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Re: [RFI] Computer speaker RFI

To: rfi@contesting.com
Subject: Re: [RFI] Computer speaker RFI
From: Jim Smith <jimsmith@shaw.ca>
Date: Mon, 01 Sep 2003 20:45:59 -0700
List-post: <mailto:rfi@contesting.com>
There is no need to try to match low Zout of sound card (or anything else) to the high Zin of an amplifier. As long as the sound card output level is sufficient to drive the amp it will be fine.

73 de Jim Smith VE7FO

Rob Atkinson, K5UJ wrote:

Hi everyone:

I'm battling a problem with RF getting into the speakers on my PC here in the shack. I had a similar
problem with rf getting into my packet tnc when I transmitted on 75 meters and I was able to fix that by putting ferrite beads on the leads going into the TNC. the speakers are a tougher problem. There
are two of them each in its own plastic cabinet. Inside of one, there is a 10 watt AF solid state amplifier that boosts the soundcard audio enough to drive the speakers. my problem is with that amp. It is rectifying the rf big-time. I was able to make some progress by pulling the amp pc board out of the speaker cabinet it was in and placing it inside the metal computer cabinet and putting chokes on
all the leads going into and out of the amp. I was giving myself a pat on the back--peace at last on 75 meters without having to kill the speakers before transmitting--then the other night I got on 40 and
experienced the same old loud growling I used to hear on 75.

Since all the shielding and chokes are just a bandaid fix for the real 
problem--transistors acting like
diodes--I got a crazy idea yesterday that I thought just might be bizarre 
enough to work:  feeding
the soundcard audio into a small simple outboard vacuum tube AF amp.  other 
than matching the low Z soundcard impedence with the hi Z amp input, does 
anyone see any other problems with this idea?  I'm thinking No transistors; no 
solid state rfi problems.

tnx & 73,

Rob Atkinson
k5uj@hotmail.com _______________________________________________
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