More suggestion to diagnosing/fixing earth hum

[Dave71]

Had some spare time today so I've shutdown and disconnected everything in my house to try find the source of a hum in my tiny home studio setup.

Was left with nothing running but my mac mini and bus powered scarlet 2i2 interface. With just a guitar, lead and headphones, i can hear a buzz which disappears as soon as i touch anything metallic in the chain - guitar hardware or even the mac mini case & apple wired keyboard.

Now the buzz isn't such a problem when playing guitar, it's when I'm on the bass as I've found I'm more likely to be on and off the strings, bringing on an audible crackle as i do.

Tried every combination of guitar and lead i have to no avail. Adding my powered monitors back into the chain reduces the buzz as I'm assuming they are providing some earth connectivity to an otherwise non earthed setup (mac mini is a class 2 device)

I don't really have anything else that gets connected other than my Qu16 desk which i will hook up at some point to experiment with. Rebooting everything else in the house hasn't made the problem any worse.

Any other suggestions to try?

Thanks

[Hugh Robjohns]

It sounds like you're on the right track.

Without a solid ground touching and exposed metal in your core system essentially injects the electronagnetic interference your body picks up (like an aerial(, resulting in audible hum.

Adding a ground via the audio cable screen to your speakers, and from there to the mains Earth will help. But if the audio ground in the speaker is decoupled from the mains earth by a small resistance — which is common and intended to minimise ground loop noises — the interface ground isn't as solid as it could be and some hum pickup may still be audible.

As an experiment, try using the tips of the plugs of an instrument cable with one end touching the sleeve of a connector on the interface, and the other on a good earth like the mounting screw of a wall socket.

[Sam Spoons]

The Mac Mini is a Class 2 device so doesn't have an earth. So if you earth the system to something metal, as Hugh suggests, or add a device with a good earth (powered speaker or similar) does that stop the buzz? Guitars are notorious for picking up hums and buzzes even in earthed systems but given the floating nature of your basic system I'd bet the lack of earth is what is causing the issue..

[Wonks]

Groundology are still going, so one of their plugs and one of their leads, a bit of soldering and you can have a nice safe ground connection.

https://www.soundonsound.com/reviews/gr ... ction-plug

You just need to find somewhere permanent on the 2i2 to connect it to. I don't know if the aluminium housing is connected to the PCB ground, but it it is, maybe add a hole and a bolt to connect a ring terminal on the end of the ground lead?

[Dave71]

Thanks Hugh, I've tested my monitors and there's no resistance between the earth pin on the IEC inlet and ground on the trs/xlr input

Have tried your suggestion of linking the exposed metal part of my interface to the local socket screw with no success. Soon as i touch it, problem goes away.

[Sam Spoons]

Is there a connection between the guitar earth and the earth pin on the monitors mains plug (you could have some dodgy wiring somewhere so you get an earth but via a high resistance)? Have you tested the earth in your power sockets, it has been known for them to be high resistance or even non-existent?

If your socket back boxes are the plastic type used in plasterboard the screws are not always earthed, unlike metal boxes used in solid walls so that could be one explanation for Hugh's suggestion not working.

[Dave71]

Great suggestions. Unfortunately (purely with reference to fixing this) I’m heading off on holiday now for a week but will get to the bottom of this upon my return. We have a fluke tester at work so I’ll borrow that to test the earth to the socket. It is a cavity box but the screws should still be earthed.

Cheers

[Hugh Robjohns]

The basic problem is that, as far as electromagnet interference is concerned, a guitar (or bass) is really a TV aerial, almost purposely designed to pick up any and all stray EM/RF radiation floating around.

In a system with no external ground collected interference can get the system's internal audio reference resulting in audible hums, buzzes and other unwanted noises.

Giving the system a solid ground reference can help enormously, as you've already found, but the geetar is still collecting and injecting unwanted noise.

If the system is quiet, hum/buzz-wise, when using a mic or an electronic source like a keyboard, then the problem is the guitar rather than the system.

If it's humming or buzzing with a just a mic or keyboard (and you know the system has a solid ground connection) then you've probably got one or more ground loops.

If the problem is the guitar/bass there are various things you can do to improve matters.

For starters you can try to find the source of interference. It will be something electrical so a methodical process of switching things off/on and moving the guitar closer/further away can often identify the problem devices. LED lighting, light dimmers, and computers and their screens are all common culprits.

One of the major original design failures of the electric guitar/bass is that are wired up as high impedance and unbalanced sources. It makes them particularly prone to picking up interference. To make it even worse, most don't even bother to screen the high impedance electronics, which is just ludicrous from an electrical engineering perspective. Many also create ground loops in the internal wiring which doesn't help, too.

So, shielding the control cavity with copper foil or conductive paint, etc, can often make a big difference. As can using humbucker pickups — a single coil pickup is designed to detect local EM fields so it's not surprising it picks up other locally radiated fields.

[Sam Spoons]

It is a cavity box but the screws should still be earthed.

Definitely not on the ones I have here.

[Wonks]

You could always fit EMG pickups in the bass.

Their single coil models are normally the quietest. Both ends of the coil are fed into a differential preamp within the pickup, so basically almost all hum picked up by the coil is cancelled out before the signal leaves the pickup. The humbucker models have the same principle applied, but because EMG tend to use mismatched coils in their humbuckers, the amount of hum cancelling is less. Still a useful amount, but not quite as silent as the single coils.

[Dave71]

Hmmm. Well, all the power suppling the first floor of my house comes up in the corner of my little room with part of the ring main and kitchen lighting ring running directly below my chair with the upper lighting ring directly above my head at the desk position. That’s not a great start I suppose. I might grab an ups from work too and that way I can switch my mains off completely then add circuits 1 at a time to see which is the worst.

I have experimented with all my guitars and found that the single coils are noisier but when I put my furniture in, I sat and found the quietest spot available considering the limited space. With fingers on strings, there’s very little noise at all.

I’m away from home now but just removed a couple of socket screws from my partners surface mount utility room sockets. They’re both Mk and have a small brass insert in the screw holes linked to the earth - I thought that was a standard?

[Hugh Robjohns]

Common, but not standard.

It might be worth checking that the guitar bridges are grounded back to the ground point in the wiring/control cavity.

[Sam Spoons]

I’m away from home now but just removed a couple of socket screws from my partners surface mount utility room sockets. They’re both Mk and have a small brass insert in the screw holes linked to the earth - I thought that was a standard?

The only ones I have which don't have the screws earthed are similar to these https://www.screwfix.com/p/british-gene ... pack/455xf

[Hugh Robjohns]

I have come across installations with metal back boxes where the earth wires from the cable aren't linked to the box at all, just the front sockets. In most cases the retaining front plate screws still pick up the earth as they pass through the holes, but not always.

[Sam Spoons]

Actually, now I think about it, all the sockets have earthed eyelets in the screw holes anyway

I must have fitted dozens over the years and when I saw that those plastic back boxes didn't have any earth connections I never made the connection (pun intended) between the two and assumed the screws would be unconnected too...

[Dave71]

Update. Swapped the socket out for a new one with the earthed screw holes and tried Hugh’s original suggestion of providing an earth to my signal chain - no joy. Ran an extension upstairs from a socket closest to my consumer unit to rule out a weak earth link in the ring main.
Still requires me to touch something to stop the buzz.

Borrowed another guitar (usa strat) expecting the single coils to be just as bad but found this to be significantly better.

Going to see which appears to be the worst guitar and experiment with cavity shielding.

[Hugh Robjohns]

Hurrah!