Electric shocks from laptop use and guitar...

Ché

I've recently set up a laptop in my room to record demos and such. 

I've noticed, when I touch my laptop casing (when it's plugged in) and touch the strings of my guitar (Godin Session plugged into a Blackstar HT1 combo) with the back of my hand I get an electric shock. This only happens on my forearms or back of my hand, not when I'm fretting strings... 

The guitar buzzes when not touching the strings normally so I'm assuming this means there's an issue with the grounding of the guitar?

What's strange is, with the guitar buzzing away, as soon as I touch the laptop the buzz gets louder. 

Not sure what's occurring here, if it's the guitar, amp or laptop that has an issue. 

Any help would be very much appreciated. 

Cheers,

Shaun

maltingsaudio

Get it all checked by a proper electrician, or at least go to your local DIY shop and get a mains tester plug. Sounds like a faulty earth either on your amplifier check the mains lead or the house mains .
https://www.screwfix.com/p/lap-ms6860d-socket-tester/91596?tc=UA7&ds_kid=92700022850517781&ds_rl=1241687&ds_rl=1245250&ds_rl=1244066&ds_rl=1249796&ds_rl=1245250&ds_rl=1249481&gclid=EAIaIQobChMIzOb9keKf6AIVj0TTCh3NwgDVEAQYAiABEgJnUPD_BwE&gclsrc=aw.ds

Ché

Thanks so much! Is there a way of checking the earth of the amplifier with the mains lead?

Danny1969

Is it a laptop with conductive finish on the palm rest ? if so then the problem is caused by a difference in the laptops virtual ground and the real ground of the amp 
It's generally so slight though that's it well within limits and not a safety issue as long as you are using a real charger as not some cheap Chinese knockoff from Amazon or Ebay 

Ché

Danny1969 said:

Is it a laptop with conductive finish on the palm rest ? if so then the problem is caused by a difference in the laptops virtual ground and the real ground of the amp 
It's generally so slight though that's it well within limits and not a safety issue as long as you are using a real charger as not some cheap Chinese knockoff from Amazon or Ebay 

It's a new MacBook Pro with Apple branded charger. The whole finish besides the screen is a metallic silver, guessing it could be that?

Danny1969

Yeah you get the same issue on Dell Latitudes and other laptops with conductive finish ... although annoying it's only normally felt from a really sensitive part on you body,..... I can feel it on my laptop if I touch the palm rest with the smooth underside of my wrist  but not with my hands 

prowla

Its an erf loop thingy.

If the amp and computer aren't plugged into the same mains socket then there will likely be a voltage differential between the computer case and the guitar earth.

Ché

prowla said:

Its an erf loop thingy.

If the amp and computer aren't plugged into the same mains socket then there will likely be a voltage differential between the computer case and the guitar earth.

They are plugged into the same socket. I made sure of that to eliminate the obvious source of ground issues.  

CeeJay

prowla said:

Its an erf loop thingy.

If the amp and computer aren't plugged into the same mains socket then there will likely be a voltage differential between the computer case and the guitar earth.

Differential between where?

springhead

prowla said:

Its an erf loop thingy.

If the amp and computer aren't plugged into the same mains socket then there will likely be a voltage differential between the computer case and the guitar earth.

Erf?  I don't think it's a loop, the laptop psu is likely class 2, i.e. unearthed.  Some of these switched mode supplies have EMI (Electro Magnetic Interference) suppressant capacitors between the low voltgae (output) and supply sides.  You can feel this if you are touching something that is actually earthed (your guitar strings) at the same time.  It's unnerving/annoying but unlikely to be dangerous (don't take my word for it if you're worried - get someone who understands switched mode psu's AND house wiring).  If possible you need to tie your laptop down to earth, maybe from a peripheral port if they have a connection to the laptop case.

Keefy

This sounds like the Switch Mode Power Supply on your laptop. The SMPS has a capacitor (sometimes called the Y capacitor) which goes between the mains input and the DC output. You can get up to about 90V from that, which can be pretty unpleasant, but the current available is insufficient to do actual harm. If you earth the DC the problem should disappear.

Ché

Keefy said:

This sounds like the Switch Mode Power Supply on your laptop. The SMPS has a capacitor (sometimes called the Y capacitor) which goes between the mains input and the DC output. You can get up to about 90V from that, which can be pretty unpleasant, but the current available is insufficient to do actual harm. If you earth the DC the problem should disappear.

It's my thinking that the laptop would be earthed through the power cable? I'll check when I get home tonight but granted Apple is a US product it wouldn't surprise me if it was a two pin cable with a plastic third prong.

Danny1969

On the older chargers some were grounded and some weren't .... although they looked like a 2 pin fig 8 cable there was also a metal stud that the plus slid over and that was the ground. Oddly the supplied extension cable used to have the earth but they stopped shipping that. On those older chargers the protective film covering the charger could go over the metal stud effectively insulating it so it was important all that crap was peeled off properly.

Yours will be a USB C charger though rather than a Magsafe which is a whole different ball game ... it’s a very complex system of voltage and current hand shaking ... it does have a ground but it’s not always connected to mains ground .  Often the only thing between the secondary side of the switch mode power supply and the primary is the Opto isolators So you’re not in any danger it’s just annoying sometimes

ICBM

The laptop isn't earthed. This is quite normal with many laptops and their power supplies, including Apple ones.

guitartango

Are you sure it is not static ?  I get this sometimes on our carpet at home. 

Ché

guitartango said:

Are you sure it is not static ?  I get this sometimes on our carpet at home. 

Definitely not static. It doesn't happen when the laptop is unplugged. As long as it's not fatal I'll just keep my sleeves rolled down when working on the computer with the guitar on my lap.