Hum

[DarkMark]

Hi folks,
Since I’ve started turning up up amp louder lately I’m aware of an unacceptable hum which dies down when I touch the strings. I’ve read enough posts in the past to know it’s probably an earth issue, but is there a likely trouble spot for me to check before I pull the guitar apart? I’ll be giving it a fret level soon and I’d like to take care of the hum at the same time.

[McCreed]

I presume you have a multimeter (hopefully anyway). I'd start by setting it on continuity and trace the ground path of each grounded part. Simply a process of elimination. A bad ground can almost anywhere, but maybe start with the bridge and work out from there.

Oh, you didn't say which guitar you're having trouble with. That might help with suggestions.

[Simon Barden]

Which guitar, or is it all/several?

Even with humbuckers, any unshielded wiring can pick up hum. It’s down to how much electrical noise there is in the locality. Some amps put out a lot more RFI than others, but sometimes it’s other things like dimmer switches or computers or phone or radio masts. Touching the strings grounds your body, so it stops acting like an antenna and inducing noise into the guitar electrics. Do the long signal cable runs in your guitars use shielded/screened cable?

You can always try switching off lights and as many appliances as you can, in case one of them causes the hum level to drop considerably. It’s noisy because there is electrical noise about.

But there’s only a little you can do to check anything before taking a guitar apart, especially when it's a hollow body. A poorly grounded pot or high resistance ground connection can increase the picked-up noise levels, but you’ll need to test each joint with a multimeter. And it may all be fine and the noise is what it is. You may have a poor connection that adds 20 ohms to the signal ground path, but with pickups with 6k-8k coil resistances, you’ll never measure that extra circuit resistance externally across the jack socket.

You can try measuring the jack ground to the exposed metal parts of the guitar. Pull the knobs off to expose the pot shafts. All the metal should have a short-circuit level of resistance e.g. same as the meter registers when you touch the probes together. Maybe only 0.1 ohm or so more. Doesn’t cover all the ground connections but it’s a start.

[Dikkybee007]

I have never used any sort of shielding on any of my kits and most of my guitars are single coil and I use the cables that came with the kits into a cheapish practice amp and the hum was terrible. I bought some good quality coax and gold plated 1/4" connectors and that reduced the hum by about 60-70%. I changed to a 100 Watt Fender Mustang amp and it removed the hum all together. I was really surprised by how much of a difference there was in just replacing those 2 items. I now buy quality cables but I added a Vox mini and with my store bought Brian May BMG special no matter what cable I use it hums but all of my guitars hum with this amp. No way was I going to pull apart that guitar to check for shielding. I bought a noise gate/filter and I can reduce the hum completely but I loose some sound quality or I reduce the hum to a minimum and it doesn't affect the sound at all. Maybe before pulling anything apart just change a cable.

[Simon Barden]

...but I added a Vox mini and with my store bought Brian May BMG special no matter what cable I use it hums but all of my guitars hum with this amp.

This is almost certainly down to the lack of a proper connection between the signal ground of the amp and the power supply ground. The external PSU is very likely a 'double insulated' PSU (there should be a square in square symbol on it if it is).



It's electrically safe, but the noise picked up by the cable screen has nowhere to go. Same thing will happen when it's running on batteries.

It's becoming more and more of an issue with laptop-based recording systems and bus-powered interfaces, where there is no linking between the system ground and earth, or else it's a fairly high resistance path.

You can normally cure this by linking an exposed metal part of the amp or cable to earth. You can buy boxes to do this if you want, but a simple DIY small metal FX enclosure, with an in and out jack (metal chassis guitar-style jacks, not the plastic type), linked straight through, with a long earth wire attached to the metal case with a mains plug on the end and the wire connected to the ground pin.

Your Fender amp will have a properly earthed chassis with the signal ground connected to earth, due to the internal power supply.

Gold-plated jack plugs aren't as good as standard jack plugs in terms of conductivity. It's got about 1/4 of the conductivity of chrome IIRC (though obviously it's a very thin layer so overall resistance is barely affected). The benefits of gold-plated connectors are for connectors that get put together once and left e.g. in a hi-fi system, as they won't corrode. Normal style guitar jacks get cleaned of corrosion every time you insert or remove the jack plug, which also scrapes the gold off quite quickly. So gold connectors are best for for fixed cables, chrome connectors for regularly used ones.

[DarkMark]

It’s my ES-3. I used the push back cloth wire. Perhaps I should redo the wiring with shielded wire?

[Dikkybee007]

My amp is only about a month old, If I let the magic smoke out I pay for it so will wait until after warranty is up.

If you use shielded wire it will remove the hum introduced by the wire but not any other parts, whereas if you shield the cavities it will remove all or you can do both and be really sure.

[Simon Barden]

My amp is only about a month old, If I let the magic smoke out I pay for it so will wait until after warranty is up.

If you use shielded wire it will remove the hum introduced by the wire but not any other parts, whereas if you shield the cavities it will remove all or you can do both and be really sure.

A) No risk of damage at all by earthing the guitar lead's ground connection.

B) The ES-3 is a fully hollow-body kit with no control cavities. Even if one were able to get inside and screen the entire body, the two F-holes would still let in a lot of RFI. A Faraday cage is only as effective as its largest gap (which obviously should be very small or non-existent). The length of the F-holes precludes any real shielding. It would have some effect for noise from the rear and sides, but maybe almost none for RFI hitting the front of the guitar. So apart from impracticality, it simply wouldn't be that effective.

The wiring harness in the ES-3 is pretty small, so the only real length of wire that would benefit from braided cable is the output jack connection.

If it were me, I'd probably start again with new pots, and use the volume pot as the 'star' grounding point. So the output wire braid is soldered to the back of the volume pot, the pickup grounds are connected to the back of the volume pot (as they are at present), and the trapeze ground and tone pot ground are also taken from the back of the volume pot. Star-wiring the ground is the best way of wiring up a harness for minimum noise, but it's only really practical on a simple arrangement like you have.

[DarkMark]

Thanks for the suggestions folks. So Simon, are you saying shielded wire would only be of benefit to the output jack and not to worry about using shielded wire between pots etc?

[Simon Barden]

Well, with the ES-3 you've only got two pots and the distance between them isn't large. So you've either got one short length of wire or a capacitor joining the volume to the tone pot. The connecting ground wire doesn't need to be shielded. Whilst you could use screened wire, for very short lengths of say 5cm/2", I wouldn't bother. You'll need to have a certain length of the wire exposed anyway at either end to prevent shorting the signal, so you'll end up with close to the same length of exposed signal wire even if you did use screened cable.

The output jack wire will be longer, so is worth using screened wire for. The longer the wire length, the more important it is that it's screened if its not in a shielded cavity.

Once you move on to two pickups and switches, then I'd use braided screened wire where possible and generally follow the method shown here.



Using heat shrink or insulated sleeving over the braid in proximity to pots or switches is very important to avoid the possibility of signal shorts to ground. I suffered from this just the other day when I found that the tone control on my open-G 5-string Vintage V130 LP special DC-style that I'd rewired didn't work, because the braid of the output lead was being pushed against a lug of the tone pot. Now fixed.