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
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
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