Odd audio feedback problem

[George Thornton]

I seem to have an elusive audio feedback problem.  It is not always
there but when it's there it's bad enough to render me unintelligible on
voice Tx.  I am posting this because maybe someone has seen this before
and can enlighten me.

 

I am operating with the LP-Pan and am using Power SDR for software.  I
am using the E-Mu (I think 0202) sound card.  My computer is an older
Lenovo T60P laptop loaded with Windows 7.  

 

For PSK I bought a second sound card, some kind of a hockey puck style
USB card that was recommended from this list.    I got the second card
because I was not able to see how the LP-Pan allows for transmit of PSK
from the computer.  I use HRD and DM780 for PSK.

 

The feedback is intermittent.  Sometimes it is there, sometimes not.  I
have tried silencing the K3 speakers, reducing MON to 0 and using
headphones, still it happens.

 

Turning off the computer eliminates the feedback.  Sometimes unplugging
one of the sound cards eliminates the feedback.   I also tried silencing
the computer microphone, presuming that the microphone was picking up
audio from the spoken Tx and feeding it back somehow through one of the
sound cards.  

 

Anyone ever encounter something like this before?  I suspect it is
something obvious that needs to be set in the computer.  

 

 

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[Jim Brown-10]

On Tue, 7 Sep 2010 10:01:26 -0700, George A. Thornton wrote:

>I also tried silencing
>the computer microphone, presuming that the microphone was picking up
>audio from the spoken Tx and feeding it back somehow through one of the
>sound cards.  

Look at this again. This is a very common problem, usually caused by a
piece of software switching the Windows Mixer in ways that it thinks you
want. Remember that there are TWO Windows Mixers -- one for record, and
the second for playback.

There is another mechanism that sounds just like RF feedback but is
actually modulation of the DC voltage as a result of IR drop when
transmitting SSB. If the V- bus in the power supply is bonded to the
chassis, that modulation, which sounds just like SSB on an AM RX, gets
added to the TX audio. W8JI was the first to alert us to this problem, and
I have done testing to duplicate it in my shack. This problem can also
happen if you get power for accessories direct from the power supply and
then connect them to the rig.

73, Jim K9YC



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