[Elecraft] K3: 6M SSB audio hash

[Joe Subich, W4TV]

Jim, 

Which is it?  In this reply you say: 

> >... but not as good as if the shield were connected to the 
> >chassis (usually PTT ground) and the mic treated as a 
> >floating/balanced input.  
> 
> WRONG! What you have just described is perfect example of a pin 1 
> problem, and a VERY common cause of hum, buzz, and RFI. 

but in the reply to Jamie you say: 

> The proper connection for that is to connect the two mic 
> wires (the hot and the shield) to the balanced input AND 
> to connect the shield DIRECTLY TO THE CHASSIS by a very 
> short path.   

Your description says the very same thing as I did ... mic wires 
floating with the shield connected directly to the chassis.  

The point is that Icom, Yaesu, Kenwood and Heil all float the 
mic return above the chassis and then connect the shield of the 
mic cable to the floated mic return!  The shield should be 
connected to the PTT return - which is connected to the chassis. 

Elecraft connect the mic return to chassis inside the K3 *BUT* 
Pins 7 and 8 (PTT and mic return) have series chokes which result 
in about 5 Ohms of resistance between both pins and the case. 
The same is true for the shield/sleeve of the rear panel mic 
jack ... the sleeve is floated by an RF choke with about 5 Ohms 
of DC resistance.  

The best solution for all amateurs is to connect the mic return 
pin on the mic connector to the shell to tie both mic return and 
shield to the chassis by the lowest possible resistance path. 

73, 

   ... Joe, W4TV 
 





> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
> Sent: Saturday, June 07, 2008 12:12 PM
> To: Elecraft List
> Subject: RE: [Elecraft] K3: 6M SSB audio hash
> 
> 
> On Sat, 7 Jun 2008 01:41:02 -0400, Joe Subich, W4TV wrote:
> 
> >That is *NOT* what is happening ... the mic connection does not 
> >simply "unbalance" the mic input because one side of the preamp 
> >is still not grounded.  What is happening is that the mic is 
> >connected to the preamp in a balanced configuration but the 
> >shield is left floating on the mic end and tied to the mic 
> >return at the transmitter input.  The mic return is then tied 
> >back to the chassis through an RF choke (DC return). 
> 
> That is an improper connection, a violation of AES48, and a 
> violation of good engineering practice. The ONLY proper connection 
> of ANY cable shield is to the shielding enclosure. A balanced 
> input can take (at least) two forms. One is a transformer. The 
> other is a differential input stage. The mic must be connected to 
> the two terminals of those inputs, and the shield must go straight 
> to the chassis, NOT through a choke. 
> 
> You say "What is happening"  -- do you mean that this is wiring 
> provided by the W2IHY box? If so, I agree it is really bad. 
> 
> >It would be better if the mic return (and shield) was connected 
> >directly to the chassis at the mic connector than the present 
> >situation 
> 
> Absolutely -- it is the ONLY right way to connect a cable shield.
> 
> >... but not as good as if the shield were connected to the 
> >chassis (usually PTT ground) and the mic treated as a 
> >floating/balanced input.  
> 
> WRONG! What you have just described is perfect example of a pin 1 
> problem, and a VERY common cause of hum, buzz, and RFI. 
> 
> See AES48. http://www.aes.org/publications/standards/list.cfm
> 
> See also the tutorials on my website. 
> 
> http://audiosystemsgroup.com/publish
> 
> The pro audio world figured this out long ago. Virtually all EMC 
> authorities agree. It's really sad that hams have kept their heads 
> in the sand about it. 
> 
> 73,
> 
> Jim Brown K9YC
> 
> 
> 
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