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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
