[Elecraft] K3 RF Feedback Problem

[Monty Shultes]

I bit the bullet and removed L4 and L7 this morning.  It is not difficult; 
they are both on the back of the front panel board.  I jumpered the pads 
with wire.  I am now getting superb audio reports from critical local 
stations that were guiding me in reducing RF on my audio.  My MC-60 mic is 
back in good graces.

It works.

Monty  K2DLJ

> Even in balanced audio systems, the same rules apply.  Had the K3 been 
> designed with a truly balanced, 3-stage instrumentation input for its mic 
> pre-amp, the inclusion of L4 and L7 on the shielded return paths would 
> have the same effect.  The saving grace in an instrumentation-input 
> circuit (or in the alternative, an audio transformer input) is the 
> inherently large common-mode rejection ratio (CMRR) across a very broad 
> frequency span that limits the presence of RF on a twisted-pair audio 
> line, even in the total absence of the cable shielding.  For nearly 100 
> years, the Bell System and its progeny have used unshielded twisted-pair 
> balanced audio systems in the presence of outrageously-high RF fields with 
> no measurable detriment to performance in many instances.
>
> Paul, W9AC