getting shocked by Tiny Giant?

[bluebunny]

I was gigging at an open mic last night, playing geetar through my Tiny Giant.  Mic was through a separate PA.  And I was getting shocks from the mic (my mic technique is very rock'n'roll - kiss, kiss!).  This wasn't happening when I switched instruments to a uke direct into the PA.  Nor indeed when I played my acoustic through a small Ashdown combo.

My thoughts were these: the TG is powered by a laptop PSU, which isn't earthed (double-insulated and no earth pin connected to the mains), so any voltages inside are floating at some arbitrary point in relation to the real dirt earth that the PA was connected to.  So there could be some arbitrary potential difference between the two so-called "grounds".  Does this sound right?  Is there anything I can do about it?  Apart from a foam pop shield, of course... 

[R.G.]

My thoughts were these: the TG is powered by a laptop PSU, which isn't earthed (double-insulated and no earth pin connected to the mains), so any voltages inside are floating at some arbitrary point in relation to the real dirt earth that the PA was connected to.  So there could be some arbitrary potential difference between the two so-called "grounds".  Does this sound right?  Is there anything I can do about it?  Apart from a foam pop shield, of course... 

You're right about the laptop power being the problem. It's powered from the AC power line, and since it's not earthed/grounded by a third-wire ground, there is always capacitive leakage. It's likely that since you're not dead now, it's not an internal breakdown of an isolation barrier, so it's probably the inevitable capacitance letting a tiny amount of the mains power through.

A really good cure for this is to run a ground conductor to the real AC mains earth/ground from your amplifier's signal ground. That is the function you were providing by kissing the microphone, and subbing in a copper wire for your body would keep the voltage difference so low you wouldn't get shocked.

Or, there's that foam pop shield. 

[PRR]

The situation inside many flaptop supplies is two 0.005uFd caps from line to DC Out Ground.

Not just stray capacitance-- these are filters for the digital crap that comes out of a laptop and might get into your TV.

This gives about 275K reactance to (assuming 120V) 60V, or 0.22mA of leakage current (0.4mA for 230V lines).

60V can sting. 0.22mA even 0.4mA through body is not fatal, but on wet lips (through neck and shoulders to sweaty fingers) sure will sting!

It would be best to re-wire the whole magilla so (as R.G. says) your amplifier chassis has a real wire to the wall-outlet ground. "Problem" is that with a laptop supply this is a real mess. Perhaps the "best" fix is to put the supply "in" the amplifier and use a 3-wire (3-pin) power cable to the wall. However not-messing with wall power is a key advantage of using the laptop supply.

A few flaptops ran true 3-pin wall cords. My antique Dell Inspiron has that. I ass*ume the 3rd pin connects to the DC out.

[mth5044]

Is there any way to measure if the third pin of a laptop supply does what we want it to do (without the 'let me get wet and sing and see if I die' technique)?

[bluebunny]

Thanks for the advice (and the explanations) guys.  Yep, "sting" is just about spot-on.    Perhaps I might rig up one of those ESD mains plug things - nothing but a 1M resistor to ground - and clamp that to the TG chassis?

Edit: Ah ha!  "Earth bonding plugs", apparently:


Anyone foresee any issues with such a setup?

Edit #2: @mth5044: just saw your post as I was typing.  The third pin is plastic for double-insulated not-earthed stuff over here in the UK.

[PRR]

> nothing but a 1M resistor to ground

Will do very little. The caps leak AC like 0.5megs. Another 1Meg gives a very slight reduction.

> any way to measure if the third pin

With wart UN-plugged, measure Ohms from wall ground pin/contacts to DC output. Should be 1 Ohm or less.

My Inspiron has a wicked 3-pin DC plug which I can not conveniently probe. But I do have the laptop. Measuring from wall ground pin (NOT in wall) to the flaptop's RS232/VGA connector lugs, I get 1.1 Ohms. (Measuring with power live I get varying readings, some impossible, due to DC leakage confusing the ohmmeter.)