Ground Topology inside a Guitar

[Steve A.]

The copper tape that I used from Stewmac had conductive adhesive, but I made solder connections between them all for added reliability.
However, my experience of trying to implement best practice has left me skeptical that shielding guitar cavities has any significant benefit, with regard to rejecting buzzy type interference.

The benefit of shielding can be much more noticeable in a guitar with humbuckers. I get a lot of RF/EMI noise when the 40" LCD TV next door is turned on- it is right next to the wall of the adjoining condo. While the humcancelling coils eliminate most of the hum without shielding there is still some noise.

On most strats the #2 and #4 positions of the selector switch are humcancelling (assuming that the middle coil is RWRP.) So it is mainly the noise that you would still get with those linkages which will be reduced by shielding the control cavity. To reduce the noise of the individual single coils pickups your best bet is to use a dummy coil, the technology of which took a big step forward with the 2005 Chiliachki patent used by Suhr and Ilitch.

B.C. [Before Chiliachki] The general idea was to use a dummy coil which was very similar to the coil in the single coil pickup that you wanted to "de-hum," but without the magnets or pole pieces. I used P-90 bobbins in several of my projects which would eliminate the hum but the sound and response was affected. The sound wasn't bad- I thought that my P-90 projects sounded much better than the Gibson and SD P-90 style stacked pickups. IMO the dummy coil added some compression to the sound as well as cutting the highs a little bit. Here is a short sample with me switching the dummy coil on and off in my Gibson LP Jr:

http://www.blueguitar.org/new/mp3/sa...90dc_noise.mp3

And here is a demo recording using that LP Jr with the dummy coil switched on:

http://www.blueguitar.org/new/mp3/samples/p90_w_dc.mp3

A.C. [After Chiliachki] The Chiliachki patent was radically different from old school dummy coil technology. Instead of using a dummy coil very similar to the sensing coil, the idea was to use a large circumference coil of a heavier gauge wire with a considerably lower DC resistance. Following the specifications in the patent a dummy coil of 39GA wire measuring 330 ohm would be used with a 7k ohm pickup using 42GA wire. Having the 330 ohm coil in series with the 7k pickup does not alter the sound very much at all- I would challenge anyone to tell the difference. To get the desired humcancelling effect the larger the circumference of the dummy coil the better. While the Suhr coil was mounted on the strat tremelo spring cover, the Ilitch coil is much larger: a circumference of 21 inches with a DC resistance of 340 ohms. I have no sound samples but here are two pictures from the Strativarius a friend built:





Steve Ahola

[Ward]

I'm gonna give this a try but instead of routing the channel I'll put a smaller coil in a swimming pool rout. Trouble now is, how do I wind the coil which will have a diameter of about 5" Making whatever I wind the coil on is gonna be a bigger job than the actual job

[Steve A.]

I'm gonna give this a try but instead of routing the channel I'll put a smaller coil in a swimming pool rout. Trouble now is, how do I wind the coil which will have a diameter of about 5" Making whatever I wind the coil on is gonna be a bigger job than the actual job

My idea (that I haven't tried yet) is to those $1.99 12" Clamps from Harbor Freight set to spread rather than clamp. I would remove the orange pads and maybe wrap electrical tape around the bare metal? Or maybe just grind off any sharp edges. With that you could make any size dummy coil up to 24" in circumference. If your swimming pool rout does extend into the control cavity area I would rout that section of the body. Since it is the number of wraps that is important there would be an advantage in using 38GA magnet wire rather than 30GA or 32GA (less bulky.) I ordered something like a 1/4 pound of green magnet wire on eBay- I'll post a link if they are still selling it.

The special routing my friend did would not be that hard if you had a router template. With one of those it is almost as easy as color by number. If there was a big demand for strat dummy coils someone ought to make the templates (probably by using *another* router template ) and sell them for something like $30 plus shipping. The price could be determined partly by how much time it would save the customer rather than based strictly on manufacturing costs. (I know I would pay $30 for a template to use on my teles or my vintage copy strats.)

Steve Ahola

[Ward]

My idea (that I haven't tried yet) is to those $1.99 12" Clamps from Harbor Freight set to spread rather than clamp. I would remove the orange pads and maybe wrap electrical tape around the bare metal? Or maybe just grind off any sharp edges. With that you could make any size dummy coil up to 24" in circumference. If your swimming pool rout does extend into the control cavity area I would rout that section of the body. Since it is the number of wraps that is important there would be an advantage in using 38GA magnet wire rather than 30GA or 32GA (less bulky.) I ordered something like a 1/4 pound of green magnet wire on eBay- I'll post a link if they are still selling it.

The special routing my friend did would not be that hard if you had a router template. With one of those it is almost as easy as color by number. If there was a big demand for strat dummy coils someone ought to make the templates (probably by using *another* router template ) and sell them for something like $30 plus shipping. The price could be determined partly by how much time it would save the customer rather than based strictly on manufacturing costs. (I know I would pay $30 for a template to use on my teles or my vintage copy strats.)

Steve Ahola

Hey Steve, well necessity being the mother of invention I have wound up my coil using 36awg wire, I put 600 turns onto a flower pot The dcr came to 350R give or take. My winder uses drill chucks on both sides so I bolted a length of threaded rod to the pot and away we went. The thing was all over the place as you would expect but I wound slow and got it on there, I liked the pot idea coz it's tapered and the coil came off easily. The circumference of the coil is 16". I haven't wired it in yet but will in the next hr or so ( need to swap the rwrp over on the middle pickup) and I'll give my findings on the hum cancelling plus any tone changes if any. So far so good, will it work. Cheers.

[Steve A.]

i just ran a wire from the copper shielding to the ground bus, which is soldered to pot cases

I do that too but if I have the time and the room I will use a ring terminal and lockwasher on the ground wires screwed tightly into the foil tape and body. If the wire is not going to move (like the wire going to the bridge) I would not hesitate to solder that directly to the foil tape.

While I solder the ground bus wire to the pot cases I do not connect the "cold" terminals of the volume pots directly to the pot cases but run a separate lead to the ground terminal of the output jack. The shielding is there to intercept RF/EMI crap in the environment and I like to keep that crap out of the guitar signal as much as possible. I figure if I connect all of the ground wires together at a single point the crap the wires has picked up might cancel each other out to some extent. And since using those tricks that I stole from the original GuitarNuts site in 1998 my guitars have been a lot quieter. Something that you not notice until you play in one of those Dive Bars from Hell which must have been wired up by Satan himself. (I used to play every Sunday at one of those bars and it was great for testing out my shielding techniques along with my dummy coil designs.)

Steve Ahola

[Ward]

Right then, just finished blasting away with the strat and JTM45 clone, I reckon the noise is cut by 80 to 90%, it's just fantastic. The tone, if I did loose any tops there were still plenty there, I couldn't really tell, it sure sounded just like a normal strat, well no it didn't did it, the noise was almost gone. Thanks go to Steve A for posting the Chiliachki thing. I guess I'll be buying more wire now coz my other guitars are just not gonna stand for this.
Thanks again Steve, this is a must do and not hard at all. Maybe some shielding might help with the remaining muck but to tell the truth I'm not worried about it right now, maybe when the honey moon's over?

[Steve A.]

Right then, just finished blasting away with the strat and JTM45 clone, I reckon the noise is cut by 80 to 90%, it's just fantastic. The tone, if I did loose any tops there were still plenty there, I couldn't really tell, it sure sounded just like a normal strat, well no it didn't did it, the noise was almost gone. Thanks go to Steve A for posting the Chiliachki thing. I guess I'll be buying more wire now coz my other guitars are just not gonna stand for this.
Thanks again Steve, this is a must do and not hard at all. Maybe some shielding might help with the remaining muck but to tell the truth I'm not worried about it right now, maybe when the honey moon's over?

My friend with the Ilitch System wired up a push-pull pot to bypass the dummy coil but could not hear any difference (although it was good for demonstrating to other guitarists how well it worked.) He guessed that it was an 80% reduction in noise but 90% is more like it. I like your idea about using a flower pot. I would pot the coils I wound in wax to keep them from falling apart but you would need a pretty big wax pot for a 16 or 21 inch coil. I was just thinking that the new silicon brushes they use for glue would work for melted wax, too- brush it over the outside of the coil to give it some body. (Once the glue or wax dries it will come right off the silicon bristles. Great invention!)

Steve Ahola

P.S. Without shielding your guitar is no different than an unshielded guitar with humbuckers- they are much better than having plain single coil pickups but in a place with a lot of RF/EMI crap in the air you will get some noise that hopefully goes away when you touch the strings (a good reason to learn to pick with the palm of your hand on the bridge.)

[Ward]

My friend with the Ilitch System wired up a push-pull pot to bypass the dummy coil but could not hear any difference (although it was good for demonstrating how well it worked.) He guessed that it was an 80% reduction in noise but 90% is more like it. I like your idea about using a flower pot. I would pot the coils I wound in wax to keep them from falling apart but you would need a pretty big wax pot for a 16 or 21 inch coil. I was just thinking that the new silicon brushes they use for glue would work for melted wax, too- brush it over the outside of the coil to give it some body. (Once the glue or wax dries it will come right off the silicon bristles. Great invention!)

I did wonder about micro but even with the amp hootin and a booster there was no hint of micro, perhaps the wire thickness. I've just wound up 2 more, these things look as bodgy as but with electrical tape all around it holds them together, great invention alright, I'm walking on air I just can't believe it, I was well prepared to accept the noise for the rest of my days.
With the tapered flower pot, it's best to put a layer of electrical tape all the way around it with the sticky side facing out, this seems to hold the wire in one place, I wound one without the tape and the wire would fall to the inner taper, I managed to tape that one up in the end but it's way easier if you put the tape around the pot first then it all slides off pretty together. This 36 wire is nice and strong so not much chance of a break unless you're an elephant

[David Schwab]

Thanks for the reply, but how do you create a star, when the whole control circuitry is encased in, and touching at many points, a copper shield?

You don't. And there is no reason for star grounding in a guitar.

Having redundant grounds does not hurt anything.

[Steve A.]

You don't. And there is no reason for star grounding in a guitar. Having redundant grounds does not hurt anything.

David: I agree that star grounding is not an issue in a guitar- in a guitar amp it is used because of possible current paths through the metal chassis. However I believe that my guitars are quieter and cleaner sounding if I keep the signal returns separate from the grounds until I connect them at the output jack. I first read about that idea on the original GuitarNuts site back in 1998 when the author was more interested in telling us stuff than selling us stuff. I think that he did bring up the issue of star grounding in guitar amps although that really is not what is involved in keeping the signal returns separate from the grounds, tying them together at one point which could be the output jack or it could be the back of one of the pots.

Of course all of that is just a hypothesis which has not been proven empirically.

Steve Ahola

[big_teee]

The benefit of shielding can be much more noticeable in a guitar with humbuckers. I get a lot of RF/EMI noise when the 40" LCD TV next door is turned on- it is right next to the wall of the adjoining condo. While the humcancelling coils eliminate most of the hum without shielding there is still some noise.

On most strats the #2 and #4 positions of the selector switch are humcancelling (assuming that the middle coil is RWRP.) So it is mainly the noise that you would still get with those linkages which will be reduced by shielding the control cavity. To reduce the noise of the individual single coils pickups your best bet is to use a dummy coil, the technology of which took a big step forward with the 2005 Chiliachki patent used by Suhr and Ilitch.

B.C. [Before Chiliachki] The general idea was to use a dummy coil which was very similar to the coil in the single coil pickup that you wanted to "de-hum," but without the magnets or pole pieces. I used P-90 bobbins in several of my projects which would eliminate the hum but the sound and response was affected. The sound wasn't bad- I thought that my P-90 projects sounded much better than the Gibson and SD P-90 style stacked pickups. IMO the dummy coil added some compression to the sound as well as cutting the highs a little bit. Here is a short sample with me switching the dummy coil on and off in my Gibson LP Jr:

http://www.blueguitar.org/new/mp3/sa...90dc_noise.mp3

And here is a demo recording using that LP Jr with the dummy coil switched on:

http://www.blueguitar.org/new/mp3/samples/p90_w_dc.mp3

A.C. [After Chiliachki] The Chiliachki patent was radically different from old school dummy coil technology. Instead of using a dummy coil very similar to the sensing coil, the idea was to use a large circumference coil of a heavier gauge wire with a considerably lower DC resistance. Following the specifications in the patent a dummy coil of 39GA wire measuring 330 ohm would be used with a 7k ohm pickup using 42GA wire. Having the 330 ohm coil in series with the 7k pickup does not alter the sound very much at all- I would challenge anyone to tell the difference. To get the desired humcancelling effect the larger the circumference of the dummy coil the better. While the Suhr coil was mounted on the strat tremelo spring cover, the Ilitch coil is much larger: a circumference of 21 inches with a DC resistance of 340 ohms. I have no sound samples but here are two pictures from the Strativarius a friend built:

[ATTACH=CONFIG]19350[/ATTACH]

[ATTACH=CONFIG]19351[/ATTACH]

Steve Ahola

Thanks Mr Blue Guitar for Sharing all of this.
Very neat stuff!
Terry

[dai h.]

in lots (millions?) of audio gear, there is an unbal. stereo out to stereo in, two hots, two shield/returns. When both (L/R) connectors are connected the two ground wires form a loop, yet this seems to be pretty much inconsequential in terms of noise. So not all ground loops do something obviously untoward (hum, buzz, cause instability, etc.).

Steve, I think separating and connecting at the jack would seem to make sense (though I don't have a particularly good explanation why). Maybe it makes the function of the return and (electrostatic or capacitive shield) more "pure". Or, if you make the "enclosure" (copper foil, etc.) big (low impedance), noise has a harder time developing (less voltage drop), and the PU return wires have more R compared to the shield ("enclosure") so easier for noise voltage to develop across the PU wires(?).

re: the giant hum cancelling coil, looks very neat, but one thing I wonder if the channel near the neck pocket would weaken the structure (maybe not a problem?). How about putting it on the back?

[Steve Conner]

in lots (millions?) of audio gear, there is an unbal. stereo out to stereo in, two hots, two shield/returns. When both (L/R) connectors are connected the two ground wires form a loop, yet this seems to be pretty much inconsequential in terms of noise. So not all ground loops do something obviously untoward (hum, buzz, cause instability, etc.).

As a part-time builder of hi-fi amps and other audio gear, I agree this loop exists, both in the interconnects, and within each piece of stereo equipment. To avoid hum, the designer has to work to minimise the area of it, and its coupling to hum fields. This isn't always easy: for instance a popular layout for a power amp is with one channel on each side of the case, and a dirty great power transformer in the middle. The left and right audio cables have to split apart and go separate ways in an area of high magnetic field.

I believe this is one reason why independent power supplies, even monoblock construction, is popular with high-end designers: it punts this ground loop problem right into the end user's lap.

Personally I get round it by building all my amps with balanced inputs.

[epis]

Hi guys, I did make this dummy coil, just as an experiment, 6" diameter, 600 turns, 41AWG, 1.4 kOhm, I connected it to single coil pickup, straight to the amp.
I expected at least a bit of regular single coil hum reduction, but it didn't work for me. No changes in hum level at all. What do you think, where did I make mistake ?

[rjb]

Hi guys, I did make this dummy coil, just as an experiment, 6" diameter, 600 turns, 41AWG, 1.4 kOhm, I connected it to single coil pickup, straight to the amp.
I expected at least a bit of regular single coil hum reduction, but it didn't work for me. No changes in hum level at all. What do you think, where did I make mistake ?

I assume you wired the coil in series with the pickup. How is the coil physically oriented?
"Perfect" location would be close to the pickup, in a plane parallel to the pickup coil, with coil centers coincident.
If it still doesn't work, try flipping it over.
For minimum affect on tone, you want to keep the dummy coil's R & L low - might try fatter wire & may not need so many turns.
The idea is to detect as much hum as the pickup coil - some folks use a trim pot to adjust the dummy coils's output.