HOW TO SHIELD & GROUND A JAZZ BASS (& other information about Jazz Bass electronics)

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This question comes up a lot on the bass-related listservs. Pasted below are some e-mails to & from luthier to the superstars (or is it superstar luthier) Roger Sadowsky of Sadowsky Guitars Ltd.

If you are in the market for a J-bass pickup replacement set, see the Bass Player Magazine "Shootout." Personal testimonial: the DiMarzio UltraJazz get a top rating for humcancelling models. They manufacture pickups for Roger Sadowsky & I believe the UltraJazzes are the same (or at least very close) to his humcancelling set. I used these to replace the single coils in my Japanese J-clone (Fernandes) &; they made an incredible difference.

Hope you find this helpful!

Andrew

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hi Roger -- I just dropped by your website; the new design is great!

Question about pickups -- I have a Fernandes jazz clone bass which is playing/sounding great, but even after shielding with copper foil I still get some hum from the single-coil pickups when my fingers are off the strings. ....snip.....

Andrew

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At 09:57 AM 11/12/98 -0500, Roger wrote:

Dear Andrew,

...snip... the hum you are describing indicates your shielding job is not >complete. Even with single coils, the hum difference between touching >the strings or not is a function of the shielding, not the design of the >pickups. Please make sure the entire control cavity is covered with the >foil and connected to ground. We also like to paint the pickup cavities >and the wire hole connection between the cavity and the control >compartment with conductive shielding paint.

>Roger
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hi Roger --
....snip.... Re. the hands-off-strings hum problem -- I lined both the control cavity & pickup cavities with copper foil, so suspect that the remaining problem is the "connected to ground...." part of the equation. Is this fixed with a ground from the shielded control cavity to the ground lug on the output jack? If so I'll open it up again & make sure it is there (& working).
As always, many thanks & best wishes,
Andrew
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Date: Thu, 12 Nov 1998 23:17:04 -0400
From: Roger Sadowsky
Subject: Re: website & pickup question
To: "Andrew R. Bonamici"

In the control cavity, if you run a little copper foil up and over one of the screw holes for the control plate, the copper foil should automatically ground through the control plate.
However the copper foil in the pickup cavities must be physically grounded. Solder a wire to the copper foil in each pickup cavity and run the wire into the control cavity and ground it to the back of one of the pots. <

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In summary:
a) add a ground wire from the copper lining of the control cavity to one of the control cover plate screws. Roger said this could also be done with a tab of the actual copper lining material
b) add a ground wire from the copper lining of the pickup control cavities to the back of the pickup volume pot. As it turns out the bridge pickup was properly grounded on my bass; the problem was with the neck pickup.

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A more detailed discussion of shielding & grounding from Cisco engineer & Austin TX bassist Josh Karnes:

Date: Fri, 29 Jan 1999 08:39:03 +0000
From: Josh Karnes
To: churchbass@ccad.uiowa.edu
Subject: Re: Trash radio in church

At 06:52 AM 1/29/99 -0600, you wrote:
>josh replied to Jet:
>[snip]aluminum >>will make a better shield (but worse ground) at FM broadcast frequencies >>than will copper, plus it's a LOT cheaper.

> >Why does it make a better shield?

has to do with the permeability of the material and whether you are shielding againist E-fields or H-fields, the impedance of the wave, etc.

ok, so the short answer is "just trust me".

the long answer follows for those of you with lots of caffiene and patience.

first, understand that an electromagnetic wave is composed of E-field energy (think of it like voltage) and H-field energy. ratio of the E field to the H field yields the wave's impedance. as waves propagate, the E field diminishes in strength linearly while the H field diminishes in strength logarithmically. the result is that in what is called the "near field", we have generally high H fields and low E fields (comparatively), and in the "far field", the H-field is almost insignificant in most cases and the wave is predominantly E-field in nature, or of a high source impedance.

now, you can also have high E-field strength in the near field. it has to do with the termination of the antenna and the antenna impedance. if an oscillator is connected directly to ground through a low impedance, you will have a high current oscillation and very low voltage, this will result in strong H-fields. for me, this would be capacitively terminating a clock line, for instance. if the oscillator is connected to ground through a high impedance (say more than air), then it's primary path to ground is air. this results in a high source impedance, high E-fields, even in the near field.

there are basically two ways that shields work. either they work by producing what are called "eddy currents", which is when an H-field "wave" hits the shield, it produces a current in the shield that's equal and opposite to the field, and this opposing current barrier prevents the wave from entering (or exiting) the enclosed space. this is the function of a "faraday shield", which is extremely effective in keeping H-field radiation from exiting the shield. so if you have something that's a radiator, and you enclose it in a faraday shield, the wave will not likely propagate past the shield.

the other way a shield will work is by presenting an impedance mismatch to the wavefront (a higher impedance) and producing a reflection of the wave. this is much more effective in E-field waves, creating a shield with a permeability that is much higher than that of air so that it does not conduct the wave, but resists the wave and the wave reflects. the better path for the wave is through the air and not through the shield.

then there is the idea of using a material with a very high permeability (like mu-metal or ferrite) to "absorb" the wave. not practical at this frequency for shielding. you'd have to encase it in a ferrite material that was pretty thick.

your radio interference is high E-field intensity, low H-field. you're miles away from the source. it's an E-field wave. the copper, as a much better conductor, will produce high eddy currents, but the wave really does not have much of a current component so the opposition to the current is going to make little difference. you need to reflect that wave. the aluminum will have more of a tendancy to reflect the wave. in fact, the aluminum will do a little of both. it will help with H-fields that you might get from, say, nearby switching power supplies or computers, etc., while also reflecting E-fields from far-away sources. with aluminum, however, you need multiple grounding points if you plan on using it as a shunt antenna.

the idea of grounding the shield basically assumes it's not going to work either reflecting or with eddy currents. you are instead assuming the shield is a big old antenna, and that by putting a big antenna around the susceptible circuit and then grounding that antenna, you are going to shunt the energy to ground. this depends greatly on the low-impedanece to ground. since the aluminum is a pretty high impedance, you need to ground it at multiple points otherwise it hurts more than it helps.

man, i could go WAY into this. this is probably enough. suffice to say, half or more of the last year and a half of my work life has been spent designing, testing, etc., an aluminum (also tried copper, paints, etc) shield that's about the size of the control cavity of a guitar against high E-field-strength waves at about 100MHz (right in the FM broadcast range). copper does not work. aluminum works a little bit if you ground it extremely well. this is assuming it's the wiring inside the cavity that's picking up the radio station, which i say it's probably not. it's almost certainly the cable or the strings, so ground the heck out of the bridge and make sure it's grounding the strings, then put a couple of ferrite beads on the end of the cable that plugs into the amp, or better yet, a common-mode choke at the input.

now, i would say that as a shield, putting any kind of foil inside a guitar control cavity won't work at all. this is because once you penetrate it with a cable or wire of any kind, you re-radiate everything that cable has on it into the enclosure. believe me, i've seen it first-hand. small openings don't matter much (unless they are a slot!), it's just when you stick a wire through them. there's NO WAY to seal up a guitar cavity without sticking a wire through the shield. worse yet, you are sticking a wire through there that's connected to a 6+ foot cable that's hanging in the wind. and if this is approaching 377 ohms or more at the frequency in question, then it's going to radiate like a mother. so a really high-quality cable is in order (like very low series inductance on the ground sleeve!)

so this is why you ground it. this might work. better yet to just kill the energy at the likely antenna source by grounding the antennas (reduce their impedance to much less than 377 ohms) or by putting a ferrite on there, at the end of the whole antenna (which would be the input to the amp, assuming the guitar is the antenna).

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Date: Fri, 29 Jan 1999 08:50:11 +0000
From: Josh Karnes
To: churchbass@ccad.uiowa.edu
Subject: Re: Trash radio in church

by the way, i know i've talked about this before on this list and it's usually followed on by a bunch of replies like "well, whatever but i tried it and it worked for me".

i don't dispute that. i just dispute that the shield was actually absorbing, blocking or reflecting much if any energy. if you shield the cavity with copper, you will probably dramatically improve the grounding. a big old copper tape plane is an excellent low-impedance ground (WAY better than simply soldering everything to a little wire that goes from pot to pot) and you'd do well to use that for all the grounding on the guitar. probably will reduce the tendancy of the strings to act as an antenna by a great factor, especially if you can do something like take the bridge off, solder a piece of braided solder-wick or something to it for grounding, then make a 1/2" long piece to go solder to the copper plane, and solder the ground terminal of the jack right to the plane. this is way more useful than thinking it will shield anything.

think grounding, not shielding. <

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Additional perspectives from Bay Area engineer & sound guru Donl Mathis:

Date: Wed, 21 Nov 2001 14:55:41 -0800
From: Donl Mathis
To: churchbass@ccad.uiowa.edu
Subject: [GEAR] Learning shielding/grounding (REPLY)
Message-ID: <4.3.2.7.1.20011121142901.00b30348@mail.concentric.net>

I think of grounding sort of in terms of weight. Grounding in the ultimate sense is a matter of electrically connecting the genuine planet earth upon which we walk -- a rather "heavy" object, indeed. ;) (I use quotes because it mightmake those of you who think about the difference between mass and weight more comfortable.)

Voltage as we use it is always with respect to some other point in a circuit. That's why a meter needs two probes: you put one at a reference point, and the other at the point you want to measure, with respect to that reference point.

If we have a piece of wire with zero resistance, then the voltage we measure at one end will be the same as the voltage we measure at the other, by definition. Wire never has zero resistance; there's always at least just a little bit. And what that means is that if we (very precisely) measure the voltage at two ends of a piece of wire in a circuit, we can find different answers. There is a voltage "drop" across the wire, and the voltage within the wire is probably turning into heat, for the most part, but perhaps being radiated in other ways.

We lay out our circuits with respect to "ground", a point that we call "zero volts." Ultimately, it's often connected to a metal rod or pipe stuck in the ground, and the electrical weight of the earth anchors the end of the wire at zero volts. But the wire is not perfect, so the ground at this end of the wire is not necessarily zero volts -- very close, but not perfectly, because there is a voltage drop across that wire. <

BR>Various other parts of the circuit are yanking the "far" end of the wire in various directions, electrically speaking, and the wire tries to remain close to zero volts, shoveling electrons up or down its length as necessary to do so, and letting the earth take care of the true reference.

We don't necessarily have to use the earth itself as a ground point, as long as we pick some point and call it zero, and then have all the other parts of the circuit produce and use their voltages with respect to that point.

So the grounds aren't perfect; if you run a wire from ground to some other part of the circuit, it will be a "nearly ground" by the time you get to use it. And if that happens badly enough, it's possible for the circuit to behave as if there are additional voltages here and there, at points of the circuit that are intended to be precisely zero volts. In audio circuits, these additional voltages often turn into noise that we can hear.

In star grounding, each important point in the circuit that needs a ground reference runs its own wire to the ground. At least that way, my ground wire isn't yanked up and down by voltages at the end of your ground wire, and we can keep our imperfections somewhat separated. If I link to your connection to ground, yours is already a little corrupt, and what I think is supposed to be zero volts is really not zero volts -- you've changed it.

Now, as to how this applies to bass: The wires in a bass are a few inches long, at best. They're typically not huge fat wires, but they're not hair-like, either. There just isn't much length over which to introduce much resistance, and consequently, the "corruption" of the ground reference is minimal at worst. My personal belief is that star grounding under such conditions is way overkill. I would expect the difference between a well-grounded bass with star grounding, and a well-grounded bass with "chained" grounding to be, for all intents and purposes, zero.

If I heard a grand success story related to someone who introduced star grounding, I would ask what *else* changed, and expect to find something else that was done as part of the project that explains why the bass is suddenly so much quieter. Coil pickups are a nightmare of noise, and there are so many other things that can introduce noise in our systems, that I expect the difference between star grounding and other grounding schemes to be inconsequential. Just use a fairly nice solid piece of wire to connect your grounds, and you'll be fine. Ain't no big thing.

Prove me wrong, and I'll listen! Until then, that's my story, and I'm stickin' to it. ;)

- donl

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last updated November 22, 2001
Andrew Bonamici
http://darkwing.uoregon.edu/~bonamici/groundjbass.html
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