How do I get rid of hum in mixing desk when guitar plugged in?

Question

I have an rcf f6x mixing desk, whenever I plug in an acoustic guitar I get a weird hum through the speakers, it seems to dissapear when I kill the bass on the guitar eq and the mixer but obviously this then sounds like you know what. Also I've noticed a slight static shock from my mic, I've noticed this in the past when I played in venues which I assumed had dodgy wiring but never with this hum noise. Interestingly using a cheap electric guitar tele style doesn't make any hum. I've tried 2 acoustics so it's not the guitar as far as I can tell. The power block has a changeable plug for eu and uk sockets and I'm using the uk one -though the prong is plastic so I don't know if it's doing anything to ground the desk. Anyone know of a quick or long term fix for this?

Answer 1

Looking at the pictures at Thomann, that looks like a cheap switch-mode PSU. They are notorious for a slight 'tingle' when you touch anything connected to them. You can't 'fix' them in any meaningful way except by swapping to an expensive 'traditional' wound transformer.

The hum is not insurmountable, though there's no single perfect fix.

You should grab a bag of ferrite beads, few quid on eBay - example. Put one on the 'input' end of every cable. They're not miracle cures but they can help a little bit with high end whine. They're cheap as chips, so you can afford a bag.

Also, a bit of DIY electricals. Grab a spare UK plug & a length of wire - mains preferably, but you're not using this for "earthing" you're going to use it for "grounding" (the US doesn't differentiate, but usefully the UK does.)
Using the screw on the back of the desk…

Take this out, scrape some of the paint from around it so it makes good contact, then wrap one end of your wire round it & tighten it back in. The other end goes into the earth pin on your plug; then plug it into a regular earthed socket. This is not dangerous so long as you can wire a plug. You might need to add a bit of cable sheath around the single wire so the cord grip holds properly.

This will properly ground the equipment.
There is still potential for ground loops, but if you plug all your gear into the same ring main, this should be minimised.
You can extend this by strapping the frames of all audio gear together in a chain.

You still might have to touch metalwork on a guitar to fully close this circuit - or like we used to do with especially errant guitars in the studio, use one of those computer grounding wrist straps round the guitarist's wrist & clipped to some part of the guitar's metalwork.

If all that doesn't work, look for a new desk… or some expensive isolating mains 1:1 transformers ;)

Answer 2

It sounds like you are using an unbalanced connection from the guitar to the desk by way of a regular guitar lead yes? This would be a cable with only 2 conductors instead of 3 like a mic cable, for a mono signal.

I checked on the RCF site and: The mono jacks for channel 1/2 is balanced input (3 conductor). The mono/stereo jacks for chan 3/4 is unbalanced input.

• Have you tried using a DI box? This almost certainly will fix it.
• Have you tried using chan 1/2 using a "TRS jack to XLR" cable? This maybe will work. If the gat has balanced out, a "stereo jack" cable may work.
• Tetsujin's fix appears to fix the instance of "stray capacitance" buildup in the chassis leaking to audio path. This is nice, but try a DI box first! Easier and likely better. The chassis sound enters the path as you plug the lead into the guitar (try plugging halfway in guitar)
• Does it have piezo or magnetic pickups? Piezo requires 10 megaohm preamp. If you buy one, ensure it has balanced XLR output.
• Only XLR to XLR guarantees balanced pathway blindly. I checked and your desk has balanced TRS on 1/2.
• I've heard that power isolators (1:1) can help with noise floor issues; I've not tried that. All DI boxes use 1:1 transformers and should provide that same "ground lift" but to your audio path not power. Good audio gear uses linear power supplies (AC normally). Low grade gear sometimes/always uses switching power supplies (DC). It maybe not trivial, but if you could get a linear power supply that outputs DC that mite work.

The simple use of a good quality DI box - one with a ground lift switch - is highly likely to take the hum to negative infinity, and let you output to balanced equipment no matter what harsh RF interference maybe about, and also over long cable runs no problem. Running that cable over power supplies should produce no hum believe it or not; a regular unbalanced should pickup some 50 / 60 hertz when laid over a lot of power cables etc.

Since it is unlikely your guitar would make use of a TRS jack, using a TRS jack to XLR mite not work if the desk (or the guitar) bridges cold to earth. A regular guitar lead bridges cold to earth because it only have 2 conductors, and the cold conductor on the bigger part of the plug is shorting to where the earth "band" would appear (on a quarter inch gat jack lead). The short happens inside the guitars female jack.

Rather than replace your guitars jack, you can use a DI box to interrupt this bridging in the jack, because the DI box isolates the guitar from the desk via a transformer. The earth on guitar is now no longer connected to the desk chassis (it's the balanced earth that touches it). The ground lift will then have next to no effect because it refers to the balanced connection (XLR pin 1) not cold, between the DI box and the desk not the guitar, and with balanced signals it is the differential inversion that kills all interference not the shield. Linking the shield to ground helps by way of the Faraday cage effect, especially if it has phantom power (a Faraday cage requires a little DC current to also flow through the cage, it's not important).

Finally, some low current 48v DC phantom power runs from hot to earth. While it can not flow through pickups / line-outs, because those circuits return via cold, it may begin to flow when you plug a 2 or 3 conductor cable like I said (TRS jack to XLR) straight from desk to guitar. Notice how just using a DI box fixes everything.

If you do not use phantom power at home you may only notice this at gigs, where phantom power runs certain condensor mics and usually all the DI boxes. I would assume your shocks come plugging guitar directly into the desk then. Again, a DI box blocks that lip tangling badness!