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Re: [RFI] Stereo/audio EMC question

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Stereo/audio EMC question
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Sun, 27 Nov 2005 12:58:26 -0600
List-post: <mailto:rfi@contesting.com>
On Sun, 27 Nov 2005 13:19:18 -0500, Jim Jarvis wrote:

>I'm dubbing all CD's into iTunes in a mac-mini,
>and feeding the stereo via a line input (vcr-2) from a 16 ohm phone/
>speaker line out of the mac-mini. 

Virtually ALL computer audio interfaces have massive pin 1 problems, and 
are thus open doors for common mode RF. In other words, those audio 
interface cables function as antennas, and the resulting RF current gets 
inside the box via the pin 1 problem. 

The easiest fix (without modifying or switching to different equipment) 
is usually to add suitable ferrite chokes to the interface cables 
(including power). 

For a lot more detail on pin 1 problems, RFI to audio gear, and how to 
use ferrite chokes, see the publications section of my website. There's 
a lot of excellent data measured by another ham. 

By the way -- the #31 Fair-Rite 2.4" O.D. toroids, by far the most 
useful part for HF suppression, are now available from Fair-Rite. It's 
Fair-Rite P/N 2631803802.  Ham resellers would call it FT240-31 and sell 
it for around $10. It's a LOT cheaper from Fair-Rite's distribution 
using the Fair-Rite part number. Several clubs got together and did a 
group purchase of 2,000 pieces. We got them for under $2.00 direct from 
Fair-Rite, including shipping. They're under $4.00 at less than 250 


Computer gear also tends to be totally unshielded, but the interior 
wiring is usually too short (as a fraction of a wavelength) to be very 
susceptible at HF. VHF and UHF is a different story -- both unshielded 
boxes and pin 1 problems are major mechanisms, and when you fix one, the 
other may still be significant. 

Jim Brown

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