[RFI] Computer speaker RFI

[Jim Smith]

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
>k5uj at hotmail.com   
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