[RFI] RFI proof speakers

Tom Rauch w8ji at contesting.com
Wed Nov 1 15:53:15 EST 2006

> 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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