[LAU] ground loop hell

[Len Ovens]

On Sun, 8 Feb 2015, David Christensen wrote:

> Installing a series resistor in the common/ reference conductor of an 
> unbalanced audio cable subject to a ground loop will reduce the ground-loop 
> current in the common conductor, but will defeat its purpose of equalizing 
> the reference potential at both ends.  Thus, increasing the noise voltage.

All I know is I put 10ohms in series with the ground line and the hum got 
much worse.

> Running extension cords so that all the equipment is powered by one supply 
> can work (if the total load is within the capacity of the supply), but a 
> ground fault at outlying equipment can create an electric shock hazard.

> You want balanced outputs going through balanced cables to balanced inputs.

I want yes :)  Right now I don't have. The snake is old and the monitor 
returns are signal and ground  :P

> If there is ground loop, add balanced input and output isolation 
> transformers.  If your monitors have unbalanced inputs, pull balanced cable 
> up to the monitor and then connect it per the manufacturer's recommendations. 
> (Out of curiosity, what is the make/ model of the monitors?)

All of our balanced stage equipment uses a DI with isolation transformer. 
The monitors are an old peavy, that is so old I can't even find mention of 
it on the peavy site. (I had the model written down but have lost it) It 
is from the dawn of powered monitors and as such expects a speaker level 
in (possibly two if there are stereo lines). We use it for keys as the 
keyboard has no other self monitor. The keyboard is floating wrt ground as 
it is powered by a wallwart. The kb players has a box I built that allows 
level setting of keyboard and monitor mix without touching the actual 
keyboard levels so the FOH gets an unchanging kb level.

The other powered "monitor" is actually a Roland keyboard monitor (KC-60) 
on a tilt stand used by the band leader. It actually works very well for 
this purpose and has great sound. It was bought used ($75 it says on the 
back... I have it home to repair the ground on the cord :P ) and one of 
the three chanels has issues. I am just noticing that it does have a 
ballanced mic level input, so I could use a standard DI for isolation in 
future if I need to.

I am not afraid to spend money, but I would like to put it to the best use 
as it is limited. Fixed grounding comes before new equipment in my book.

> You are also correct that the best solution is to fix the electrical power 
> system.  This means hiring a competent electrical engineer and electrical 
> contractor to ensure proper grounding, voltages, phases, and equipment/ load 
> separation throughout the facility/ site.  Isolated ground techniques might 
> be in order.

It looks like this will happen. There have been two very large breaker 
pannels installed already and there are many inspections to go through 
ahead. I am going to set up a plan for power, signal and data for audio. I 
will do a second for stage lighting as well, if for no other reason than 
keeping the audio power from being used for lights  :)  If I need more 
than one breaker for audio power, I will make sure it is on the same 
phase.


--
Len Ovens
www.ovenwerks.net