Ahola

[Bruce Johnson]

In my experience, the MG Super Shield bonds very well to existing paint, as well as to bare wood. I think the base of Super Shield is an acrylic, similar to Krylon paints. Joe's right that you do need to make sure that you get good coverage to get good conductivity throughout. I've found that two coats is plenty. You MUST swirl the can for every 5 seconds of spraying to make sure the nickel is coming out.

On my basses, I spray the clear base coats of the paint on the body first, including all throughout the cavities. This makes sure that the wood inside is all sealed up against moisture. Next, I put on a masking board and spray two coats of Super Shield in the cavities. Then I mask off the cavities and continue on with the color coats and top coats.

I used to fight with the Stew-Mac conductive paint some years back. It was a mess to paint it well, and some would always come off, leaving conductive carbon powder floating around inside the electronics! I used copper tape for a few years too. It was effective as shielding, but was a lot of labor to put in, and messy looking. Also, the tape has a habit of peeling up in some corner down in the cavity, and then shorting against some component. That can be a nightmare to troubleshoot! The trick to avoid that is to put down the copper tape first, then cover it with a layer of clear packaging tape, which sticks very well and is non-conductive.

The Super Shield is expensive (about $40 per can), but it's so much faster and neater and more foolproof.

[David Schwab]

I wonder what the secret technique was... The basic Guitar Nuts idea of star grounding is really simple: the first step is recognizing the difference between the signal returns (the black wires on strat pickups) and shield grounds. Yes, they all end up in the same place but the path there does make a difference.

No, it doesn't make a difference. All grounds in your guitar are at the same potential.

Any difference that might have been created by the star ground is lost as soon as it connects to the output jack.

The whole premise of that shielding tutorial is flawed. It assumes that the buzz you hear from electric fields is caused by a ground loop, and that the star grounding will some how magically solve that. It wont. Also you cant have a ground loop with a single ground path... your cable to the amp.

Proper grounds and shielding are important though, but they can be standard daisy chain connection to the pots, and the pots can touch the shielding. That's just like everything being in a metal enclosure.

_________

Going back to dummy coils... I recently had a customer bring me a Music Man Axis Sport to install a new neck pickup. The guy also told me the output jack wasn't working right. I noticed the guitar had an empty battery box, and asked if it had a preamp. He just bought it used, and said it worked without the battery. Someone had already changed the single coil neck pickup for a Duncan Little '59, and he was having me install an Air Norton S. After I opened the guitar up I could see why he didn't like the neck pickup...

What I found was a small active circuit wired to a very unusual 5 way switch. It had a patent number (5569872) on the side, so I looked it up and it's an active dummy coil, just for the neck pickup. I can't imagine what it did with no battery installed.

I used to have the exact schematic for this, as they had them posted on their web site. Now they just show the basic switch wiring block diagram. But the idea is the dummy coil goes to an op amp, and the output of the op amp goes to the pickup coil.

I don't know how well it works, since by the time the guitar came to me it had a new pickup installed. Since I had to rewire the guitar with a new 5 way switch, I kept the dummy coil unit. When I get some time I'm going to play with it.

[Mike Sulzer]

David, that looks like the idea I have presented here a couple of times, although I might be interpreting that patent schematic wrong. I believe the coil on the left is the dummy. It is amplified and the resulting signal is inserted in series with the ground connection of the pickup (the coil on the right). I did not realize that anyone had patented it. I like my way better; it is simpler, just one FET, but the dummy has to be wound just right with somwwhat more sensitivity than the pickup.

[David Schwab]

Yes, I thought of your circuit when I saw this. Actually I saw this first, so I thought of this when I saw yours, but didn't realize they were so similar until now.

I had the actual schematic as it applied to one of their guitars, and it seemed much simpler. I lost it a couple of years ago when I erased a hard drive I thought I had backed up. Now they don't show the circuit in their guitar diagrams, just a flow chart.

They have a transistor version in the patent, but I put the op amp version since that's what I saw in the guitar schematic.