[Elecraft] [K3] Serial interface noise issues

[Don Wilhelm]

I will only comment on the part reproduced below - audio return is NOT 
audio ground.  In the case of the  Motorola amp cited, I suspect the 
audio amp output is a bridged amplifier, and for those, neither speaker 
lead should be grounded (it shorts half of the audio amplifier.

BUT, Jim Brown was talking about unbalanced audio inputs, not speaker 
outputs.  In the ham world and the world of consumer audio, most audio 
inputs are unbalanced, with one conductor (the shield) connected to 
"ground".  That ground used to be the equipment enclosure, but more 
recent implementations circumvent that and put the connection to the 
shield onto the PC board rather than the enclosure - and often the 
enclosure is plastic which creates yet another set of potential problems.

We used to connect our audio (and RF) jacks directly to the enclosure 
and then inside the enclosure, we would route that connection to the 
circuit contained in that enclosure - no "pin 1" problems because the 
shield of connected cables were connected first to the outside of the 
enclosure..  In today's implementations, pin 1 problems abound because 
jacks for any and every purpose are designed to connect to the PC board 
and may or may not have any connection to the equipment enclosure.
That allows noise and other undesired signals to be introduced onto the 
device circuit board without being first stripped off by the (grounded) 
metallic enclosure for the device in question.   We have come a long way 
in convenience for manufacturers, but several problems have occurred 
along the way, and they are not easy to correct.

73,
Don W3FPR


On 11/1/2011 9:58 PM, Rick Bates wrote:
> In the case of Motorola radios, you'd have to ask them why, but never assume
> that either side of a speaker (for example) is chassis grounded, or you'll
> blow up a very expensive audio amp (like on the popular GM300).  A careful
> resistance test should be done before connecting audio ground to chassis.
>