Shielding - What's to know?

[Turnaround]

Looking at the "Requests" thread, I see it has been 5 years since member @sissy kathy asked for a thread about shielding. I guess it's about time.

First, let's make sure we are on the same page about shielding. The idea is to prevent "noise" from entering the circuitry of your bass. The noise we are talking about are stray bits of electrical energy that get into the wiring and get converted into unwanted sound. Where do these bits of electrical energy come from? There are 4 sources we need to be concerned about:

1. Electromagnetic Induction
2. Electrostatic coupling
3. Radio frequency interference
4. Direct electrical injection

We don't need to worry about the fourth one since it only occurs if we connect an electrical current directly to the wiring in our instrument. But isn't that what a pickup does? Well.... no. A pickup works by inducing an electrical current - item #1. But electrostatic coupling and radio frequency are also means of inducing current. And they are the problem ones.

So the problem is that we need to allow some induction while eliminating all the rest. A string vibrating in the magnetic field of the pickup induces a current in the pickup (Item #1) so we cannot eliminate it or we get no sound at all. This is the largest single problem we face in this regard - we cannot eliminate all sources of noise and get output from our pickups too.

What we can do is attempt to eliminate all induction that does not come from the pickups. But we have to recognize that some unwanted induction will happen at the pickups despite our best efforts.

So now we get to the strategy we will use to lessen unwanted induced electrical signal - we will prevent induction from happening everywhere but through our pickups. We will create a shield (Kirk: "Shields up!")

A electrical shield, as with the shield on the Enterprise, should completely surround what it is we are trying to protect. Actually this is not absolutely essential since the shield a gladiator would carry would protect him from the adversary he is facing. And in the gladiator games, he would be facing his attacker. However, in a melee it would do nothing to protect him from an attack from the rear. So if you know that you will always be facing the source of noise induction when you play your bass, you only need to shield the side that will be facing the noise. You probably have experienced this - the noise goes away/appears as you rotate in different directions. But in the world most of us know, we have no idea what angle the noise will come from, and indeed if there will be more than once source coming from different directions. So best protect from all directions simultaneously. We want to surround our electronics with a shield.

But as I pointed out, we cannot do this because we still need the pickups to see the disturbance in their magnetic field caused by the vibrating string. So we will create a shield with a gap (or gaps) for the magnetic fields of the pickup(s) - no other gaps. That's important - often there are places in many a shielding job where there are other overlooked gaps (more on this later).

We probably understand the concept of a shield. We can imagine one. But now we are shielding against something we cannot see, and sometimes that gets a bit trickier to understand. First let's make some noise. Plug a lead cord into you amp but leave the other end unplugged. Turn on the amp with the volume low. Now grab the tip (just the very tip) of the free end of the lead cord - BUZZZ! We all know that sound. But what exactly is it? It's you! You happen to be a source of a lot of noise that gets induced into the lead cord. Now grab the shaft of the plug in one hand (pinch tightly) without touching the tip and touch the tip with the other hand. Little or no buzz. Why? Because you are grounding yourself to the shaft of the plug and that "drains" your noise to ground leaving nothing to enter the tip of the plug.

The noise we are trying to eliminate with our shielding is just like the noise we induced when we touched just the tip of the lead cord. And our strategy will be the same as grabbing the shaft (ground) of the cord - we are going to send the noise to ground, diverting it from the signal generated by our pickups. We do this by building a conductive cage around our electronics and connecting that cage to ground. Induction noise will encounter the cage and get carried away before it reaches our electronics.

OK enough about the theoretical part. Let's get on to the practicality.

Our instruments have one or more "cavities" that hold the electronics. We need to line these cavities with a conductive material, then connect the lining to ground. Let's look at a typical passive J-bass. It has two pickups, each in its own cavity. And there's the control cavity. There is also a hole from each pickup cavity to the control cavity. And there is a hole from the bridge to the control cavity. We don't need to worry about that last one since it does not carry any signal - it's just ground. But we do need to be concerned about all the cavities and the other holes because there are wires carrying electrical signals there.

For the moment we won't concern ourselves about what conductive material we use for our cage. It could be aluminum, copper, conductive paint. But somehow we need to fabricate a complete cage. And we need to connect that cage to ground. Where is ground? It's in your amplifier. One side ( the sleeve or shaft) of your lead cord is connected to that ground and at the other end it connects to the frame of the jack on the bass. Everything connected to the grounded lug on the jack will be connected to ground at the amp and that's the only real ground we are concerned with.

So we need to line the pickup cavities, the control cavity and the holes that carry the wires from the pickups to the control cavity with conductive material. All of this material must be electrically connected. But we also need to completed the cage by putting conductive material on the underside of the pickguard to complete the cage that surrounds the electronics (except where the pickups poke through). That material on the pickguard needs to connect to the rest of the cage. To be effective it does not need to connect all around the perimeter of the cavities, just one point, but it needs to be a good positive electrical connection. This is most often done by laying a thin strip of copper foil from the inside of one of the cavities over the top edge of the cavity just enough to contact the underside of the pickguard - best done right where one of the pickguard screws will be so that the screw will ensure good contact with the copper strip. It's not a bad idea to do the same thing at the metal control plate, though it's not essential since the plate will be grounded from the frame of the jack, which you will remember is connected to the ground of your amplifier.

I'll leave it here for now and start a separate post about the materials and methods used.

 

[sissy kathy]

Great start. I hope you address star grounds, ground loops, and other nonsense,

 

[testing1two]

This is destined to be a sticky next to the setup thread.

 

[96tbird]

There’s a good number of pro techs here, our author being one, that every answer they give should be in the stickies. Thanks @Turnaround and you others for all you do here.

And a good read to boot.

 

[Turnaround]

Here's a quick look at the ground plane.

In the case of an amplified instrument, there is only one "ground" that counts, and that's the ground plane of the amplifier. One of the connections in our lead cord is connected to that ground and at the other end of the cord our instrument's ground plane is connected to that. Everything is brought to the same level.

You can view the inside of our instruments as having two grounds - one signal ground and one chassis ground. The two grounds are connected (remember - there is only one ground in reality), but we can imagine a functional difference. The signal ground is the return of the signal generated by the pickups. Our volume pots bleed some of that signal off to ground (signal ground) and so does the tone pot. The chassis ground is for all the non-signal related connections, things like the bridge ground, the shielding ground, the pot casings ground, etc. In large part the chassis ground is the point where noise is drained. If you connect a live wire directly to ground you have a short circuit. Electricity will always take the path of least resistance to ground, and that's what we are doing with our chassis ground - short circuiting the noise to ground.

As I said, there is only one ground and both the signal and chassis ground planes are connected to it. And so why make a distinction about these two planes? Well, that's where there is some disagreement. There are those that advocate the two should be kept separate then brought together at a single point. Others claim that it is not necessary, but all ground wires should be brought to one single point (star grounding). The point of these schemes is to avoid "ground loops".

Ground loops are simply two or more paths to ground at any point in the circuit. But why would two paths to zero volts be a problem - they both arrive at the same level? A problem can occur if one of the paths runs into some resistance in its route. There will be a voltage difference in the two paths upstream of resistance and that voltage difference will be heard as noise. The bit of resistance causing this voltage difference could be as simple as the residual resistance in the wire where one path to ground goes through a longer wire than the other. In our case though it is not likely to be simply wire length that is the source of the problem, but other factors that can cause resistance in the circuit.

A "solid gold" approach to the wiring in our basses is to connect all the components requiring chassis ground to one point, all the signal grounds to another single point, then bring the two together, connecting them to the ground of the amplifier (at the output jack). That would ensure that there are no ground loops in the chassis connections, no ground loops in the signal circuit and no loops between the two.

However, this solid-gold approach may be over-kill. For every proponent of this scheme there is at least one who would argue that ground loops are not significant in the audio signals in our instruments. The wiring runs are very short and the potential for uneven ground lines is close to zero. And many (most) manufacturers wire the chassis ground together with the signal ground and have multiple paths to ground. Look at the wiring on a standard Fender Precision bass for example. On the volume pot, one of the tabs is bent back and soldered to the case of the pot. That takes the signal ground and connects it to the chassis ground. There is a wire soldered from the case of the volume pot to the case of the tone pot. Then there is a wire from the case of the tone pot to the ground lug of the output jack.

Not only does this mix the chassis ground with the signal ground, it also sets up the possibility of different voltages in the ground plane. Consider the signal ground at the volume pot. To get to the ground plane of the amplifier, it goes through the wire from the case of the volume pot to the tone pot, then along the wire from the tone pot to the jack. Each wire has a bit of resistance (there is no true 100% conductor). Let's call the amount of resistance in the wire a "smidgen". From the volume control to ground the resistance is the sum of the two pieces of wire, i.e. 2 smidgens. But signal ground from the tone pot travels through only one wire - 1 smidgen. Therefore there is a difference in the ground plane between the volume and tone controls, and that can cause noise.

If we were being purist we would not bend the lug of the volume pot back to the case but rather run a wire from that lug to the ground on the output jack. Similarly we would run a wire from the case of the volume pot to the output jack and a separate one from the tone pot case.

If we believe that star grounding and separation of the chassis and signal grounds is necessary, we introduce a lot of wiring hassle to our bass-building project. Looking at a passive J-bass, we would have these connections that would be made to one chassis ground point: shield, bridge, each pot case (3). That's five wires going to one point. And on the signal ground side we have one from each pickup (2), one from each volume pot (2) and one from the tone pot, for a total of 5. Then each group of 5 would need to be brought together into one great wad of 10 wires all connected at the ground lug of the jack. If the bass is an Active Jazz, there are a whole lot more wires to be bundled .... ouch!

The classic wiring of the J-bass is much simpler and cheaper to produce. It mixes signal and chassis grounds freely. And many advocate that it certainly is good enough for our instruments. And the most common complaint about noise in J-basses is not from the wiring, but from the inherent noisiness of single coil pickups. Next most common complaint is related to inadequate shielding. Noise as a result of the wiring scheme used, be it ground plane separation, star-grounding, is not the major contender in the noise war.

 

[Turnaround]

Now to the act of installing shielding.

To be the most effective the shield should completely encase the electronics in our bass. Except that we need the pickups to poke outside of the shield or else they won't work. We must shield the control cavity, the pickup cavities plus any channels between the cavities where there are signal wires running. We don't need to shield any chassis wiring since it is already tied to ground. But we want to catch all the noise from getting to any of our signal wires. There are some areas that often get overlooked such as holes that are drilled between the pickup cavities and the control cavity - signal wire run through those holes. We need not worry about shielding the battery cavity in an active bass because there are no signal wires going there. We don't need to worry about the channel that carries the ground wire from the bridge unless a signal wire runs through that channel (I've never seen that).

So the next question is what material we should use to do the shielding. The common options are copper foil, aluminum foil and conductive paint. There are pros and con's to each.

Copper Foil
Copper foil can be had as a simple foil, or an adhesive-backed foil. Whichever we use it should stick to the interior surfaces of the cavities. The last thing we ant is some loose foil that can come into contact with the signal wiring shorting it out. So you can use some glue with plain foil or choose to use the self-adhesive kind. But regardless which you use you need to ensure electrical conductivity through every joint you create. So you may choose to use copper foil with "conductive" adhesive on it.

Sometimes, more often than I would like, the conductivity of the adhesive on copper foil deteriorates over time, and when it does you may lose electrical connection between the pieces of foil in your shield. So it is recommended that you solder the pieces of foil together. They don't need to be soldered their entire length, just tacked together with spot soldering on each piece. As long as there is electrical connection between all the pieces it is good.

The upside of copper foil is that it will form a very effective shield so long as it covers the entire surface of the cavities and connecting channels that carry signal wires, and so long as there is electrical continuity throughout the shield. The downside is that it is not as readily available as aluminum foil (you probably have some in your kitchen). It's more costly than aluminum foil, especially if you are using the copper foil with conductive adhesive.

Aluminum Foil
You can use aluminum foil rather than copper. It can form a perfectly good shielding cage. The challenge is ini its application. Self adhesive aluminum foil is generally harder to find than self adhesive copper, and it may be very difficult indeed to source aluminum foil with conductive adhesive. If you can't find adhesive aluminum foil you will need to glue it in yourself, and finding a good glue for aluminum is the next challenge. Your best bet is some rubber cement though you may find something else that will do. But you still need to ensure electrical conductivity between all the pieces you use. The problem is that it is not easy to solder to aluminum. And when you do get the solder to stick it often has less than optimum electrical conductivity - not good. So in the end there is little to recommend aluminum foil for our shielding efforts.

Conductive Paint
Conductive paint can be every bit as good as a copper foil shield. There are some "buts" though. First, there are various types of conductive paint, some of which are very expensive. The paint can be carbon based, nickel based, silver-plated copper based or silver based (from least expensive to most). It turns out that even the cheapest (carbon -based) is good enough for our requirements. So long as there is enough carbon density in the application it will do a good job. If you want to be super-sure you could step up to nickel-based or copper-based paint, but given equal density they will be almost equally effective.

There are commonly 3 binders used in conductive paint, acrylic, water-based and epoxy. For our purposes it makes no difference since we need not concern ourselves about things like abrasion resistance (it's not likely that anything will be rubbing the inside of our basses). However, the epoxy is a 2-part system that you have to mix together, and it is usually the most-expensive binder as well as being the most toxic. The water-based binders are far less toxic as is the acrylic. And they are cheaper.

With conductive paint you don't need to worry about creating electrical conductivity between sections. If you have covered all surfaces conductivity will be there. But as with any other form of shielding you must cover the entire cavity surfaces and the channels between the cavities - wherever there is a signal wire. Actually this can be easier to do with conductive paint than with foil - just think of those holes that go from a jazz pickup cavity to the control cavity. Getting copper foil in there to completely line the hole can be a challenge - less so with paint.

And, of course with paint there is no soldering to do.

Very often we see shielding done with both paint and aluminum foil - the paint in the cavities and the foil on the underside of the pickguard and control cavity cover. That is an effective way to go about it and saves a bit on the cost of the paint by using the cheaper aluminum where it's really easy to apply. Or you could use the conductive paint on the underside of the pickguard/control cavity cover. Perhaps you're not into saving pennies.

The downside, if you call it that, with paint is that it may well take two or more coats to provide the same level of shielding as one layer of copper foil. You need to be thorough with the application of the paint, but you also need to be thorough with the application of either foil, so I would call it a wash.

Regardless of the material you use, the cage must completely enclose the electronics of the bass, except where the pickups poke through. We need to take special care where the covers of the shield are attached (the bottom of the pickguard and control cavity cover). The shielding on these pieces must make electrical contact with the shielding inside the cavities. In the case of the pickguard this is done by extending the shielding material from the cavities up over the edge onto the upper surface. Then when the pickguard is screwed down the shield on its underside will connect with the material of the shield in the cavity. To be a good positive connection it is good practice to create this overlap right where a pickguard screw goes. When the screw goes in it will ensure good contact. In the case of a control cavity cover, they are usually inset into the surface of the bass so they sit flush with the surface. Here the shielding material should extend into the recess lip so that it can make contact with the shielding on the underside of the cover plate when screwed down.

A note should be made about mixing shielding materials. It's pretty common to see shielding paint in the cavities and aluminum foil on the underside of the pickguard. That works fine. But when two different metals are in contact with each other there is a chemical reaction called "galvanic action" between the two that can cause corrosion. It's not going to be a big deal in our case, but as corrosion builds up, electrical conductivity can suffer along with the effectiveness of our shield. The process is accelerated in the presence of moisture (sweat) so we should pay attention to it. You can avoid this problem in large part by using the same material for shielding throughout your bass.

Next we need to ensure that our shield is connected to the ground plane of the amplifier. We don't actually need to plug into our amp to check this. If we check the ground lug on the jack we can assume we are good to the amp - unless we have a faulty lead cord which is an entirely different matter. On a standard P-Bass our shield will be grounded because the jack attaches to the pickguard which is shielded, and the ground lug on the jack is part of the frame of the jack. And on a standard J-Bass the control cover is metal which will connect with the shield in the control cavity, an the jack frame is directly connected to this metal plate. but in cases where the jack is not in contact with the shield directly, we need to provide that connection. A simple wire soldered to the ground lu on the jack with the other end making contact with the shield is all it takes. You can connect to the shield if it is copper foil by simply soldering the wire from the jack to the copper foil at any convenient point. Same is true for aluminum foil except that it is difficult to create a good solder connection to aluminum. and in the case of paint soldering is out o the question. So for both aluminum or paint a small lug if screwed into the cavity. It will make good contact with the shield since we completely covered the cavity (right?) and we solder the jack ground wire there.

And this brings up another often-overlooked point of our shield. Some basses have jacks mounted on the lower edge of the bass (commonly referred to as side-mount jacks). Some of the side-mount types use barrel jacks which are shielded their entire length. Others use regular frame jacks mounted on metal plate screwed to the edge of the bass. In the latter case we should be shielding that hole from the mounting plate to the control cavity. It's barely half an inch long, but it's one more point where noise can enter.

It is good practice when doing shielding to test your shield. A visual inspection is first - see that you covered all cavities where there is a signal and all connecting channels. Then do an electrical test. A simple ohm-meter, multimeter or continuity checker will do fine. Check for connectivity from extreme ends of the cage - for instance, on a jazz bass, check for continuity from the neck pickup cavity to the frame of the output jack. Do the same from the bridge pickup cavity, and any other cavity we have shielded. We should have close to zero ohms resistance from any given point to another. Don't fuss if you find a bit of resistance here or there, but i you find a place where there is a sudden large increase in resistance you might want to check how thorough you have been.

One final note in this post. There is an alternative to all this shielding, but it has a bit of a compromise. Remember we are trying to shield the electrical signal from stray impulses that produce noise. And there is no point in shielding anything that is not carrying our audio signal. If we use shielded wire everywhere there is audio signal in our instrument there will be no point where noise can enter. So if you replace the wires coming from your pickup with a shielded wire and do the same for the entire audio path, we don't need copper or aluminum foil or conductive paint. The compromise with this scheme is that there are some areas that won't be shielded, such as all connection points. The capacitor that is commonly used as part of the tone circuit will not be shielded. And there may be other components that are left exposed as well, especially in the case of active basses, where the pre-amp and connective wires may be exposed. But many guitars use shielded wires rather than cavity shielding to good effect.

Next time we will consider what we might do to calm noisy pickups.

 

[sissy kathy]

Outstanding! Wonderful dissertation.

 

[James Collins]

Great write up. Two edits I would recommend to be very particular:

1) A pickup transduces an electric current.

Transduction is converting one form of energy (mechanical) into another (electromotive).

2) The shield isn't grounding the outside noise.

The shield acts like a Faraday cage. The shielding acts to neutralize the electromagnetic field on its interior, but the field passes through to the other side. Even if it was not grounded the shield would prevent the electromagnetic interference. I believe the reason for grounding the shield is actually because the interior of the shield if not grounded to the same ground as the output will have a potential that can create its own noise. The noise in this case is not caused by the outside electromagnetic Fields but by the electric potential between the Faraday cage and the instrument wire leaving the instrument.

 

[Nahuel Rosso]

Thank you Turnaround!! Please, could you tell us more on the subject of high en roll off/tone compromises when shielding? I've just realised that a new hand built bass that i've got, which has an active preamp, with it's cavity fully shielded with copper, and two humbucker pickups (SD's NYC, i believe that the cable is protected). Thing is that the back of the pickups also have copper attached to them, and the pickups cavity has no shield at all.

And I'm pretty sure that this bass has no real highs, it has a "tone roll off" compared to my Am Dlx Jazz bass 2013, both in active or passive.

I should send some pictures, but i don't have it with me right now... :/

Now to the act of installing shielding.

To be the most effective the shield should completely encase the electronics in our bass. Except that we need the pickups to poke outside of the shield or else they won't work. We must shield the control cavity, the pickup cavities plus any channels between the cavities where there are signal wires running. We don't need to shield any chassis wiring since it is already tied to ground. But we want to catch all the noise from getting to any of our signal wires. There are some areas that often get overlooked such as holes that are drilled between the pickup cavities and the control cavity - signal wire run through those holes. We need not worry about shielding the battery cavity in an active bass because there are no signal wires going there. We don't need to worry about the channel that carries the ground wire from the bridge unless a signal wire runs through that channel (I've never seen that).

So the next question is what material we should use to do the shielding. The common options are copper foil, aluminum foil and conductive paint. There are pros and con's to each.

Copper Foil
Copper foil can be had as a simple foil, or an adhesive-backed foil. Whichever we use it should stick to the interior surfaces of the cavities. The last thing we ant is some loose foil that can come into contact with the signal wiring shorting it out. So you can use some glue with plain foil or choose to use the self-adhesive kind. But regardless which you use you need to ensure electrical conductivity through every joint you create. So you may choose to use copper foil with "conductive" adhesive on it.

Sometimes, more often than I would like, the conductivity of the adhesive on copper foil deteriorates over time, and when it does you may lose electrical connection between the pieces of foil in your shield. So it is recommended that you solder the pieces of foil together. They don't need to be soldered their entire length, just tacked together with spot soldering on each piece. As long as there is electrical connection between all the pieces it is good.

The upside of copper foil is that it will form a very effective shield so long as it covers the entire surface of the cavities and connecting channels that carry signal wires, and so long as there is electrical continuity throughout the shield. The downside is that it is not as readily available as aluminum foil (you probably have some in your kitchen). It's more costly than aluminum foil, especially if you are using the copper foil with conductive adhesive.

Aluminum Foil
You can use aluminum foil rather than copper. It can form a perfectly good shielding cage. The challenge is ini its application. Self adhesive aluminum foil is generally harder to find than self adhesive copper, and it may be very difficult indeed to source aluminum foil with conductive adhesive. If you can't find adhesive aluminum foil you will need to glue it in yourself, and finding a good glue for aluminum is the next challenge. Your best bet is some rubber cement though you may find something else that will do. But you still need to ensure electrical conductivity between all the pieces you use. The problem is that it is not easy to solder to aluminum. And when you do get the solder to stick it often has less than optimum electrical conductivity - not good. So in the end there is little to recommend aluminum foil for our shielding efforts.

Conductive Paint
Conductive paint can be every bit as good as a copper foil shield. There are some "buts" though. First, there are various types of conductive paint, some of which are very expensive. The paint can be carbon based, nickel based, silver-plated copper based or silver based (from least expensive to most). It turns out that even the cheapest (carbon -based) is good enough for our requirements. So long as there is enough carbon density in the application it will do a good job. If you want to be super-sure you could step up to nickel-based or copper-based paint, but given equal density they will be almost equally effective.

There are commonly 3 binders used in conductive paint, acrylic, water-based and epoxy. For our purposes it makes no difference since we need not concern ourselves about things like abrasion resistance (it's not likely that anything will be rubbing the inside of our basses). However, the epoxy is a 2-part system that you have to mix together, and it is usually the most-expensive binder as well as being the most toxic. The water-based binders are far less toxic as is the acrylic. And they are cheaper.

With conductive paint you don't need to worry about creating electrical conductivity between sections. If you have covered all surfaces conductivity will be there. But as with any other form of shielding you must cover the entire cavity surfaces and the channels between the cavities - wherever there is a signal wire. Actually this can be easier to do with conductive paint than with foil - just think of those holes that go from a jazz pickup cavity to the control cavity. Getting copper foil in there to completely line the hole can be a challenge - less so with paint.

And, of course with paint there is no soldering to do.

Very often we see shielding done with both paint and aluminum foil - the paint in the cavities and the foil on the underside of the pickguard and control cavity cover. That is an effective way to go about it and saves a bit on the cost of the paint by using the cheaper aluminum where it's really easy to apply. Or you could use the conductive paint on the underside of the pickguard/control cavity cover. Perhaps you're not into saving pennies.

The downside, if you call it that, with paint is that it may well take two or more coats to provide the same level of shielding as one layer of copper foil. You need to be thorough with the application of the paint, but you also need to be thorough with the application of either foil, so I would call it a wash.

Regardless of the material you use, the cage must completely enclose the electronics of the bass, except where the pickups poke through. We need to take special care where the covers of the shield are attached (the bottom of the pickguard and control cavity cover). The shielding on these pieces must make electrical contact with the shielding inside the cavities. In the case of the pickguard this is done by extending the shielding material from the cavities up over the edge onto the upper surface. Then when the pickguard is screwed down the shield on its underside will connect with the material of the shield in the cavity. To be a good positive connection it is good practice to create this overlap right where a pickguard screw goes. When the screw goes in it will ensure good contact. In the case of a control cavity cover, they are usually inset into the surface of the bass so they sit flush with the surface. Here the shielding material should extend into the recess lip so that it can make contact with the shielding on the underside of the cover plate when screwed down.

A note should be made about mixing shielding materials. It's pretty common to see shielding paint in the cavities and aluminum foil on the underside of the pickguard. That works fine. But when two different metals are in contact with each other there is a chemical reaction called "galvanic action" between the two that can cause corrosion. It's not going to be a big deal in our case, but as corrosion builds up, electrical conductivity can suffer along with the effectiveness of our shield. The process is accelerated in the presence of moisture (sweat) so we should pay attention to it. You can avoid this problem in large part by using the same material for shielding throughout your bass.

Next we need to ensure that our shield is connected to the ground plane of the amplifier. We don't actually need to plug into our amp to check this. If we check the ground lug on the jack we can assume we are good to the amp - unless we have a faulty lead cord which is an entirely different matter. On a standard P-Bass our shield will be grounded because the jack attaches to the pickguard which is shielded, and the ground lug on the jack is part of the frame of the jack. And on a standard J-Bass the control cover is metal which will connect with the shield in the control cavity, an the jack frame is directly connected to this metal plate. but in cases where the jack is not in contact with the shield directly, we need to provide that connection. A simple wire soldered to the ground lu on the jack with the other end making contact with the shield is all it takes. You can connect to the shield if it is copper foil by simply soldering the wire from the jack to the copper foil at any convenient point. Same is true for aluminum foil except that it is difficult to create a good solder connection to aluminum. and in the case of paint soldering is out o the question. So for both aluminum or paint a small lug if screwed into the cavity. It will make good contact with the shield since we completely covered the cavity (right?) and we solder the jack ground wire there.

And this brings up another often-overlooked point of our shield. Some basses have jacks mounted on the lower edge of the bass (commonly referred to as side-mount jacks). Some of the side-mount types use barrel jacks which are shielded their entire length. Others use regular frame jacks mounted on metal plate screwed to the edge of the bass. In the latter case we should be shielding that hole from the mounting plate to the control cavity. It's barely half an inch long, but it's one more point where noise can enter.

It is good practice when doing shielding to test your shield. A visual inspection is first - see that you covered all cavities where there is a signal and all connecting channels. Then do an electrical test. A simple ohm-meter, multimeter or continuity checker will do fine. Check for connectivity from extreme ends of the cage - for instance, on a jazz bass, check for continuity from the neck pickup cavity to the frame of the output jack. Do the same from the bridge pickup cavity, and any other cavity we have shielded. We should have close to zero ohms resistance from any given point to another. Don't fuss if you find a bit of resistance here or there, but i you find a place where there is a sudden large increase in resistance you might want to check how thorough you have been.

One final note in this post. There is an alternative to all this shielding, but it has a bit of a compromise. Remember we are trying to shield the electrical signal from stray impulses that produce noise. And there is no point in shielding anything that is not carrying our audio signal. If we use shielded wire everywhere there is audio signal in our instrument there will be no point where noise can enter. So if you replace the wires coming from your pickup with a shielded wire and do the same for the entire audio path, we don't need copper or aluminum foil or conductive paint. The compromise with this scheme is that there are some areas that won't be shielded, such as all connection points. The capacitor that is commonly used as part of the tone circuit will not be shielded. And there may be other components that are left exposed as well, especially in the case of active basses, where the pre-amp and connective wires may be exposed. But many guitars use shielded wires rather than cavity shielding to good effect.

Next time we will consider what we might do to calm noisy pickups.

 

[Turnaround]

Great write up. Two edits I would recommend to be very particular:

1) A pickup transduces an electric current.

Transduction is converting one form of energy (mechanical) into another (electromotive).

2) The shield isn't grounding the outside noise.

The shield acts like a Faraday cage. The shielding acts to neutralize the electromagnetic field on its interior, but the field passes through to the other side. Even if it was not grounded the shield would prevent the electromagnetic interference. I believe the reason for grounding the shield is actually because the interior of the shield if not grounded to the same ground as the output will have a potential that can create its own noise. The noise in this case is not caused by the outside electromagnetic Fields but by the electric potential between the Faraday cage and the instrument wire leaving the instrument.

Thanks James. I mispoke when I suggested that the shield grounded the noise. A Faraday cage does not need to be grounded to be an effective shield. But, as you pointed out if not grounded it may be at a different potential than the wiring and thus be the cause of noise. And thanks for the correction on the terminology.

 

[Turnaround]

Thank you Turnaround!! Please, could you tell us more on the subject of high en roll off/tone compromises when shielding? I've just realised that a new hand built bass that i've got, which has an active preamp, with it's cavity fully shielded with copper, and two humbucker pickups (SD's NYC, i believe that the cable is protected). Thing is that the back of the pickups also have copper attached to them, and the pickups cavity has no shield at all.

And I'm pretty sure that this bass has no real highs, it has a "tone roll off" compared to my Am Dlx Jazz bass 2013, both in active or passive.

I should send some pictures, but i don't have it with me right now... :/

I would like to comment on this but have limited first-hand knowledge. All my basses have shielded cavities, and none of them suffer from depressed high frequencies. But there are sources that indicate that some shielding adds capacitance to the circuit which bleeds off higher frequencies.

The copper on the bottom of your pickups is likely there to ground the pole pieces. If they are not grounded they may get really noisy if you happen to touch any of the pickup poles - most pickups do not have grounded poles. If the cable from the pickups into the control cavity is shielded, there is no need for shielding the pickup cavity. Still there still may be some benefit from shielding the control cavity unless all of the wiring there is shielded as well.

The consensus is that shielding the pickup cavities may cause some attenuation of the higher frequencies, but that wrapping the pickups with a shield will likely cause more attenuation. What I have directly experienced is that the attenuation tends to remove only very high frequencies that are the centre of overly edgy glassy frequencies. It's a matter of taste I suppose, but any effects I have found have not been undesirable.

One of the theories that has been forwarded is that as the shielding/grounding removes high frequency noise the perception is that some highs have also been removed from the audio signal, but it only seems that way. Nice idea, and perhaps it's true.

 

[DeathSlanger]

Incredible write up, thank you so much.

 

[Nahuel Rosso]

It seems to be that the shielding job is properly done. I think i just don't like these pickups that much. That must be it. Thank you so much!!

I would like to comment on this but have limited first-hand knowledge. All my basses have shielded cavities, and none of them suffer from depressed high frequencies. But there are sources that indicate that some shielding adds capacitance to the circuit which bleeds off higher frequencies.

The copper on the bottom of your pickups is likely there to ground the pole pieces. If they are not grounded they may get really noisy if you happen to touch any of the pickup poles - most pickups do not have grounded poles. If the cable from the pickups into the control cavity is shielded, there is no need for shielding the pickup cavity. Still there still may be some benefit from shielding the control cavity unless all of the wiring there is shielded as well.

The consensus is that shielding the pickup cavities may cause some attenuation of the higher frequencies, but that wrapping the pickups with a shield will likely cause more attenuation. What I have directly experienced is that the attenuation tends to remove only very high frequencies that are the centre of overly edgy glassy frequencies. It's a matter of taste I suppose, but any effects I have found have not been undesirable.

One of the theories that has been forwarded is that as the shielding/grounding removes high frequency noise the perception is that some highs have also been removed from the audio signal, but it only seems that way. Nice idea, and perhaps it's true.

 

[walterw]

followup to @Turnaround's fantastic intro for the TL;DR crowd:

-no such thing as ground loops inside a guitar, "ground is ground" in there

-shielding won't touch actual single coil hum, it only reduces the extra buzz that happens when you let go of the strings

-shielding that's too "good" (too conductive) should not be too close to or covering the pickups themselves or you get eddy currents induced into the shield from the pickup's magnetic field that dull the tone. that means don't copper foil-line your jazz bass pickup covers!

 

[Turnaround]

followup to @Turnaround's fantastic intro for the TL;DR crowd:

-no such thing as ground loops inside a guitar, "ground is ground" in there

Actually it is possible to have ground loops inside a guitar. It's not that there is the opportunity for differing voltage potentials, but that a loop can act as a single turn coil and thus pick up noise.

 

[walterw]

It's not that there is the opportunity for differing voltage potentials, but that a loop can act as a single turn coil and thus pick up noise.

do you have a source for that one?

i have a hard time believing it, the entire "loop" including both ends would be grounded together, how could that become any kind of "coil"?

it's not like inside an amp where you might have hundreds of volts difference between one end and the other and thus where you make the grounds matters.

 

[Turnaround]

do you have a source for that one?

i have a hard time believing it, the entire "loop" including both ends would be grounded together, how could that become any kind of "coil"?

it's not like inside an amp where you might have hundreds of volts difference between one end and the other and thus where you make the grounds matters.

Check what Lindy Fralin has to say about it, part way down, under "More is Better"...

Fralin Pickups: Understanding Guitar Grounding And Common Mistakes

 

[walterw]

Check what Lindy Fralin has to say about it, part way down, under "More is Better"...

Fralin Pickups: Understanding Guitar Grounding And Common Mistakes

that's a nice, clear, well-written explanation.

i'm just not sure i believe it.

with shielding around a pickup you might have eddy currents induced by a moving magnetic field, and i have heard one rare tale where a circular pot ground arrangement in an LP picked up radio noise that disappeared when the circle was broken, but i'm more inclined to suspect one of the solder joints was bad creating some kind of weird partial connection.

i've done "belt and suspenders" wiring of fenders forever, connecting pot grounds via actual soldered wire as well as whatever plate or foil the pots are bolted onto, and never noticed any issue. what is an issue is if a pot comes loose from the plate and loses its ground and all of a sudden a pickup stops working! the soldered ground prevents that.

 

[Turnaround]

that's a nice, clear, well-written explanation.

i'm just not sure i believe it.

with shielding around a pickup you might have eddy currents induced by a moving magnetic field, and i have heard one rare tale where a circular pot ground arrangement in an LP picked up radio noise that disappeared when the circle was broken, but i'm more inclined to suspect one of the solder joints was bad creating some kind of weird partial connection.

i've done "belt and suspenders" wiring of fenders forever, connecting pot grounds via actual soldered wire as well as whatever plate or foil the pots are bolted onto, and never noticed any issue. what is an issue is if a pot comes loose from the plate and loses its ground and all of a sudden a pickup stops working! the soldered ground prevents that.

I'm not sure either. Between that and eddy currents I'm a bit asea. If eddy currents are a problem, then surely grounding their source would drain the effect. No? And if a loop is grounded, wouldn't that drain any noise as well?

OTOH, I just grounded the poles on the pickups in my Jazz bass and I have lost some high-end bite. I thought it had to do with capacitance, but apparently since the grounding strip is at right angles tot he magnetic field I have an Eddy current problem.

 

[walterw]

I'm not sure either. Between that and eddy currents I'm a bit asea. If eddy currents are a problem, then surely grounding their source would drain the effect. No?

i know, right? me too.

as i understand it, the eddy current thing around a pickup is not about noise but high end loss;

move a magnet through metal and electricity forms in that metal, the electricity moving through the metal forms its own magnetic field, and that new field "fights" the original field, blocking some signal. this happens even when the shielding is totally grounded.

the trick there is to indeed have the shielding be partially "interrupted" so the eddy current can't form a full loop, or to have the shield be made of something not as "good" (conductive) as copper or brass, but rather something like the "nickel silver" that gibson uses on their guitar pickups.

here's a fantastic exploration of that principle, including actual tests: Invalid Link Removed

thing is, i don't know that any of this has anything at all to do with grounded loops down in the control cavity away from the pickups and their magnetic fields.

OTOH, I just grounded the poles on the pickups in my Jazz bass and I have lost some high-end bite. I thought it had to do with capacitance, but apparently since the grounding strip is at right angles tot he magnetic field I have an Eddy current problem.

now that's weird, since those magnets should already be right against the grounded end of the coil wraps, so there shouldn't be a big difference. you didn't wire the pickups reversed did you, to where the magnets were now against the hot end of the coil?

that would have made for noisy poles if touched, which grounding them would solve.