[RFI] Headphone Hum

[Jim Brown]

On 12/10/2013 2:36 AM, Gary Smith wrote:
> The problem is a hum generated when I interconnect the sound card & K3,

Do you hear HUM (pure 60 Hz), or do you hear BUZZ (triplen harmonics of 
60 Hz)?  The causes are VERY different, and so are the solutions.

Pure 60 Hz interference is the result of MAGNETIC coupling of stray 
magnetic fields into the interconnecting shield wiring. Two common 
sources of these fields are 1) leakage flux from big power transformers, 
like those in a ham power amp; 2) wiring errors in your home power system.

BUZZ is the result of leakage current flowing on the "green wires" of 
power wiring (the third pin). It consists of harmonics of order divided 
by 3 -- 180 Hz, 360 Hz, 540 Hz, 720 Hz, and those harmonics are 
generated in 3-phase power distribution systems. Even though most of us 
don't have 3-phase power to our homes, nearly all power distribution 
outside (on the street, poles, underground) IS 3-phase.

The IR drop on the green wire creates noise voltage on the chassis of 
equipment, and because every green wire has a different length, and 
every piece of equipment sees different leakage current, there is a 
DIFFERENCE between every chassis, and there is also a difference between 
every chassis and another ground connection (like the rods outside your 
shack).

The cure for BUZZ is quite simple, and nearly free.  It requires two steps.

  1) Get power for all interconnected equipment from the same outlet, or 
from outlets that share the same green wire (in the same multi-outlet 
box). If you must use more than one outlet box, bond those outlet boxes 
together with short, fat copper. A length of copper braid stripped from 
a junk piece of RG8 or RG213 is roughly equal to #10 copper, and works 
well.

2) Bond from chassis to chassis of every piece of interconnected 
equipment with short fat copper.  The chassis of laptop computers can 
usually be found at the retaining screws of D-connectors.

Step #1 reduces the difference in the IR drop in the green wires to the 
length of the power cord between the equipment and the outlets, and is 
typically good for about 20 dB of buzz reduction. Step #2 shorts out the 
smaller difference that remains by simple Ohm's Law, and is typically 
good for another 20-30 dB.

Not long after I moved to CA in 2006, I was invited to guest operate at 
N6RO, which at the time had buzz problems that were truly awful. The AC 
distro for Ken's shack had been done VERY well, and I found nothing 
wrong with it. The problem was that power for gear for the six operating 
positions were taken at random from lots of outlet strips, which in turn 
were randomly plugged into different wall outlets.  I solved the 
problems in an afternoon by doing nothing more than what is outlined above.

Another suggestion. The voltage difference between interconnected 
equipment is a problem for two reasons. First, because we are using 
UN-balanced wiring, and second because lots of equipment is built Pin 
One Problems. Pin One Problems are excited by SHIELD current, so if 
there's no cable shield, there's no shield current.

Cable shields do NOT provide magnetic shielding at audio frequencies. 
Unshielded twisted pair cable connecting balanced output circuits with 
balanced input circuits is VERY resistant to hum, buzz, and RFI. 
CAT5/6/7 in all its variations is excellent unshielded twisted pair.

The Mackie HR824s are excellent loudspeakers (I've used a pair in my 
small professional mixdown studio for many years), and they have 
balanced inputs. I don't know whether the EMU has balanced outputs, but 
it it does, I suggest that you use them to drive the Mackies.

Like most powered loudspeakers, the HR824s are quite susceptible to RFI 
at certain frequencies. A 5W 2M talkie at 10 ft can cause full 
deflection of the woofer. They are much less susceptible at HF, so I 
suspect the root cause is a layout issue on the circuit boards. .

Note that the simple bonding protocol I've outlined requires more and 
shorter bonding between equipment. This flies in the face of tired, old 
thinking like "ground loops" as the cause of all ills. Nothing could be 
farther from the truth -- the whole idea of a "ground loop" is false, 
and based upon false logic. Loops are NOT the cause of hum and buzz - AC 
leakage currents and unbalanced wiring are the root cause.

The ONLY interference coupling mechanism involving a LOOP is MAGNETIC 
coupling, so we're back to that leakage flux.

Magnetic coupling is directly proportional to the loop area of the 
wiring involved, both on the sending and receiving end. Thus, magnetic 
coupling can be reduced by compressing the loop (running bonding 
conductors in close proximity). It can also be reduced by rotating 
either the source (the power transformer) or the receiving loop so that 
the field is at right angles to the loop, and by moving the source 
farther from the victim.

There are several tutorials about all of this, including several about 
"The Pin One Problem," on my website. k9yc.com/publish.htm, and on Bill 
Whitlock's Jensen Transformers website.  Bill and I are Chair and 
Vice-Chair of the AES Standards Committee's Working Group on EMC, and 
principal authors of all AES Standards on EMC.  Bill's work on analyzing 
a balanced interface as a balanced Wheatstone bridge caused the IEC to 
re-write their Standard on the definition and measurement of CMRR. It 
was Bill who opened my eyes to leakage current as the cause of buzz, and 
I've applied that to our ham shacks in the Power Point pdf about Ham 
Interfacing (on my website)

73, Jim K9YC