8 Feb
2015
8 Feb
'15
11:57 p.m.
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