[LAU] ground loop hell

[David Christensen]

On 02/08/2015 09:06 AM, Len Ovens wrote:
> Beware old building with additions  :P  It appears we have power coming
> from two power entrances with two different earth grounds to the panels.
> I had found two power cords with the ground pin pulled which I
> replaced... big ground loop noise. I tried (as read from a number of
> sources) making unballanced cables with a small resitor in the ground
> path but this made things worse not better. I will be running a power
> cord from the stage back to the mixer next. All of our signal in paths
> are already isolated. It is only the two monitor mixes to powered
> monitors that are unballanced and causing problems. The outputs from the
> mixer are ballanced, but the lines to the stage are not... I may make
> cables to allow me to use two of the input ballanced lines instead and
> two more isolation boxes as a longer term fix. There will be a new stage
> going in and I have already been asking that power and grounding be
> corrected for the whole signal flow.

I've had similar experiences with a temporary public address system I 
oversee at an annual festival.  There is a mixer sending a balanced 
line-level signal to four powered loudspeakers around the site via ~750' 
of 3-conductor XLR cable (e.g. hot/+, cold/-, and ground).  The mixer 
and speakers are each connected to the nearest outdoor 120 VAC 
receptacle.   Several years ago, the owner added a new building with a 
new electrical service and new outdoor receptacles.  The owner might 
have also changed the feed to another existing building and it's outdoor 
receptacles. Since then, we've had audio problems and equipment failures 
(burned out speaker internal amplifiers).  I finally realized what was 
going on last year, when I put a Fluke DMM on all the receptacles and 
found that half the site was running at one voltage and the other half 
at another (~117 and ~120 VAC?).  I need to put an isolation transformer 
in the line at the boundary between the two systems (or a transformer at 
each speaker).


Electrical power extension cords without ground pins are a deadly 
hazard.  You are correct to replace them.


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.


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.  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?)


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.


HTH,

David