[Elecraft] K3s Line Out (stereo)

[K9MA]

There's no mystery at all. The headphone outputs of two radios go to a 
switching system with a common ground. There inevitably is a small AC 
voltage difference between the chassis of the two radios, and from the 
radios to the rest of the system ground, which results in some current 
flowing in the headphone cable ground wires. Bonding everything together 
with braid might have helped, but it might have been hard to get the 
impedance low enough. The common ground was an oversight on my part, but 
avoiding it would have required third relay pole and isolated headphone 
jacks. Rather than rebuild the switching system, I inserted the 
transformers. They serve exactly the same purpose as the K3 internal 
ones in the line outputs.

I'm not sure how other SO2R systems deal with the problem, but it has to 
be either isolation transformers of switched headphone grounds. Or, as 
does one of my friends, completely isolated left and right headphones.

73,
Scott K9MA

On 1/16/2019 09:36, Don Wilhelm wrote:
> The K3/K3S Line Out already has isolation transformers internal to the 
> K3.  I do not understand any need to put additional transformers in 
> the audio path(s).
>
> What happens if you cable to LINE OUT signals to a computer soundcard 
> (with both left and right channels - some only are one channel if the 
> input is set for MIC)?  You should hear the main RX in the left 
> channel and the SubRX in the right.
>
> Your external transformers/cabling raises the question of "is it the 
> K3S or is it your external devices that are causing the problem".
>
> Time to strip everything down to the essentials and begin to determine 
> where the "failure" is occurring.  Test one thing at a time.
>
> 73,
> Don W3FPR
>
> On 1/15/2019 8:00 PM, Dave Sublette wrote:
>> My K3s arrived over the weekend and I have been setting it up. I am 
>> using
>> the rear Audio jacks to drive either headphones, or the Line out to 
>> drive
>> an amplifier and two speakers in the stereo mode.
>>
>> The headphone jack has stereo output ok.  But the line out only has 
>> right
>> channel audio.  Left channel has nothing but a low hum.
>>
>> I am testing them one at a time, using the same cable. So it is not a 
>> case
>> of a bad cable.  The plug inserts to the same depth in both the 
>> headphone
>> and line out jacks, so I believe the plug is properly inserted,
>>
>> I have been through both the Main Menu and Config menus, item by 
>> item.  I
>> see nothing that would give the option to turn off the left channel 
>> of the
>> line out.
>>
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-- 
Scott  K9MA

k9ma at sdellington.us