Little Shocker

[needsmorecowbel]

Hey guys I've been doing a bit of recording this week and I've been noticing that if I have my hands on the strings of the guitar and accidentally touch my computer case or touch the usb recording interface case (metal/ alloy) I get a light shock. I've heard of this with live mics if your guitar is poorly grounded or the venue has crap wiring. I've checked the grounds on the guitars and they are fine.

Can this just be down to:

Old House Wiring [it is an old house]?

Any ideas?

Stu

[GregL]

Hi Stu,

Could you please give a bit more detail about what you're doing - electric guitar or acoustic? Recording directly into the usb interface (guitar->cord->usb interface) or via a mike (guitar->mike->usb interface OR guitar->cord->amp->speaker->mike->usb interface)?

Thanks,
GregL.

[needsmorecowbel]

Sorry Greg I'm recording with these old matched mono "Sharp" Japanese Mics from the 60s/70s They plug into the usb interface with a 1/4 inch jack with no phantom power.

Electric Guitar> Cord> valve amp - Mic in front of valve amp > Interface > USB > Computer

The room is carpeted if that is of any importance...

Stu

[GregL]

Hi Stu,

If you are getting a shock, that means there's a voltage difference between you and what you touch. Static electricity (via the carpet) *might* be the problem. Can you "earth" yourself (eg touch the back of your hand to a bathroom tap) before you touch the computer?
My guess is that the mics are not the problem.
The grounds on your guitar might be fine, but do you know how good the grounds (and other electrical connections, for that matter) are on your amp?
You mentioned that it's an old house, has the house wiring been checked in the last 10 years?
How old is the computer?
Do you get a little shock from the computer when the guitar cable is not plugged in but everything else is the same (ie amp on, computer on, etc)?
(I'm not an expert, these are just some guesses)

Thanks,
GregL.

[Taffy Evans]

I got two big shocks off of our stainless draining board at the kitchen sink, and I wasn't even holding a guitar. Must be the weather.

[needsmorecowbel]

If I turn the amp off, hold the strings, and then touch the interface/ computer case I get a light shock. If I unplug the guitar lead from the amp but still have the amp on, just hold the guitar then I don't get a shock. I tested the guitars with a multimeter, there was some initial fluctuations but then it settled to 0.0 [should there be no initial fluctuations?] (on the Ohm setting) when putting the black lead to the grounds and then the red lead to the jack ground...

Stu

[GregL]

Hi Stu,

So many variables! Try to troubleshoot by eliminating one variable at a time (while taking apropriate precautions!).

As Taffy said, it could be the weather (static electricity) - however, I've heard that it's been moist in Melbourne recently, which would decrease the likelyhood of weather causing static buildup.

>If I turn the amp off, hold the strings, and then touch the interface/ computer case I get a light shock.

That sounds like an earthing problem to me. Most likely between amp and wall. Note that "problem" might just mean that their is an "earth loop" caused by different earthing techniques in the amp and computer;

> If I unplug the guitar lead from the amp but still have the amp on, just hold the guitar then I don't get a shock.

Adds weight to the "amp earthing" scenario. What happens if you plug a guitar lead into the amp, but not into the guitar, amp on, touch the guitar lead to the back of your hand while you touch the interface/computer?

> I tested the guitars with a multimeter

I'm sorry, I'm a bit confused as to how you are doing this - is the guitar plugged in, and you have bared the cable to access the black/red wires? Are you just testing the guitar earth?

Of course, the simple answer is "don't touch the interface while the amp is on"

Thanks,
GregL.

[needsmorecowbel]

Sorry Greg I'm particularly poor at explaining myself today as I didn't sleep very well. I tried your suggestion, amp on, lead on the back of my hand, still get a light zap. Sorry with the multimeter I was testing from the ground points [black wire] (bridge, grounds on pots) to the input jack ground [red cable]. Guitar was not plugged in. I really need to go over some basic multimeter stuff I'm sure there are tutorials on the anzlf somewhere.

Haven't been able to find out much about the mics. 1975 Sharp MDL-126, and MDL-127 Japan. One is faulty and just produces a really irritating hum, but the other is pretty good. They were destined for the bin, but managed to save them before they got turfed.

Here's a sound sample (no post effects except some volume tweaking)


youtu.be/

Stu

[GregL]

Hi Stu,

>testing from the ground points [black wire] (bridge, grounds on pots) to the input jack ground [red cable]

In that case, it would seem that guitar ground is probably OK.

> amp on, lead on the back of my hand, still get a light zap.

Then it would seem that the issue is more likely to be amp-related, not guitar-related. The issue *might* be that there is a (small) voltage between "earth" on the interface/computer and "earth" on the amp - an earthing issue.
Are you using the Hughes&Kettner Statesman amp? Does the amp have a "ground flip" switch? If so, use it and see if that makes a difference. NOTE: DO NOT DISCONNECT EARTH!
(Of course it might be some other reason! I'm just having a "slightly educated" guess.)

Is the issue *really* bad? If not (if it's just annoying) it might be easier to just avoid touching the interface/computer when recording. If it is really bad, maybe you need to get the house wiring checked by a qualified electrician?

The old Sharp mic sounds good!

Thanks,
GregL.

[simso]

You need to buy a small voltage detector, this will identify where you have a leakage.

Is your amp a two pin or three pin amp, if two pin then theres your answer, if three pin, is the earth stake on your house still connected.

Do you have your amp leads laying across power cords etc, the induced voltage will be enough to do it as well.

Lots of things can cause it, and it can be incredibly deadly too.

Steve

[jeffhigh]

Are both devices plugged into the same outlet, they should be?
With devices unplugged
Check the amp for continuity of earth from the plug ground pin to the chassis and to the sleeve on the input jack(easiest way is to plug a metal bodied plug in and measure to the metal body)
Do the same on the computer
With both on
Check for voltage between computer chassis and amp chassis with multimeter rather than your body.

[jeffhigh]

duplicate

[needsmorecowbel]

Thanks for the replies guys. It's more of an annoying thing but as Steve points out it can be very dangerous. (I'm not planning on doing anything drastic about it other than talk to my electrician and seeing what he things or doing some multimeter troubleshooting with the power off (taking into account residual charges as well)

Greg - Don't think there is a ground flip switch (I looked at the manual as well), It's more that I have to alter the mic volume from the interface so I have to inadvertently touch the metal casing of the interface.

Steve - It is three pronged, Will check the earth stake, The power cables at no time cross over the guitar leads

Jeff - Forgive my ignorance Jeff but is there a reason why they should be plugged into the same outlet? Does this create some kind of voltage/ phase difference? If so then that is probably the issue.

Stu

[jeffhigh]

Makes sure they are both grounded to the same point

[needsmorecowbel]

Thanks Jeff, I feel like an idiot.

[needsmorecowbel]

such a noob

[dshaker]

I'm not an electrician by any means, but I am remodeling my house and I just read a book on wiring.
My amateur opinion is that you have two problems.

1) There is something wrong with the wiring in your amp. One of the wires somewhere in there has electrical contact with the chassis and that needs to be fixed. If you don't feel qualified to do it, take it to a repair shop. Until then, don't use it.

2) Your computer and your amp are on different circuits and the different circuits have different paths to ground (a.k.a. earth) probably because they are connected to different panels, though possibly because one circuit is not properly grounded. If it was me, I would go to the hardware store and get a ground-testing device and see if the tester thinks each outlet is properly grounded. If one isn't, then it's call the electrician time, because badly grounded circuits are recipes for disaster: shock or fire.
If both circuits show up as well grounded, then it's probably just that they are grounded through different paths and the different paths have different non-zero resistance.

Again, I'm not an electrician, but as far as I can tell, that's what is going on. It's just an amateur guess, but if I was in your house, I'd be unplugging the amp.

[needsmorecowbel]

Cheers Doug, Jeff's idea was on the money. As I had the amp and computer plugged into a different outlet on the other side of the room it was grounding to two different points. I did this because I wanted to spread the load (didn't realise that was potentially dangerous). Having said that I'll still get the electrician to check the grounds or get one of those plug in testers.

Is there anything else like this that I should know to plug into the same outlet so I don't do the same?

Stu

[kiwigeo]

such a noob

Better being a live noob than a dead noob

I'm no expert either but if all outlets in your house aren't earthed to the same point then I'd say there would be serious doubts as to how effective your earth leakage breaker is going to be.