[Elecraft] K3 RF Feedback Problem

[Tom Hammond]

Hi Monty:

I just added a jumper around (well, actually over) my L4. No need to risk
possible PCB damage by completely removing the RFCs.

73,

Tom   N0SS

At 17:50 11/02/2008, Monty Shultes wrote:
>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
>
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