Hello brundle,
Thanks for the question.
What you are experiencing is commonly called RF interference. This
commonly occurs on wireless devices such as radios, wireless
microphones, cell phones and TV's, but in your case, if you have no
wireless devices in your system, then something is "rectifying" the
broadcast signal and allowing it to enter your audio chain. Some
people, you may have heard, can even pick up radio stations through
fillings in their teeth. What you are experiencing is similar.
Sometimes, the solution is a simple as moving the location of the
equipment, or moving cables so that they do not run parallel, but
perpendicular to each other. Sometimes it is next to impossible to
track down the source of the problem.
Having built a number of recording studios, my first reactions are
two-fold:
1. Grounding and
2. Ferrite beads.
It is always important to have a good grounding scheme, particulalry
when using unbalanced equipment, i.e., cables with a hot conductor and
a shield that is being used also for the ground. Balanced equipment,
such as microphones, for example, use three conductors and suffer less
from RF interference as they have a separate hot, negative and ground
wires. When using unbalanced, only one device, either the mixer or
pre-amp or sometimes power amp, should have three pronged AC
connections and the connections to that device through interconnect
cables provide "one path to ground".
I'll look for some good links to grounding in just a moment.
Ferrite beads are devices you might have seen on some computer gear,
for example. They appear on the cable and look like donuts that
surround a short length of the cable. These actually act to block some
RF.
Ok, I'll search first for eliminate RF interference:
Frequently Asked Questions About RF Shielding
http://www.lbagroup.com/associates/lbaafaq.htm
"Where does RF interference come from?
Radio frequency interference (RFI) can originate from a wide number of
sources from natural phenomena such as lightning and sun spots to a
host of man-made generators. Typically, those trying to guard from
interfering signals are concerned more with man-made sources. These
sources include high power broadcast systems like AM/FM radio and TV
transmitters and a multitude of other communications systems,
including 2-way radio, paging, mobile telephone, and emergency and
public safety communications systems. There are also unintentional
emitters that are sometimes a source of RFI. Power lines,
transformers, medical equipment, electro-mechanical switches and many
others produce RF energy as they perform their functions."
Although this page is targeted to wireless systems, there is some good
background information here.
Just in case you ARE using a wireless guitar transmitter/ receiver or
microphone, do read this page:
Audio Technica - Types Of Interference
http://www.audio-technica.com/using/wireless/advanced/types.html
As Imentioned, something is rectifying this RF signal and can even be
an oxidized connector on a piece of your gear. A product called
Stabilant 22 claims to help eliminate this possibility:
http://www.stabilant.com/appnt11h.htm
"RF interference can also be a problem in recording, especially with
the number of illegal RF power amplifiers being used with CB's. With
the passage of time, connectors often build up thin films that act as
crude rectifiers. This source of RF interference can often be
eliminated by using Stabilant 22."
Here is a page with very good, albeit technical, information on
ferrite beads:
Using Ferrite Beads to Keep RF Out Of TV Sets, Telephones, VCR's,
Burglar Alarms and Other Electronic Equipment
http://www.antennex.com/shack/Dec99/beads.htm
"There are several ways to tackle the problem but most of them involve
opening the affected equipment and adding suppressor capacitors,
filters, and other circuit modifications. Unfortunately there is a
serious disadvantage associated with this approach. Any modifications
made to domestic entertainment equipment can - and often are - blamed
for later problems that arise in it. Modifying your own equipment is
not so bad, but taking a soldering iron to your neighbor's stereo is
risky. An alternative approach is to use ferrite beads to reduce the
amount of RF entering the equipment. If the equipment is in a metal
box, or even if it's in a plastic box, if RF is prevented from
entering the box on the antenna lead, the power cable, the speaker
leads, the phono pickup leads, and on any other wires entering the
box, it is possible to solve the problem without any modification to
the equipment. Ferrite beads just slip over the wires and stop RF from
going in."
The page continues with VERY good information.
I was going to save this advice for last, but this page states the
case fairly well:
http://www.proguitar.de/TippsTricks/PickUpSchaltungen/RfInterference/RfInterference.html
RF INTERFERENCE OR YOUR EQUIPMENT TURNS INTO A RADIO
If you heard a radio station in your amp all of a sudden, you're not
alone. This pops up now and then. A cure in most cases is installing a
10 to 50pf mini capacitor across the output of the device causing the
racket. If you unplug one piece of gear at a time until the noise
stops, that is the bugger to fix; it happens to be sensitive to the
radio waves.
Take it apart and solder the cap across the output jack of the device.
Start at 10pf. and go up if the noise is still there. It may be your
guitar too. Solder a cap across the guitar output jack if necessary.
If the radio noise is coming from your amp, DO NOT SOLDER THE CAP TO
THE OUTPUTS OF YOUR AMP. Try gluing or stapling some thick tin foil to
the top or inside walls of the amp too.
If these tips don't help, get help from a good tech.
Indeed, if it is coming from either your guitar or guitar amp, the
above tips may help. And, you may have to take the offending device to
the audio doctor if you can isolate which piece is the culprit and can
not solve it yourself.
Searching ferrite beads for more information for you on this potential
fix:
http://www.palomar-engineers.com/Ferrite_Beads/ferrite_beads.html
And "How Stuff Works" at http://www.howstuffworks.com/question352.htm
has this to say:
"Question
Many of the cables attached to my computer have "bumps" on them near
the ends. What are they for?
Answer
In a typical computer system found in a home or office, you normally
see these "bumps" on the mouse, keyboard and monitor cables. You can
also find them on power supply wires when a device (like a printer or
scanner) uses an external transformer.
These "bumps" are called ferrite beads or sometimes ferrite chokes.
Their goal in life is to reduce EMI (Electro-Magnetic Interference)
and RFI (Radio-Frequency Interference).
A ferrite bead is simply a hollow bead or cylinder made of ferrite,
which is a semi-magnetic substance made from iron oxide (rust) alloyed
with other metals. It slips over the cable when the cable is made, or
it can be snapped around the cable in two pieces after the cable is
made. The bead is encased in plastic -- if you cut the plastic, all
that you would find inside is a black metal cylinder. "
And last, a page or two on good grounding techniques:
http://www.chebucto.ns.ca/~ak621/Lighting/Instruct/Ground.html
GROUNDING and BUZZ PROBLEMS
"...The most common cause of interference being picked up is a ground
loop. This is usually caused when the house audio console is plugged
into an AC outlet in the audience area and the power amplifiers are
plugged into power at the stage. Each is safety grounded through the
third prong on the plug. However, actual ground is not at these
outlets. The ground circuit must run through the building wiring to
the main electrical input at the service entrance where it is attached
to a cold water pipe or ground stake. Therefore, electrical resistance
is added because of the resistance in the building wiring which puts
the circuit shields at some potential above actual ground. There is
resistance in the ground line from each piece of equipment to their
common point and then there is resistance from that common point to
actual ground...
One solution to this problem is to disconnect the shield at one piece
of equipment; another is to lift one line cord's third prong from its
ground connection. Although, this second method will break the loop,
please do not do it - This grounding prong is required for safe use of
electrical equipment.
A better and safer solution is to attach all the audio equipment
ground prongs to the same point and then attach only that point to the
building ground. The building ground you attach to should be as close
to actual ground as possible..."
and...
http://www.trinitysoundcompany.com/grounding.html
and...
http://documents.epanorama.net/documents/groundloop/audiosignal_grounding.html
Audio signal grounding in audio equipment
And another good page "Sound System Interconnection" at
http://www.stagetec.co.uk/SOUNDCON.html
As you can see, there is no simple answer to your problem. And of the
solutions above may remove the interference and yet it may take a trip
to the repair technician is the above fails.
If you would like to learn more, search the following terms to find
many pages on the subject:
ferrite beads
audio system grounding
RF interference
Good luck solving this annoying problem. If anything above is unclear,
please do ask for clarification.
Regards,
-=clouseau=- |
