[Elecraft] Digital Mode Interface for the KX3 for WSJT modes

[Jim Brown]

On 7/11/2012 9:48 AM, Lance Collister, W7GJ wrote:
> I am planning to connect the computer LINE OUT to the tip and sleeve of the KX3 MIC
> jack.  However, in K9YC's interface manual, he suggests, putting a 4700 ohm resistor
> in series with the tip, and putting a 470 ohm resistor from tip to sleeve.  Has
> anyone tried this yet on digital modes with the KX3?  Does this provide enough
> isolation and prevent hum?

Hi Lance,

A clarification.  The 4.7K/470 ohm network is a simple 20dB pad (voltage 
divider), and almost any values in this range and with this ratio will 
work just fine. Use what's in your junk box.

Hum and buzz are caused by leakage currents from the AC mains, which 
create IR drops in the green wires and other conductors that connect 
equipment to "ground." Those IR drops are DIFFERENT for each piece of 
gear because the resistance of the grounding path is different, and, 
because the connection between gear is unbalanced (that is, one side is 
connected to the chassis at each end), that DIFFERENCE is added to the 
signal.  The leakage currents are 60 Hz and the harmonics of 60 Hz, and 
they sound more like buzz (the harmonics) that hum (pure 60 Hz).

The solution is VERY simple, but because we have for decades used the 
wrong words -- "ground loop" -- to describe the problem, is almost 
always overlooked.

That simple solution comes in two steps. 1) Get all power for 
interconnected equipment from the same AC outlet, or from outlets that 
share the same "green wire" (that is, they are in the same steel 
backbox), or from outlets whose green wires are bonded together by short 
fat copper.  2) Bond from chassis to chassis of all interconnected 
equipment with short, fat copper.

Be sure to complete all this by the long established good engineering 
practice for safety and lightning protection -- bond from that 
collection of chassis connections to everything else in your building 
that's grounded.  That tends to help minimize the presence of leakage 
current on the wiring of our equipment.

73, Jim K9YC