Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!optilink!elliott
From: elliott@optilink.UUCP (Paul Elliott x225)
Newsgroups: sci.electronics
Subject: Re: PC speaker to stereo (update)
Summary: whine sound due to power supply noise
Message-ID: <3387@optilink.UUCP>
Date: 11 Apr 90 16:08:41 GMT
References: <6062@jarthur.Claremont.EDU>
Distribution: na
Organization: Optilink Corporation, Petaluma, CA
Lines: 43

In article <6062@jarthur.Claremont.EDU>, rspangle@jarthur.Claremont.EDU (Randy Spangler) writes:
> Well, Heath Roberts suggested I have the leads that were going to my
> stereo go to one side of an audio transformer, and then take the other
> side and attach that to my stereo. 
> 
> Result: massive improvement in sound quality
> 
> Problem: I still get a high whine (mostly >2KHz) even when the speaker isn't
> doing anything.  I figured it was probably RF interference from the monitor

The whine is probably due to the switching power-supply in the PC.  There is
a significant amount of ripple in the +5V supply, and it will show up on the
speaker output (translation of 'significant' = 'I don't know how much').

Here are some things to try:

*   Connect the transformer inputs closer to the timer chip to reduce
    ground-loop problems (the ripple current causes a voltage difference
    between different ground points).

*   Instead of connecting the 'common' input of the transformer to ground,
    try connecting it to +5V (at the timer chip).  This might improve things,
    depending on the idle output state of the chip.  Use a seperate
    coupling capacitor so you don't connect +5V to the speaker via the
    transformer winding.  This one is a long-shot, but it might help.

*   If you don't need high-freq response, consider an R-C filter between
    the speaker connection and the transformer.

*   It is possible that the transformer itself is picking up the magnetic
    fields generated by the PC switching power-supply.  Re-orienting the
    transformer might help.  Electrostatic coupling is less likely, but
    grounding the transformer frame might help (where to ground it?).

*   For $799.95 I can sell you a whiz-bang DSP card that will filter out
    the noise.  Such a deal! ;-)



-- 
      Paul M. Elliott      Optilink Corporation     (707) 795-9444
            {uunet, pyramid, pixar, tekbspa}!optilink!elliott
    "Less than perfect, that's what I've been aiming for all along."