Shielding a single coil

[voodoochild]

Hey guys,

I am building a strat for someone who wants single coils but with the least hum possible. He doesn't want humbuckers either. I tried talking him into the Suhr backplate but it was too pricey. That said I am thinking of shielding the coils but I don't know if I should just wrap copper shielding around the outside and connect to ground or should I wrap the inside around the magnets and the outside of the coil connecting both to ground. I would think that shielding inside and outside of the coil would be best although I have just done the outside before with decent results. Has anyone tried this before and if so how did it sound? I also don't want to kill the tone of the pickup.

[guitician]

Blocking EMI/RFI noise and hum will require total coverage. Like a base plate with a nickel cover. And yes, it will sound different. There are all kinds of noiseless "Strat" pickups, why bother trying to partially shield? If your playing this into a sensitive input you'll want humbucking "single coil" types, or the backplate.

[voodoochild]

I have conductive adhesive shielding tape. I was thinking of wrapping around the coil and connecting another piece from that to the underside of the pickup across all 6 magnets. That should do the trick no? I know it changes the sound but I want to be able to reverse this if he doesn't like it. If I wrap the inside with copper and wind over it I'm screwed.

[Mike Sulzer]

The type of shielding that you are planning to do is good for shielding out electric fields. Humbuckers cancel out magnetic fields. You cannot make a single coil do that. (So-called noiseless single coil pickups are some kind of humbucking design that fits in a single coil slot.)

[Mark Hammer]

Dummy coils can be your friend, but a lot of folks have unrealistic expectations about them, and occasionally counterproductive mindsets about what a dummy coil has to be.

The advantage of the Suhr system is that the hum-rejection is decent without incurring a tonal penalty. Does hum-rejection need to be "perfect"? Nah. I imagine most players of traditional SC-equipped guitars would be happy (though not necessarily satisfied) with 6db reduction in hum. So, if you can try a few experiments in whipping up your own ersatz Suhr coil on either the backplate or pickguard, why not? It won't be as good as the Suhr coil, but hey 6db less hum is 6db less hum, right?

[voodoochild]

Originally Posted by Mike Sulzer

The type of shielding that you are planning to do is good for shielding out electric fields. Humbuckers cancel out magnetic fields. You cannot make a single coil do that. (So-called noiseless single coil pickups are some kind of humbucking design that fits in a single coil slot.)

I know it isn't humbucking. I need to try and cut the noise down as much as possible on the single coils. I completely shield my guitars anyway but I don't shield the pickups. This is more of a customer request than anything else.

[Sir Cuitous]

No matter what you do with shielding single coils and the cavities of the instrument, you still have the poles of the pick-up just hanging out there like antennas picking up more than just the string vibrating - ahhh but the tone.......

[David King]

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.

[Arthur Dent]

Low-impedance!

[David Schwab]

Originally Posted by Arthur Dent

Low-impedance!

Low impedance single coils also hum.

[StarryNight]

I thought this might help. Seems like a pretty easy design. I wonder how strict/specific the patent is on it.

[David Schwab]

Originally Posted by StarryNight

I thought this might help. Seems like a pretty easy design. I wonder how strict/specific the patent is on it.

The patent is for a "...large free shaped low impedance coil for noise cancelation"

The actual invention was intended as a coil embedded in the body around the pickups, but the trem cover coil falls within the invention. The coil is shown round, square or freeform.

It says the closer to circular the coil is, the better, and it must be at least 5 times the "average square surface area of a regular single coil style signal picking coil". The dummy coil is said to be "100 to 1000 turns wound with wire gauge #41 or heavier." It goes on that the preferred coil has an overall diameter of 8" and be wound with 200 turns of 36AWH wire.

There is more to it than just a coil though, it also has some accompanying passive circuitry.

This is listed as a "simplified schematic."

[voodoochild]

Ok. I just tried shielding the single coils and let me tell you they sounded TERRIBLE! It cut back on the hum a lot, but it sucked the life/tone right out of the pickup. It was as if the low mids were knocked right out which is the best part IMO. I told my customer and he wanted to hear them anyway and agreed they didn't sound as good as the unshielded ones. Once took the shielding off, it opened them right up and they sounded fine. So lesson learned shielding single coils ain't for me. So I think he is going to save a few pennies and go with the Suhr backplate down the road which I think is the only design that makes sense for single coils without altering the sound.

[Mike Sulzer]

Did you remember not to complete the loop around the coil? You do not want a complete path forth the current to flow around.

Originally Posted by voodoochild

Ok. I just tried shielding the single coils and let me tell you they sounded TERRIBLE! It cut back on the hum a lot, but it sucked the life/tone right out of the pickup. It was as if the low mids were knocked right out which is the best part IMO. I told my customer and he wanted to hear them anyway and agreed they didn't sound as good as the unshielded ones. Once took the shielding off, it opened them right up and they sounded fine. So lesson learned shielding single coils ain't for me. So I think he is going to save a few pennies and go with the Suhr backplate down the road which I think is the only design that makes sense for single coils without altering the sound.

[voodoochild]

Originally Posted by Mike Sulzer

Did you remember not to complete the loop around the coil? You do not want a complete path forth the current to flow around.

I'm not sure I follow you. I wrapped the coil with paper tape and then I wrapped copper foil around that and soldered it to ground. Are you saying I should leave a gap or a break in the copper somewhere around the coil? If so wouldn't that defeat the purpose of shielding it? I'm a newbe when it comes to shielding pickups themselves but I have been shielding guitars for 20 years. It sounds like there is a different method for pickups?

[David King]

Voodoochild, do a quick search of the term "eddy currents" to get the lowdown.
You don't want a continuous loop of metal around a pickup. An overlapping loop that's insulated between the ends is going to still give you 100% coverage (on the sides anyway) and there won't be a major sucking path for the eddy currents to form. (They will still be there in the foil but you will have stymied them somewhat). Eddy currents = significant losses at higher frequencies. Not so sure about your low-mids -though that may be an auditory sensation from losing highs...

[Joe Gwinn]

Originally Posted by voodoochild

I'm not sure I follow you. I wrapped the coil with paper tape and then I wrapped copper foil around that and soldered it to ground. Are you saying I should leave a gap or a break in the copper somewhere around the coil?

Exactly. The usual method is to have an insulated overlap.

If so wouldn't that defeat the purpose of shielding it? I'm a newbe when it comes to shielding pickups themselves but I have been shielding guitars for 20 years. It sounds like there is a different method for pickups?

A shield on the pickup is far closer to the action than the shielding in the guitar pickup cavity, so even if there are eddy currents in the copper foil cavity shield, it's too distant from the pickup to make much difference.

A nickel-powder loaded conductive paint cavity shield has too high a resistance (~20 ohms) for eddy currents to matter as well, even though the resistance is low enough to be totally effective against electrostatic fields.

[Mike Sulzer]

Originally Posted by Joe Gwinn

A shield on the pickup is far closer to the action than the shielding in the guitar pickup cavity, so even if there are eddy currents in the copper foil cavity shield, it's too distant from the pickup to make much difference.

And it is very easy to understand why the distance makes a difference. What you need to do is "add up" all the field lines that pass through the conducting loop. Field lines are continuous: those that pass through the core loop around and come back in the other end. But if your loop is big enough to include a field when it loops around as well as when it passes through the core, the effect of that line cancels out. That is because the line points "up" once and "down" the other time. If the loop is big enough, nearly all lines are included both times.

[David King]

So it's better to shield the cavity than the coil? Or it's OK to shield the coil but you just have to be careful about how you go about it?

[Joe Gwinn]

Originally Posted by David King

So it's better to shield the cavity than the coil? Or it's OK to shield the coil but you just have to be careful about how you go about it?

If you shield the cavity, you also protect the wiring and pots and switches, more or less for no added cost.

[voodoochild]

Thanks for all the good info. I always like to keep it old school and simple, and this time I stepped out on a limb and broke it. But now I know. I'll have to try it again soon and do it right next time.

[David Schwab]

Originally Posted by David King

So it's better to shield the cavity than the coil? Or it's OK to shield the coil but you just have to be careful about how you go about it?

I think it's better to shield the pickup. If you shield the pickup cavity you are only shielding the back and maybe the sides of the pickup. Shielding the control cavity is always a good idea, of course.

Here's a good way to do it. This is a Duncan MM pickup. Note the way the foil is wrapped around the coils... they have it closed in the back, and open in the front where the poles come out. This grounds the poles, and the magnets in this case are glued to the foil. Of course if you are using rod magnets, than that doesn't apply.

This shields the coil, while not being a closed loop. It doesn't even go around the coil in the same direction the wire does.

I completely shield my pickups, but I do something different for shielding. I can hold them a few inches from my CRT monitor without all that usual buzzing you get.

[voodoochild]

So I tried this again on my own time to see what effect it had. I finally got the technique right by insulating the ends so it didn't create a loop. I found that it still took something away from the pickup that made it sound different in a worse sorta way. It was even more noticeable when I shielded across the magnets. I think I would rather stick with the old school single coil design without shielding. It just plain sounds better to me. On a positive note I think this will get me experimenting with dummy coils isolated further away from the single coils to reduce hum without noticeably changing the tone.

[Mike Sulzer]

Originally Posted by voodoochild

It was even more noticeable when I shielded across the magnets. ... On a positive note I think this will get me experimenting with dummy coils isolated further away from the single coils to reduce hum without noticeably changing the tone.

Shielding across the magnets provides a better path for the current than an unconnected loop around the coil; so your result makes sense.

A dummy coil can work well, but it is not the same as a shield. The shield is for electric fields; the coil for canceling magnetic fields. In my experience, a minimum amount of shielding is adequate for the electric fields, and the cancellation of the magnetic field is more important. So I think that you are headed in the right direction.

[David King]

Voodoo,
I say skip the foil and try the paint, you might just like that better.

[Joe Gwinn]

Originally Posted by voodoochild

So I tried this again on my own time to see what effect it had. I finally got the technique right by insulating the ends so it didn't create a loop. I found that it still took something away from the pickup that made it sound different in a worse sorta way. It was even more noticeable when I shielded across the magnets. I think I would rather stick with the old school single coil design without shielding. It just plain sounds better to me.

You may have raised the self-capacitance of the coil enough to matter. Some layers of insulating tape between top of coil and the shield foil can help.

On a positive note I think this will get me experimenting with dummy coils isolated further away from the single coils to reduce hum without noticeably changing the tone.

As others have mentioned, humbucking and shielding solve different problems

[belwar]

Originally Posted by Joe Gwinn

A nickel-powder loaded conductive paint cavity shield has too high a resistance (~20 ohms) for eddy currents to matter as well, even though the resistance is low enough to be totally effective against electrostatic fields.

BTW Joe - I tried that MG Chemicals Nickel Super Shield you recommended .. WOW! I'm now on my fourth can. I made a batch of guitars this week with P-90's, played them sitting on top of the amp, with a 4 ft floresant light over head and I couldn't detect hum at medium volume. I was blown away at how quiet the guitar was. I had the guitars setup RWRP and it really didnt need it. Thanks for the recommendation.

[Joe Gwinn]

Originally Posted by belwar

BTW Joe - I tried that MG Chemicals Nickel Super Shield you recommended .. WOW! I'm now on my fourth can. I made a batch of guitars this week with P-90's, played them sitting on top of the amp, with a 4 ft floresant light over head and I couldn't detect hum at medium volume. I was blown away at how quiet the guitar was. I had the guitars setup RWRP and it really didnt need it. Thanks for the recommendation.

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. The test is easy. Using an ordinary DC ohmmeter, probe around, trying all combinations of surfaces. If you always get ~20 ohms or less, no problem. If you get a much higher value anywhere, inspect under bright light to see where the painting technique needs improvement. Once you have developed a good painting process, the ohmmeter tests become unnecessary.

Priming the raw wood cavity before applying shield paint may reduce shielding paint consumption enough to be worthwhile.

The dried paint film is thin and somewhat delicate, so for long life the electrical connection to the film should involve a wide area of contact, such as a brass washer, versus just a wire and a small wood screw. Another good approach is where the film extends out on the surface of the guitar body, with area contact being made all around the routed cavity by some metal foil attached to the underside of the pickguard.

[TGD]

Vodoochild, my 2 cents.

this is an article I wrote for an italian magazine concerning Stratocaster shielding, including pickups. It' s in italian, but you should easily translate it using Google translator. Pictures are also quite eloquent

Hope it can help you.

How to shield a Strato

[Possum]

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