[RFI] Headphone Hum

Bob at N2SCJ.net Bob at N2SCJ.net
Tue Dec 10 08:08:25 EST 2013

Is this a 60 cycle hum?  Perhaps run some braided ground wire from the 
computer case and the K3 to your single point ground?


-----Original Message----- 
From: Gary Smith
Sent: Tuesday, December 10, 2013 5:36 AM
To: rfi at contesting.com
Subject: [RFI] Headphone Hum


I've got a problem I just discovered and whilst its not RFI to my
radio, this forum is likely a good source for a resolution. The
problem is hum in my headphones. I've switched headphones and
verified the hum is present in both my noise cancelling Bose QC-15
and a standard cheap headset that collects dust. The QC-15 are far
better headphones and the hum is louder in the Bose.

Here's how things unveiled and all the details I can offer; I use a
creative E-MU 0404 USB sound card to do recording work and to play
music from my computer. I have two self powered Mackie HR824 studio
monitors connected to the outputs of the sound card. The "card" is a
self contained device with controls which sits on the desk next to my
K3. When I plug the headsets into the sound card headphone out there
is no hum. When I plug the headsets into the K3 there is no hum.

The K3 is connected to the computer via a RS-232 cable to a RS-232
PCI card. The sound card is connected via a metal shielded USB cable
with included ferrite shield next to the computer connection.

Normally I wouldn't want to listen to sound from the computer while
listening on the K3 but I've found a reason to use the sound card and
the K3 simultaneously and that is my Logic 9 logging software and
when a needed spot for DXCC on CW comes through, I have it set to
announce that alert. Rather than leave the self powered speakers on
all the time I figured I'll turn them off, put a stereo Y cable from
the sound card to the front phones connection on the K3 and then use
the open female from the Y to connect the headset to. This way when
the alert is announced I can hear it while listening to the K3. There
is no other reason to listen to both the sound card & K3 at the same
time and this sounded like a simple solution.

The problem is a hum generated when I interconnect the sound card &
K3, it' a constant low hum and it does not increase/decrease by any
change in volume from the sound card or K3. I re-checked my grounding
& found no apparent issues, jumpers between bits made no change. I
started unplugging everything connected in the shack and the sound
continues until I unplug the USB cable from the sound card to the
computer and the hum stops. I killed the power to everything and I
still hear the hum in the QC-15 headphones, removing the USB cable
from the computer while all the power is off resolves the hum.

So I'm on the floor in the basement (carpet with cement underneath)
trying to unplug things to see if that makes a change and I noticed
the hum varied in intensity as I moved about. Working with this I
found if I bring my head next to the floor the hum completely stops
and when I pull up the hum increases. With the power off I did not
try this with the non-powered headset but with the computer running I
did try it and I get the same stoppage of hum when the standard
headphones are near the floor. I re-verified it with the QC-15s and
though the hum is markedly louder in the QC-15s, when I put my head
next to the floor the hum goes completely away. Apparently when the
headphones are next to true ground the problem resolves.

Any suggestions on what might be happening and a straight forward way
to resolve it? As it is, I'm leaving the powered speakers on so I can
hear an alert when it's sent and just plugging straight into the K3
with the headsets. sticking my head in the sand might actually work
in this case...

Thanks for any thoughts on the matter.



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