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Re: [RFI] Computer speaker RFI

To: "Rob Atkinson, K5UJ" <k5uj@hotmail.com>, <rfi@contesting.com>
Subject: Re: [RFI] Computer speaker RFI
From: "Michael Tope" <W4EF@dellroy.com>
Date: Mon, 1 Sep 2003 11:12:44 -0700
List-post: <mailto:rfi@contesting.com>

Make sure what your battling isn't a ground loop between your
computer and your rig. I have +12V rig here (Ten-Tec), and
its chassis potential seems to bounce a little with modulation
due to the high current in the power supply return (~20 amps
on voice peaks). This will drive audio components nuts if
you try feeding line level audio between the computer sound
card and the rig. Any connection between the computer
ground and the chassis of the rig results in big trouble.
I found audio isolation transformers to be a good solution.
Just a thought.

73 de Mike, W4EF...................................

----- Original Message -----
From: "Rob Atkinson, K5UJ" <k5uj@hotmail.com>
To: <rfi@contesting.com>
Cc: <k5uj@hotmail.com>
Sent: Monday, September 01, 2003 8:14 AM
Subject: [RFI] Computer speaker RFI

> 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@hotmail.com
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