[RFI] RFI In My Shack

[Jim Brown]

On 12/2/2017 9:25 AM, Joe wrote:
> A little different question.  Perhaps some suggestions here:
>
> I have a new amplifier in the shack, a Drake L7, that is being used in place
> of a European version of the Henry 2K D Classic.

The description of your system sounds like a recipe for RFI. I've done 
three tutorials on the topic. They are at k9yc.com.  The most 
theoretical on is k9yc.com/RFI-Ham.pdf   BTW -- I tore up a lovely 18-in 
JBL sub-woofer about 15 ft from a dipole. It was in shreds before I 
realized it.

Several highlights.

AC line filters are a complete waste of money. They only work on 
differential mode RFI (even though they say they do common mode) and 
nearly all RFI induced on wiring is common mode.  SO -- Ferrite common 
mode chokes are the solution that works.

Poorly balanced antennas are a MAJOR cause of RF in the shack and the 
house.

Almost any antenna fed with 2-wire line will put common mode current on 
it, and off-center fed antennas are the worst. The solution is a serious 
common mode choke at the feedpoint, BUT off-center fed antennas put HUGE 
common mode on the line, so any decent choke will FRY.

Any antenna that WORKS will put RF current on any conductors near it. We 
may call them speaker cables or audio cables, but Mother Nature calls 
them ANTENNAS. Nearly all equipment is built with a design defect called 
"The Pin One Problem," which causes any current flowing on cable shields 
to get into the equipment. If it's AF (power line fundamental and 
harmonics) we hear it as hum and buzz; if it's RF, it gets detected and 
amplified.

The solution is pretty simple -- start with proper bonding of all 
equipment in the shack and any home entertainment setup. Add to that 
multi-turn ferrite chokes on all wiring coming into those systems, and 
if you still have problems, chokes on interconnecting wiring. Study this 
set of slides for a talk I've done for several ham clubs and at 
Pacificon and the Visalia DX convention. It has been a large part of the 
basis for N0AX's new ARRL book on the topic.

http://k9yc.com/GroundingAndAudio.pdf

Another mechanism couples RF from speaker cables to power amps. All 
audio amplifiers have feedback around every gain stage to reduce 
distortion and noise. The feedback network around the output stage of an 
audio power amp couples any RF on the speaker cable back to the input, 
where it is rectified and amplified.  Two solutions for this. First, 
replace all zip cord used as speaker cable, glorified or plain, with 
twisted pair. Twisted pair has been used for more than a century to 
resist crosstalk, and is about 30 dB better than zip cord in rejecting 
differential mode pickup.  Second, if you still have a problem, put 
serious ferrite chokes on the speaker cable.

As to why you have problems with one amp and not the other, I'd suspect 
issues with bonding, or some combination of monkeys and typewriters 
writing Shakespeare.

73, Jim K9YC