[Elecraft] K3 SO2R audio problem

[Rob]

I have two K3s set up for SO2R using a DX Doubler. Each radio has the Top Ten cable for a K3 with a PTT, key, mic and phones plug on one end. When everything is set up and I plug my headphones into the DX Doubler's phones jack, I hear a bit of hum on the audio regardless of whether the audio switch is in Radio 1, Both, or Radio 2 position. When I send with the right radio or left radio, I hear a hummy audio. However, if I plug the headphones into either radio's phones jack, everything sounds perfect. No hum. No hummy sidetone audio. If I plug it back into the DX Doubler, and key radio 1 while the audio switch is in radio 1, and I pull the interface cable from radio 2 out of the DX Doubler socket, it all sounds fine. Same with the opposite situation, key and monitor radio 2, pull the interface for radio 1, and have headphones plugged into DX Doubler phone socket.

Looking at the DXD schematic, the audio from each radio is fed to the DXD phone socket with no active devices in the audio chain. It is strictly a switch and relay.So, physically, the two radio's audio outputs (audio 1, audio 2 and ground) are fed into the Radio 1 and Radio 2 interface sockets. The grounds on both radios are joined and the DXD's audio switch controls two DPDT relays that either serve just radio 1, both, or just radio 2 audio to the phone socket.

Clearly what I am hearing on the DXD is coming from stereo phones jack back into the DXD, or somehow getting created in the DXD. Monitoring the phones jack on the front panel I hear only clean stuff. Does anyone have any clues as to why the audio coming out of the stereo phones rear jacks should be interacting this way when being routed through the DXD? 


Rob K6RB