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
More Parts. Less Waiting.
; Semiconductors
; Capacitors, resistors
; Heat shrink tubing,
solder products
; Chemicals, much more
High Quality NTE and ECG Products
; No minimum order
; Cross-referencing for
thousands of parts
;;Prepaid ground shipping
on orders over $50