Topband: Mother of all ferrite common-mode coaxial chokes

[Paul Christensen]

> One of the dumbest things I've seen recently from a very good company is
> RF chokes in series with the shield connections on analog and RS232 I/O
> boards for the Elecraft K3.

Right.  I think Joe, W4TV, was one of the first to identify that problem 
back around 2008.  Many of us quickly jumpered the handful of RFCs used on 
shield leads of the RS232 I/O Board.  Until revised by the factory, that 
board was the Achilles Heel of the K3.

> Modern pro output stages have a very low source Z (typically
> 100 ohms for line level) and high input Z (typically 10K for line
> level), and consumer stuff is roughly 3-5X those values.

Somewhere in my files from the late '80s is a white paper authored by 
Richard Cabot.  I believe Richard was the chief designer of the Audio 
Precision brand of audio test equipment.  The focus of the article was on 
the standarding of all audio output stages, balanced or unbalanced to a 
value of 40-50 ohms.  He created models showing the effect of changing drive 
Z from 1-ohm through 600-ohms into long audio cables (and independent of 
terminating Z) that start to take on transmission line qualities.  His 
conclusion was that a target of just under 50 ohms was optimum.

Paul, W9AC