Some sound cards, especially SoundBlasters, have an output from the pre-amp
and one from its on-board amplifier.  The pre-amp output is to be used for
connecting to an external amplifier as you are doing while the on-board
amplifier is used to drive earphones and small speakers.  The connection on
the stereo can also accept different input levels.  A stereo input that is
supposed to be used with low-power inputs will show this symptom if you
connect it to the sound card's amplifier output.  Make sure that you are
using the output on the sound card and the input on the stereo that you
used to use before you saw this problem.  If you are still having problems,
please give us the kind of sound card you are using and the type of
connectors it has.  You should be able to find this information in the
manual.  Also, what input on the stereo are you using?

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> From: Staggers, Kevin <[log in to unmask]>
> Subject: [PCBUILD] Soundcard Has Loud Hum in output
> Date: Monday, August 17, 1998 3:43 AM
>
> I have fixed my soundcard up so that I can plug it into my stereo when I
> play games, or to record midi's I like onto tape, etc.  This has worked
> fine in the past with no problems.  Now when I plug it in, I get a very
> loud hum.  This sound is also noticeable on the pc speakers, but you
> have to turn them up quite a bit before you hear it.  (They're not
> really good speakers, that's what the stereo's for. :)