[Elecraft] K3 SO2R audio problem

[Jim Brown]

On 12/28/2010 6:21 AM, kz5d at aol.com wrote:
> The fix is to go to the Top Ten website and build the audio isolator for each radio that is described. I believe that it is shown as an addendum.

If there is HUM (pure 60 Hz) as opposed to BUZZ (triplen harmonics of 60 
Hz), it is magnetic coupling between some 60 Hz source and the 
unbalanced shield wiring. The induced voltage is added in series with 
the shield (and thus in series with the signal). There are two common 
sources of 60 Hz in most ham shacks.  One is the power transformer in 
big linear power supplies, and the other is a field caused by wiring 
errors in the AC power system.

W4TV has described the proper, simplest, and most effective fix for 
problems like this -- simply implement proper bonding and grounding 
within your station.  That approach is described in

http://audiosystemsgroup.com/HamInterfacing.pdf

and in http://audiosystemsgroup.com/RFI-Ham.pdf  in the chapter on 
Solving Problems in the Shack.

The executive summary of this bonding is to bond together, using short, 
fat copper, the chassis of all interconnected equipment.  That 
collection of bonded equipment must, of course, also be bonded to all 
grounds within the building -- ground rods for the shack, ground rods 
for the power system, ground rods for CATV, ground rods for TELCO, etc. 
It must also be bonded to the green wire(s) at the outlets that power 
equipment in the shack.

BUZZ is also greatly reduced by getting power for all equipment in the 
shack from a single AC outlet. When that is not practical (for example, 
one or more 240V power amps), the green wires of all the outlets that 
feed equipment in the shack should be bonded together with short fat 
copper.

What I have described here is nothing more than proper grounding and 
bonding that satisfies the requirements of the National Electric Code, 
and of equivalent safety codes in virtually all of the developed world.  
This simple practice is also the simplest and most effective method of 
preventing "ground BUZZ."  And it's almost free -- the only cost is the 
short lengths of copper wire (#10 stranded copper or the heavy copper 
braid stripped from transmitting coax).

Some other fundamental points.

1)  If the magnetic field is coming from power transformers, the 
coupling can be reduced by moving the power supplies further from the 
magnetic loop that is coupling it to the system.

2)  Coupling can also be reduced (and sometimes eliminated) by rotating 
either the loop or the source of the field so that the loop and the 
field are at 90 degrees to each other.

3) Magnetic coupling is DIRECTLY PROPORTIONAL to the LOOP AREA -- that 
is, the area of the loop that is receiving the field.  That means 
reducing the area by a factor of 2 reduces the hum by 6dB. Thus, by 
simply lacing the cables tightly together with the bonding conductors, 
the loop area is reduced and the hum is reduced.

4) Nearly all cheap transformers are unshielded, and unshielded 
transformers can pick up magnetic fields. Thus, while a transformer can 
break the magnetic loop and prevent coupling from the loop to the 
system, cheap transformers can also CAUSE hum coupling.  The 
transformers used in the K3 are unshielded. Shielded transformers are 
expensive.

73, Jim Brown K9YC



.