[RFI] RFI proof speakers

Rob Atkinson, K5UJ k5uj at hotmail.com
Wed Nov 1 18:33:22 EST 2006

>From what I have heard, Tom Rauch is absolutely right.  Here's what an AM 
station engineer told me this afternoon:

"You can solder a small cap (.001 to .005) across the audio chip's input pin
to gnd, right at the chip. That usually does it."

i.e. open the enclosure and get the amp board out where you can work on it 
and either find the  audio amp's input lead with a scope or the chip's spec 
sheet pin diagram.   it might be a bit tricky but get a cap from that pin to 
gnd as close to the chip as possible without frying it.

the problem is that you don't have the problem; ur neighbor does?  someone 
else I think.  so you may have to fix a second set of speakers up and give 
them up instead of risking the trashing of the neighbor's property.

I have to confess I have not tried this--my problem was in my shack so I had 
the luxury of simply turning off the speakers.  I'd like to know how this 
fix works out if you try it.


rob / k5uj

From: "Tom Rauch" <w8ji at contesting.com>
Reply-To: Tom Rauch <w8ji at contesting.com>
To: <rfi at contesting.com>, <k3lr at k3lr.com>
Subject: Re: [RFI] RFI proof speakers
Date: Wed, 1 Nov 2006 15:53:15 -0500

 > At 11:17 PM -0600 10/31/06, Tim Duffy K3LR wrote:
 >  I really need to find a solution for 40 and 15 meter RFI
 > (caused by my near
 >  field RF) that gets into my neighbors computer speakers.
 > I have asked this
 >  before, but I need to find a good set that is close to RF
 > proof. I need to get
 > them fixed once and for all and a new set of RFI proof
 > speakers is the way to
 > go.
 > I can't believe that these speakers can't be fixed by
 > putting _good_ ferrite
 > common-mode chokes on _all_ of their cables (perhaps as
 > many as five cables: one
 > for
 > the right speaker, one for the left speaker, one for the
 > subwoofer, one for the
 > wallwart, and one for the USB or audio cable to the
 > computer)


The idea a bead or even a box of beads is always a major
step just isn't accurate.

A bead adds a series common mode impedance on the line, but
we have no idea how much SHUNT impedance there is or what
the common mode impedance is at that point in the system. If
the common mode impedance is high already either on the line
or on the sensitive device port, then adding a bead of any
reasonable impedance can be almost useless. The bead also
does nothing for differential impedance.

By far the single most effective thing anyone can do on
anything like this is connect the inputs, outputs, and power
port to a common point. If dc or low frequency isolation is
required, then the common connection can be through an
appropriately sized capacitor.

When working on TVI problems a common connection like this
between  the antenna cable shield and the power mains safety
ground is significantly more effective than anything else
that can be done.

For differential problems (which are usually less common), a
bypass capacitor works very well on audio lines.

Plastic case or not I can make almost any computer speaker
system settle right down, but not by just throwing beads
into the system.

73 Tom

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