Nuts and Volts - November 2014

November 2014 9

QUESTIONS and ANSWERS

buy a new manufactured device than to try to repair an old one. Be sure to recycle the old device with your local community recycle system rather than sending it to the landfill via the garbage system. Die-hard electronics buffs can salvage parts from old VCRs (gears for robotics, capacitors, or even entire power supplies) which is the ultimate form of recycling: REUSE.

Sound System Hum

QOur local theater group has me as its sound technician, even though I have no experience in this area. Our sound system consists of a main sound board, a monitor amplifier, house speakers, monitor speakers for the stage, and a series of wired and wireless microphones. The problem we are having is a 60 Hz hum on all of the monitor speakers.

We are using shielded cables from the wired mic to the main board; from the main board to the monitor board; and from the main board to the speakers. We have tried replacing the cables from the wired mics to their plugs and installing power line filters on the two sound boards (replacing the house cables will be fairly expensive and time-consuming), but this has not helped. Can you give me any ideas on how to solve this problem?

— Bill W.

AThe 60 Hz signal originates from the power lines, but is not necessarily coming into your system directly from these. Since power filters on the incoming lines to the sound boards have not helped, this means the signal is not coming in directly which is a very rare occurrence. Unless you have shielded mic cables running close to a power line that is pulling heavy amperage, this hum is most likely not being introduced into the mic cables.

(Tip: Don't run low level signal cables near power lines, but if you must, run them perpendicular to the power lines instead of parallel to minimize picking up hum.) Since you have two line powered audio devices connected together (sound board and monitor amplifier), I suspect that you are experiencing a "ground loop."

A ground loop refers to a current in a conductor connecting two points in an electrical circuit that are supposed to be at the same voltage (ground, in this case) but, in reality, are at different voltages. Two signal ground points (main sound board amplifier output and monitor amplifier input) could have a small resistance between them so as to create a voltage drop due to magnetically coupled currents from the power lines in the signal ground conductor which causes the "ground current" to flow in the ground conductor which then causes a voltage variation in the input of a device such as the monitor amplifier.

In instrumentation signal lines, we were always told to disconnect the ground (shield) conductor at the load end

(monitor amplifier), but audio equipment will give a terrible level of hum (if you have had a broken shield connection on a guitar cable, you know this is true), so that is not a good idea. Placing a small resistor (around 10 ohms) between the ground of the cable and monitor amp plug may reduce the hum to acceptable levels without inducing the "broken ground" problems. My choice of solution is to insert an in-line isolation transformer into the audio line (in your case, at the monitor amp input). These devices fit right into the cable and amp input so there is no soldering or opening up the amp — just in case you are not an experienced electronics person (this is a not good time to start learning electronics).

These devices are $40 to $50, and you’ll need to specify the type of connector input and output needed (phone, XLR, RCA, etc.). The sound systems I have worked with have a 1/4" stereo phone plug on the audio cable and a 1/4" jack on the input to the monitor amp. So, I would purchase a 1/4" stereo phone jack input on the transformer with a 1/4” plug on the output. There are many vendors in this magazine who can supply the isolation transformer you need, or check out a local electronics store. NV