[RFI] RFI to a Home Theater System

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Fri Feb 27 12:22:18 PST 2009


One comment regarding the Line Conditioner:  Even if the unit actually 
provides 75 dB common mode rejection at 1 MHz, that all goes away as a 
useless expense if you do not preserve the "cleaned up line" between the 
Line Conditioner and the sub-woofer.  In order to keep things clean, the 
sub should be connected directly to the conditioner by a shielded power 
cord, made as short as possible, with BOTH ends of the shield tied to 
earth ground (meaning that the Line Conditioner and the sub would have to 
have a common chassis ground).  Since these are conditions that are not 
likely to occur, it would be a lot more useful to put the common mode 
filtering directly at the power input of the sub. 

One more thing:  not only does it sound as if the power line needs common 
mode filtering, but the audio lines feeding signal to the sub should also 
be fitted with common mode filtering or you may not notice any useful 
effects at all.  Good luck.

Since the victim (sub) is not yours, it is obvious that your choices are 
limited.  Keep in mind that there are 2 other ways to help resolve the 
problem that you do control:  distance from your antenna (relocate antenna 
or use different type of antenna) and power applied to the antenna.  Short 
of buying your neighbor a sub that has better RFI characteristics, those 
two factors are about the only other way to mitigate the problem if the 
applied filters are not enough.  From what you say about 75 watts being a 
trigger point, and assuming that you wish to use 1 kW to 1.5 kW, you have 
a roughly 13 dB problem.  That should be doable, but applying that much 
filtering/reduction at the sub may entail  some "ugly" fixes on the back 
of the unit.

73, Dale


Ed Richardson <ed_richardson at shaw.ca> 
Sent by: rfi-bounces at contesting.com
02/27/2009 01:09 PM

rfi at contesting.com

Re: [RFI] RFI to a Home Theater System

Well this problem does not appear to be going away any time soon. As a 
recap, RFI to neighbor's Polk Audio subwoofer on all bands with any more 
than about 75 watts. Separation is 75-100' from antennas.

Neighbor is not very cooperative but has
granted me a small window to install some filters after he returns from a
Mexican vacation. I want to make optimal use of this time so I want to 
as much as possible ahead of time. I have read Jim Brown’s article as well 
as W1HIS’s
web article.  I have
armed myself with 100 of the 2.4”OD mix 31 rings as well as 30 of the big 
on units.

 I am also
considering bringing over a Tripplite Line conditioner, that supposedly 
has a 75
db of common mode rejection at 1MHz on the AC line.This sub
woofer is so sensitive, that it howls when a 5 watt UHF portable is keyed 
up in
the same room. The engineers at Polk Audio have been able to reproduce the 
in their lab and short of placing the amp in a Faraday cage, they have no
suggestions. (Jim Brown I have passed along your name and article to 
them-thanks for the offer)

What methods have others found most useful? Using binocular configurations 
or simple wound torroids with 10-14 turns? Are we looking at multiple 
fiter types to solve the problem?
In this case, the audio input to the sub is via a shielded coaxial cable 
(The higest quality/cost bestest in the world, Monster cable). Removing 
this cable from the sub does not correct/improve the problem. This makes 
me think that the RFI is coming in via the AC line or directly through the 
non shielded case.
Would differntial mode filters likely also be required?
 Any other hints
on placement of filters and the configurations you found most useful would 
be appreciated/


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