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[RFI] Magnetic Field Interference

To: RFI List <rfi@contesting.com>
Subject: [RFI] Magnetic Field Interference
From: Brendan Minish <ei6iz.brendan@gmail.com>
Date: Sat, 13 Mar 2010 14:56:47 +0000
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
Recently I adapted my Beyer-Dynamic headset (DT109) for use with my K3
the DT109 has a Dynamic low impedance hypercaroid Microphone element in it.
To drive my K3 adequately this required the addition of a peamp. I 
constructed one one based on this design.

This is constructed in a metal tin with the ground side connected at the 
point where the output to the K3 connects to the tin, My issue is not a 
'pin1 problem'.

I noticed I was having some issues with low level Hum pickup. At first I 
though this might be due to the input shield decoupling being inadequate 
   . I increased the capacitance of C1 to no avail. then I tried a 1:1 
Isolation transformer at the input. this made things much worse, then 
the penny dropped!

The actual cause of the issue is pickup of the magnetic field from the 
PSU of my Acom1000 Linear amplifier. Once I removed the 1:1 transformer 
the only remaining source of pickup seems to be the mic element it's 
self. the closer it gets to the amp the worse it gets. If I go far 
enough away there is no residual hum.
My Typical operating position places the mic element at a distance of a 
little over 1M away from the amplifier.
Placing the amplifier further away is not a good option for me. it's 
already as far away as I can place it and still reach the controls (at a 

My questions are

1/ Is there anything I can do to increase the magnetic shielding of the 
amplifier? Will using 'Mu metal ' type material on the outer case of the 
amplifier in the vicinity of the PSU help?

2/ Is there anything I can do to improve the immunity of the the dynamic 
microphone element to the magnetic field ( I suspect there isn't.)?

I would really like to be able to continue ot use this headset, the 
DT-109 headset is very comfortable, provides great isolation and the 
microphone element produces excellent SSB audio with plenty of presence 
  and not too much in the way of LF, Ideal for Dxing

Brendan Ei6IZ
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