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Re: [RFI] Magnetic Field Interference

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Magnetic Field Interference
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Sat, 13 Mar 2010 16:41:55 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Sat, 13 Mar 2010 14:56:47 +0000, Brendan Minish wrote:

>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?

It is VERY unlikely that you can fix this with shielding. There are two 
good and simple fixes that you MAY make work. 1) Increase the distance 
between the field source and the mic. 2) Rotate the field so that it is 
perpendicular to the pickup field of the mic. 

>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.)?

This is solved with pro mics by adding a "hum-bucking" coil that is in 
series with the mic element and out of polarity with it. I don't think it's 
practical with the DT-109. I don't know of any other good solutions. 

>I would really like to be able to continue ot use this headset, the 
>DT-109 headset

I know those headsets well -- for many years, I've specified them for 
production intercom systems in the theaters for which I design sound 

Something else you can (and should) do with the K3 is roll off the low 
octave bands in TXEQ. For most mics and most voices, the two lowest bands 
should be full cut on the K3. That's 16dB for each band, and if they're 
minimum phase filters, they'll combine to add a bit more. There's no useful 
speech content in those two bands, but that's the spectrum where breath 
pops and handling noise are prevalent. Indeed, many mics should also have 
the next higher band cut as well. 

Another headset that works VERY well with the K3 (and most other ham rigs) 
is the Yamaha CM500, which sells in the US for under $60 (I've seen it 
discounted at $40). If that is available to you, it's worth considering. 
Although some catalogs (including Yamaha's) list it as a dynamic, it's 
actually an electret. 


Jim Brown K9YC

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