Is K3/0 Audio pass through or processed in K3/0?

[W1AN]

Hello group, I've found minimal information on whether or not the K3/0 actively handles audio from the front 8 pin microphone jack before exiting the rear 1/8" connector feeding the mic input on the RRC Control unit. I can change settings for the K3/0 such as Bias, FP-H/L etc., when disconnected from the network but don't think the changes actually do anything. Is the front 8 pin mic connector hard wired "DC connected" to the rear 1/8" connector and simply a pass through? I know that when connected through to the remote K3 the settings are normally to the K3. I wasted several hours trying to eliminate some "buzz" audio noise when connecting my Heil headset through the 8 pin connector on the K3/0. I finally made up a connector to go from the Heil directly into the RRC control unit. The noise then was gone! So noise is being picked up inside the K3/0, a ground loop possibly? The noise is not 60 Hz, but varies in frequency with the Advances settings in Microbit for the Remote rig.

Ideally some minimal settings could be configurable from the K3/0 such as Bias ON or OFF for the local 8 pin mic input. But if the wiring is hardwired and simply pass thru where is the buzz coming from? Documentation and an internal schematic would greatly help!
73, John W1AN

[John K3TN]

John - I bought a K3/Mini last year and tried connected mic/headset to the front panel 8 pin circular connector and the front panel headphone jack. I had hum on xmit audio and very low receive audio.

I sent it back to Elecraft, they sent it back to me saying it was working - to eliminate digital noise getting on the audio, later K3/Minis have a big honking low pass filter in the audio chain between the rear panel connector and the front panel connectors - that introduces a lot of loss. Their recommended solution was to turn up the audio level in the RRC, but I'm never in favor of amplifying then attenuating. This also would not solve the TX hum/distortion, which the Elecraft tech didn't hear.

So, I just connected mic/headset directly to the RRC. Makes the cabling ugly but the audio is FB now.

If you have an older Mini (I don't know the serial number cutoff) you might have the noisy audio board. If you have a later Mini, you have the attenuating/hummy audio board. Either way, I think cabling to the RRC directly is a work around.

I quickly regretted buying the Mini, should have went with a bare bones K3/10 - I don't really need the KX3 like form factor, have never actually taken it on the road.

73 John K3TN

[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 the K3/0 to the Remoterig control unit shows no conformity to the schematic provided in the Remoterig 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

[John K3TN]

If you are using the K3/0, not the K3/Mini, then I'm not sure if the cable is the same. However, I will look at the cable that came with my K3/Mini from Elecraft to see if it is similarly constructed.

Thanks and 73 John K3TN