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In this blog we’ll look at what causes pickups to hum and how to deal with it.

guitar control panel mod modification hum

We’ve had a couple of guitars in the workshop recently which have had hum issues, so it’s worth recapping how and why this happens. How pickups convert the vibration of strings into sound via an amplifier is another blog, so let’s take that as a given and look at the reasons why hum occurs and how it can be tamed.

For reasons we will come on to in the next blog, hum is normally only an issue with ‘single coil’ pickups. By single coil, we mean a pickup which contains one magnet with one length of wire wound around it. Single coil pickups are typically found, for example, in classic Stratocasters and Telecasters. ‘Humbuckers’, which contain two separately wound magnets, were invented to overcome pickup hum and first appeared in Gibson guitars in the 1950s.

Unwanted pickup noise is normally produced by two things: 50 cycle AC power (or 60 cycle, if you are in the US); and electromagnetic interference (EMI). You can only hear it when the pickup is idle and not generating its own sound. Usually, a guitarist will notice the hum and EMI gets worse in a rehearsal studio or at a gig. This is because at these venues, there are a lot of devices plugged in to the power network, reinforcing the 50 cycle oscillation, and usually also producing their own EMI. Lighting and computers are also a known problem. Sometimes, in extreme cases, your guitar will pick up the local cab drivers or other radio transmissions. A rehearsal studio I use has an electrified railway track above it, which produces an uncomfortable amount of noise through the amps when it rains!

The way to tackle hum is to first ensure everything that should be grounded in the guitar is joined properly (i.e. strings, via the bridge, and all the electronic cases in the control cavity), but only joined once – creating ‘ground loops’ can in itself produce hum. There are various ways to test this with multimeters, which we won’t go into here, but there are plenty of online resources on this. Once you have eliminated poor grounding you can look at what surrounds the pickups and their controls. Single coil pickups and their controls should ideally be ‘shielded’.

By shielding we mean creating a metal ‘compartment’ around the controls (and, if you really want to, around the pickups too). You will often see partial shielding in the form of an aluminium sheet applied to the back of Stratocaster-style guitars’ scratchplates. This is only partial EMI shielding - it isn’t very effective because the cavity is generally not shielded, but it’s better than nothing.

copper shielding prevents emi hum

You can do proper shielding with either: metallic shielding paint; or copper foil. Paint is easily applied and you can get it into the wire cavities as well. However, it is not as good a conductor as copper and can lose its properties over time. We see a lot of guitars which are ostensibly painted with conductive black or grey paint which, when tested with a multimeter, conduct nothing. That is why we prefer adhesive copper sheeting and tape, which is conductive on both sides and lasts forever. If applied properly, copper will create a ‘Faraday cage’ around a control cavity which will block EMI. Copper shielding also eliminates the need to solder ground wires between potentiometers, because your pots will be grounded via the copper (as long as the pots are tightened sufficiently!).

Another manufacturer tactic is to use shielded cable. This is most often seen in Gibson guitars. The centre of the wire carries an insulated ‘hot’ signal and the outer metallic braid carries the ground. The braid can be soldered to the back of potentiometers, meaning the whole wire path is prevented from acting as an EMI antenna. In our opinion this is a bit OTT, and you are limited in this by the wiring that comes with your pickup (unless you want to get really serious and change the wires on the pickup yourself).

It’s rare to be able to eliminate hum completely, but shielding will definitely make a big contribution. Some pickups seem more or less prone than others. Shielding Fender or Squier Jaguars seems to make a tremendous difference, although possibly this is also because those pickups have a bit of built in shielding.

Whatever you try to do, you will be lucky to completely eliminate hum and EMI with single coil pickups, but shielding will definitely help tame the beast. In our next blog we will look at how to use the mix position to turn two single coils into a humbucker and eliminate hum and how polarity effects pickup sound.

You can get your guitar shielded at String King Guitar Works. Check the Services page for pricing.

We didn’t do an update last month because of Christmas and a short shut down. But here’s some of what we’ve been up to over the last couple of months at String King.

Aria Bass: New saddles, Neck Shim and Set Up

We’ve had quite a few basses in recently. This one was another which hadn’t been used for a while but the owner remembered he loved the feel of the neck and wanted to get it set up properly. One of the bridge saddles had missing grub screws and they were worn away, so they were replaced by new Wilkinson brass saddles. The action was not right on this bass so we put a shim in the neck which sorted out the problem. Finally we did a set up and the bass was ready to go.

Takamine Acoustic: Fret Level, Neck Repair

Paul brought in a lovely Japanese Takamine for us to repair. Someone had previously replaced a broken truss rod by removing the fretboard rather savagely and re-gluing it poorly. On top of that a fret levelling had been attempted and not completed, which left the guitar just about unplayable. The first thing we did was take out the nut and re-level the frets. This was the first chance we had to use the new neck jig and it worked a treat. Then the back of the neck was cleaned up and smoothed out – the owner did not want a full cosmetic repair, just to get it feeling right. We set the neck relief and the nut action and put on some new strings. The owner was delighted with the job and it was definitely satisfying to get such a great instrument back in good order.

Squier Strat: Re-Wire and Copper Shielding

Another guitar from Mark. This time he brought in his lovely Japanese Squier Stratocaster. This one has been sitting around unplayed for a few years and the electrics needed an overhaul. Everything was replaced – pots, switches and wiring. We use quality full-size potentiometers which will last several years of hard playing. We also fully copper shielded this Strat. The result is an ultra-quiet, sweet sounding instrument.

Kit Guitar: Fret level and set up

Alberto came back with a kit guitar he’d put together. It needed a fret level and a full set up. We got the work done for him quickly and he was pleased with the much lower string action. I liked the finish he got on this one.

Ibanez Silver Cadet

This is a nice vintage Ibanez, from a much under-appreciated manufacturer. I am usually impressed with these guitars and pound for pound they are great value. It’s not an opinion the purists would agree with but I don’t think you can beat Japanese instruments for general build, although sometimes they skimp on hardware quality. This one was generally fine but the electrics needed an overhaul . The tuners were not very good so they were replaced with a new Wilkinson Deluxe set.

Westone Concord I Bass: Full overhaul

We like to get old classics and overhaul them. This is an ongoing project which we are doing as time permits. The guitar was a bit of a wreck but the Concorde series were good guitars in their day. Well. it's Japanese, isn't it. So far we have stripped the horrible paint work off and begun to respray. All the parts have been removed and cleaned ready to be put back in. We can’t wait to see what this plays like.

And finally… A Workshop Re-Organisation

It’s really important when working on an instrument to have everything within easy reach and there’s nothing more frustrating than losing a tool when you need it. We’ve had a bit of a tidy up and created new storage, which will help us be more efficient.

This month we’ve been sorting out a lot of different problems for guitarists. Some of them were straightforward and some were a bit more complex. Here’s a few of them.

Ibabez Joe Satriani Series Neck Re-Finish

Now this is a lovely guitar. I’m not a big fan of the Floyd Rose style guitars but their JS Series instruments are really very well made. The owner needed the neck re-finished, and the guitar generally cleaned up, so we re-sprayed it with lacquer and polished it to a high finish.


Gibson SG Faded 3 Truss Rod Adjustment

It always surprises me when I see a guitar from a big name that has not been set up from the factory properly, but really you would expect better from Gibson. This guitar came in with a neck with three times as much relief as there should be. The owner wasn’t able to fix this because for some reason Gibson had fitted the guitar with a non-standard adjustment nut.

We sorted out the relief problem, but I also noticed that Gibson hadn’t set the nut up properly. Still, it’s a lovely guitar despite these easily remedied issues.


Luna Acoustic Headstock Break

Probably the most dramatic thing that can go wrong with your guitar is the headstock snapping off or developing a severe crack after it gets dropped.

Most people would think that it’s bin time, but a broken headstock is usually one of the more straightforward jobs. This acoustic guitar came in with a clean break and it was relatively unproblematic to clamp and glue it back on again. A good wood glue, properly applied, can result in a stronger joint than the wood itself.


Fender Telecaster Re-fret

Eventually, every guitar is going to get to the point where the frets have worn down, making the guitar unplayable. Sometimes this can be sorted out with a fret level, which is not an easy job but it is quite quick to do. But there are times where so much material is missing that the only option is a full fret replacement. Changing the frets is a long job and not inexpensive, so it’s got to be an instrument that you really value to make it worthwhile.

This Fender Telecaster is ten years old and we replaced the frets as there really was nothing left to work with. Luckily we had the correct fret size in stock so there was no hanging about. The rosewood was quite soft, which gave us some issues, but the end result is the restoration of a lovely guitar.


If you've got a guitar problem you'd like us to have a look at, contact us for a free assessment.


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