[RFI] RFI to a Home Theater System

[Jim Brown]

On Fri, 12 Dec 2008 09:16:49 -0600, Ed Richardson wrote:

>Contacted Polk Engineering and their initial solution was to 
>install a high pass filter from the signal input to ground. What 
>ground, there is no shield or even safety ground on this item.

Send the following link to Polk Audio's "engineers." You should 
tell them that the author (me) is a Fellow of the AES, Chair of 
the AES Technical Committee on EMC, and Vice-Chair of the AES 
Standards Committee Working Group on EMC.

http://audiosystemsgroup.com/RFI-Ham.pdf

Much of the RF comes in on speaker wiring. The rest of it is due 
to lousy shielding. It also comes from pin 1 problems and lousy 
filtering of the signal wiring. I've seen a LOT loudspeakers with 
built-in power amps. I've only seen one (a Genelec) that didn't 
have serious RFI problems.

Several specific recommendations: 

1) First, if any of the wiring is zip cord (parallel wires), 
replace it with #12 twisted pair. This often helps a lot. If your 
neighbor has a problem with #12 copper, refer him the the classic 
AES Paper by Prof. R.A. Greiner on the subject of Loudspeaker 
wiring. He's now retired, but at the time was prof in the EE dept 
of U Wis. 

2) To kill RFI coupled on the loudspeaker wiring, you need chokes 
tuned to the frequency of the interference. Follow the 
instructions in my tutorial. A few clampons do NOTHING at HF. The 
WILL help on 2M. 

3) The only cures for shielding are the bucket or returning the 
product to the mfr as defective. It IS defective if it picks up 
radio transmissions. It's not supposed to be a radio receiver, but 
it is. 

4) RFI is quite frequency sensitive. You can use a VHF/UHF talkie 
as an injection probe to learn some things about how stuff is 
being coupled at those frequencies, but very different mechanisms 
may be at play at HF.  

This summer, I fixed a similar RFI problem to a home theater 
system using five or six of the "big clamp-ons", one on each 
loudspeaker line. The ham is a serious contester, running legal 
power on a small city lot, with antennas within about 30 ft of his 
neighbor's theater system. 

Emphasize to Polk Audio that this is THEIR problem, NOT the ham's 
problem. It's a result of THEIR design errors and compromises. 

73,

Jim Brown K9YC