>Date:    Mon, 17 Aug 1998 04:43:12 -0400
>From:    "Staggers, Kevin" <[log in to unmask]>
>Subject: Soundcard Has Loud Hum in output
>
>I have fixed my soundcard up so that I can plug it into my stereo when I
>play games...  Now when I plug it in, I get a very loud hum.

Since it breaks through on other signals it's clearly a large
signal. it sounds like some kind of hum loop or grounding fault,
but first...

... test the speaker output of the soundcard using headphones
with a decent bass response (eg walkman style or large phones).
Since this is a "floating" load, if a strong hum is present
then either the soundcard is faulty or there is hum through
an unintended input: check with the internal CD audio cable
disconnected and look for stray wires touching the card.
(PSU loading has changed with CPU upgrade so fault is
 a possibility but they usually fail drastically!
 Could check with an AC millivoltmeter for excessive ripple.)

Assuming a clean output on headphones, very likely
the hum is superimposed on the soundcard output,
either because of:
* faulty grounding allowing impressed mains voltage on the signal, or
* different ground potentials leading to large ground-loop currents
  in the shields which induce hum voltages on the signal.

Try the following tests:
* switch off computer by its power button - if still hum, must be wiring!
* check that computer and stereo are properly grounded to mains
* if either is on a two-prong reversible plug try the other way round
* power both units from the same mains outlet using distribution strip
* check the earthing on your mains outlet (careful!)
* check the shield continuity on your audio cables (again ;-)
* waggle the audio cables - if the stereo crackles
  you have an intermittent cable fault.


Len Warner <[log in to unmask]> http://easyweb.easynet.co.uk/~len/ ICQ:10120933