[Elecraft] Is K3/0 Audio pass through or processed in K3/0?

[W1AN]

John, I've done some further experimentation. My units are the full size
K3/0s, so it's unlikely any low pass filtering is in place. I checked the
continuity between pin 1 of the 8 pin front mic connector and the tip
connection on the rear mic connector and found they are tied together
measuring less than 10 ohms. The very fine traces on the circuit board might
account for this resistance.  Sadly no schematic or board layout is
available to confirm. The PTT from pin 2 on the 8 pin mic connector also
checks to be directly connected to the rear PTT RCA jack. So this tells us
the "local" K3/0 settings do nothing to change the mic audio. I also
determined the problem is a ground loop where noise from the RRC control
unit is being picked up by the interconnecting mic leads. After adding
further ground loop isolation directly at the rear 1/8 mic jack with a
600/600 ohm transformer removes all the noise. This might have been my final
solution but you cannot pass DC mic bias through a transformer if you need
it.

But next the real answer!

Close inspection of the "factory" AUX/MIC cable from Remoterig shows no
conformity to the schematic provided in the manual! The cable provided is a
lengthy CAT5/6 4 pair cable with short breakouts at the end with the PTT RCA
connector and 1/8" mic connector. This factory cable is unshielded. Wires
used for mic audio and mic return are from separate pairs, not from one
twisted pair. Consequently, shielding is close to non-existent. PTT wiring
is similar. Making up my own cable with longer shielded wires connected to a
very short piece of CAT 5/6 at the RJ45 works much better! Leaving a long
length of exposed mic cable is not good practice. This may work OK with high
level signals but is a no-no for low level mic signals. A shielded piece of
CAT 5/6 might also work.

73, John W1AN



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