In article om,
wrote:
The tweeter of my powered event tr8 monitors are picking up radio. its
very quiet but in silence its annoying.
Technically, this is more properly called "undesired reception" - that
is, a device which should not be sensitive to RF, is
receiving/detecting RF.
I took one of my tr8's and moved it to different rooms and plugged in
the power with ground line connected and not connected, also with no
audio cable connected. I always got the same annoying radio signal
coming out of the tweeter, with the radio signal being the same volume
regardless on what the gain of the powered monitor was set to, since
the tr8's have a knob thats from -20db to 0db. The "shhhhhh" sound
which all speakers have would change volumes as the db knob turns, but
the RFI volume would not. whether the monitor was grounded or not, I
received the same results.. RFI!
Good test procedure! You've eliminated a lot of possible culprits.
I took the other other tr8 monitor since I have a pair and did the same
test and received the same results. I never opened these monitors. they
have never been dropped. I've had them for a little over 2 1/2 years.
maybe its just my ears but sometimes its louder than depending on
location. I really dont know what it is, and what I can do to prevent
it.
Your tests strongly suggest that the RF is being "detected" (converted
back into audio) in a relatively late stage in the amplification
circuitry... after the volume control. Odds are very good that it's
coming in through the power cable... possibly by direct pickup by that
one cable, or possibly by conduction from the house wiring.
The first thing I would suggest is adding a ferrite "choke" to the
power cable. These come in a variety of sizes and shapes and types...
toroids (donuts), cylinders, and rods.
There's probably already a ferrite RF choke on each of your monitor
cables (look for a cylindrical swelling of the cable, near the monitor
and/or near the plug which connects to the computer.
Probably the easiest thing for you to find, and try, is a "snap-on"
cylindrical ferrite bead. For examples, see
http://www.allelectronics.com/cgi-bi.../Ferrites.html
I suspect that Radio Shack may still carry beads of this sort (at a
rather higher price), although their online catalog / product index is
so useless I can't find 'em.
The right way to use one of these beads, in your application, is to
snap the bead around the power cord as close as possible to the
speaker cabinet. If you can, loop the cord around one half of the
bead (i.e. through the center and then around the outside) several
times before you snap it closed - the impedance of the resulting choke
goes up roughly proportional to the square of the number of turns, and
this will improve the RF blocking.
Since your speakers are known to be sensitive to RF pickup, I'd
suggest buying four beads - one for each monitor's power cord, and
another for the audio interconnect cable you plug into the monitor.
It's possible to do more extensive internal modifications to an
amplifier to reduce its RF sensitivity, but I think the chances are
good that snubbing the power and audio cables with ferrite beads will
eliminate the problem.
--
Dave Platt AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
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