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 _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
