PA Question

[Dax196]

Hi Guys
I seem to have an issue with voltage coming back from a QTX 15 pa I was getting a lot of hum (I have a pair and confirmed it's the same on both) so I purchased a Di box so far do good the box has a hi z input 1/4 and a low z xlr output it has a ground lift feature my issue is when I plug in my computer sound card id14 I get a tingle/shock off the xlr input and the screw holding the xlr socket I measured the voltage and it's 112v if the box is in ground lift mode it's fine but the hum returns, it's completely silent when the ground is set to normal but that's when you get a tingle, I'm new to pa and have heard of 100v lines but this doesn't seem right to me. I have checked the earth's on the equipment and tried a second computer running a different sound card interface with the same results. Hope someone can pleaee point me in the right direction at the least I don't want to damage any equipment and I'm quite attached to my life also. Thanks in advance D

 

[Avo Mk8]

Your PA system has the usual IEC 3-pin mains input connector, and I'm willing to bet there is a traditional mains filter inside the unit behind that. A mains EMI filter typically has capacitors between L and E, and N and E, the net effect of which, if you disconnect the earth wire externally, is to give you approx half mains voltage on the ground of the PA and all the signal input grounds. It is the reason you get a tingle, but there's not enough current flowing to give a harmful shock.
So that is a feature built into the design of the equipment, and is not a fault.

As far as hum is concerned you seem to have an earth loop problem. I suggest you continue experimenting, but lifting ground on the PA will always give you the 112V!

The "Y" capacitors below are responsible:
(And in your case the CY 'centre tap" is going to the chassis of your PA!)

 

[Dax196]

Your PA system has the usual IEC 3-pin mains input connector, and I'm willing to bet there is a traditional mains filter inside the unit behind that. A mains EMI filter typically has capacitors between L and E, and N and E, the net effect of which, if you disconnect the earth wire externally, is to give you approx half mains voltage on the ground of the PA and all the signal input grounds. It is the reason you get a tingle, but there's not enough current flowing to give a harmful shock.
So that is a feature built into the design of the equipment, and is not a fault.

As far as hum is concerned you seem to have an earth loop problem. I suggest you continue experimenting, but lifting ground on the PA will always give you the 112V!

Thank you so much I have spent hours and hours looking for this answer, appreciate your time and comprehensive reply. I will look further into the hum but at least I won't worry about it cheers.