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Ever noticed a nasty hum when you plug in your new kit guitar, especially a guitar with P90 or single coil pickups?
In the following article we’ll discuss what the source (or sources) of the hum are, why it occurs, what you can do to fix it, and why it’s only a problem in some guitars. So, if your new guitar kit is a little noisy, stay tuned, the information below should help you fix, or at least reduce the extent of the problem with minimum fuss or expense.
Before we start shielding, it's a good idea to check the hum you are hearing isn't due to a grounding issue. This is simple enough to check. If the hum stops, or reduces significantly when touching the guitar strings, you can assume there’s a problem with your ground circuit. In this case you will need to check your wiring. If the guitar is correctly grounded we can move onto a more common source of hum, known as electromagnetic interference.
If not a ground wire issue, that hum you are hearing is likely to be electromagnetic interference (EMI) often referred to as 60cycle hum.
It’s called this because mains power alternates at 60hz, and in many cases is the source of the interference.
Audio and electrical equipment within the same room, e.g. mains power, powering your amp (or other electrical appliances), or just the lights in the room, even if not on the same circuit can interact with your pickups.
The source of interference doesn’t need to be on the same circuit as EMI travels throughout the air in waves and is picked up by the coils in your guitar's pickups, serving as a type of antenna.
The best way to prevent, or at the least reduce the amount of hum you are hearing is to shield the guitar’s pickup cavity using a conductive material. In the majority of cases copper shielding tape is used, which is adhesive making it easy to apply.
Copper effectively reduces the amount of EMI. As a result it's commonly used for shielding audio equipment e.g. you may notice your guitar cable utilizes copper braiding.
You don't have to use copper tape however, aluminum foil can work, and often works just as well. The other option is to use shielding paint. It’s tidier than using copper tape or foil but can be expensive (unless you are working on multiple guitars) and takes a bit more work (multiple coats in most cases) to shield the cavity.
If you own a guitar with humbuckers (especially humbuckers with metal covers) you more than likely won’t need to bother with shielding your guitar. Humbuckers are designed to buck interference, hence the name humbucker.
If you play a guitar with single coil pickups or P90’s (which are essentially oversized single coil pickups) it can be a good idea to shield the cavity if you are hearing hum when turning on your lights or running appliances in the same room. Otherwise, if you are assembling a new kit guitar with P90 or single coil pickups, it can be a good idea to do this as part of the assembly process.
In terms of equipment required the basic list below is all you need to complete the job.
Masking tape is optional. I’ve used it in the past to apply directly to the cavity and then lift out any dust or other residue which sticks to the tape, but as long as you clean out the cavity to allow the adhesive backed copper tape to make solid contact you will be fine. Otherwise a sharp blade for cutting the tape into sections and of course copper shielding tape are all you need.
We’re basically lining the cavities of the guitar to shield from EMI. So if you are doing this on a kit guitar, and are yet to install the pickguard and strings, it's simply a matter of lining the cavities with the copper tape (or foil).
Otherwise, remove the strings, and if applicable to your guitar, undo the screws for the pickguard and carefully lift the pickguard up taking care not to break the wires from the pickups. If your guitar doesn’t have a pickguard, remove the pickup/s and rear control cavity plate.
Should you shield the output jack cavity?
You can, but in most cases it’s not necessary. The pickup cavity must be shielded as the pickups are highly susceptible to interference, but any potential interference affecting the wires running from your output jack cavity will be minimal and in most cases won’t be detectable by ear.
Once you have removed the pickguard (if required) and the output jack cover (if you are doing this also) begin shielding the guitar by following the simple instructions below.
1. Clean the pickup and control cavity and input jack cavity
You can use compressed air to do this, or use masking tape and press it against different parts of the cavity. Dust and grit will stick to the masking tape and then be discarded.
2. Cut sections of the copper tape (into shapes that can then be used to cover the cavity).
Stick the sections of copper tape to the inside of the pickup cavity and make sure no bare areas are left remaining. If the guitar has a pickguard you should overlap the cavity slightly so when the shielded pickguard is installed they are in contact with one another.
3. If you are shielding the output jack cavity (see above for why this often is not necessary) connect the output jack cavity to the control and pickup cavity.
You can use wire (soldered to both cavities), or a thin section of copper tape, as long as you have a conductor between the two cavities. Alternatively you can also use shielded ground wire for your ground circuit which will take care of connecting the two cavities without requiring a separate wire.
We need to shield our pickup cavities, but we also need to shield the pick guard itself.
You can buy shielding plates which are basically thin sheets of foil designed to fit a standard S style guitar's pickguard. In many cases pickguards will come with this already installed, or use a smaller section of shielding near the holes for the controls.
If you need to shield the pickguard, it’s a simple process:
1. Unscrew the volume and tone pots and remove the grund wire. It’s best to use a soldering iron to free up the ground wire if you have already installed one.
2. Clean the underside of the pickguard
3. Apply lengths of copper tape to the underside of the pickguard. It can often be easier to cover the pickup cavities and control holes and then cut them out using a sharp blade (Make sure the tape is overlapped and no bare areas are left remaining).
Once the pickguard is shielded and installed, the shielding on the underside of the pickguard connects to the shielding within the cavities and creates what is known as a Faraday Cage (an enclosure used to block electromagnetic fields, named after scientist Michael Faraday, who developed the Faraday cage as far back as 1836).
This might seem confusing, as the top of the pickups are still exposed, meaning the pickups are not completely isolated. But as long as the shield is bled off to ground then the amount of hum should be greatly reduced.
From there, the job is complete. All you need to do from here is reinstall the pickguard and restring the guitar.
Shielding your electric guitar is an easy task to do. It isn’t costly, doesn’t require great skill, and while I have outlined the cause for EMI above you really don’t even need to understand the cause.
So if your guitar is noisy, get hold of some EMI copper shielding tape and shield your guitar's pickups and controls. The next time you plug your guitar in, while it may not be completely eradicated will be far less noisy allowing you to focus on playing the guitar instead.