[Elecraft] [K3] Shorting K3 PHONE jack outputs (WAS OT: Stereo to mono adapters)

[Paul Christensen]

James,

You just discovered that some mono phase cancellation will occur when 
combining two audio channels with AFX engaged.  That's normal.  The effect 
is the same as when two channels on an analog tape deck are combined, and 
those two channels are not phase coherent.  If the tape heads are not 
aligned to within a small degree, phase cancellation occurs beginning with 
the high frequencies, then begins to comb-filter with greater misalignment.

I've not done this, but it would be interesting (and cool) to look at the 
K3's headphone audio into a scope using the Lissajous method.

http://www.jmargolin.com/mtest/LJfigs.htm

Left ear channel to the vertical Y input, right channel to the horizontal X 
input.  With AFX disengaged, there should be a straight 45 degree line on 
the display.  Then, with AFX engaged, what will be seen is a fuzzy oblong 
ball that reflects the deliberate phase change (the reason for AFX!) between 
left and right channels.  Any deviation from a straight line shows the 
degree of phase change.  The reason for the 45 degree line is that equal 
amplitude/phase from left and right inputs places the line between both X 
and Y axis.  If only left channel audio is monitored, you get a straight 
vertical line.  Right channel only?  A flat, horizontal line is shown.

Looking at the K3 schematic, when both headphone channels combine into your 
22 ohm ANEM source Z, each channel sees about 14 ohms.  I Thevanized the 
inputs and outputs and got that value.  What's interesting is that with AFX 
engaged, one headphone amp channel may be going negative while the other is 
going positive.  Also, there's little isolation between outputs with the 8 
ohm resistors separating channels.  I doubt this is a concern, but the 
National Semiconductor datasheet doesn't address instability concerns with 
resistively combined output channels.  Myself, I see no problem with it as 
you have it configured.

Paul, W9AC