A sound system consist of four basic kinds or components:  inputs,
amplifiers, attenuators (volume controls) and outputs.  (Feedback occurs
when an amplified signal output is presented as an input and gets amplified
again.  For any given length of "feedback loop", some family of frequencies
is going to be in synch, and the amplitudes (volumes) at those frequencies
will rapidly build toward infinity.)

  "Hum" refers to an extraneous input into the system.  Michael Faraday
discovered that it was easy to turn a fluctuation electrical current into a
moving magnetic field, and vice versa.  In the case of most hum, the
fluctuating AC power from a wall outlet is producing a weak magnetic field
which int turn induces a small fluctuating current in wires carrying the
sound signal (which is ALSO a small fluctuating current).
  This won't usually be noticeable unless it occurs on the input side of one
or more amplification stages.  If these are powered speakers, then there is
an amplifier built into one or more of the speakers, and so it may be the
speaker *wire*, not the speaker itself, that is passing too close to a power
cord or other device.  (A fluorescent light could be the culprit.)

David Gillett


On 7 Sep 2003, at 16:11, Brady C Burke wrote:

> I put to together a system epox mobo kl133 750mhz everything ran ok I
> delivered it and they called me with bad hum in the speakers.  The sound in
> built into the mobo and I was using the drivers downloaded from epox.  I
> made sure the speakers are not next to the monitor or electrical wiring I
> rebooted the system using a boot disk  to dos and I still have this hum
> when the speakers are turned up all of the way.  I even installed a new pci
> sound card same problem.  Any and all ideas on how to proceed appreciated.
>
> Brady
>
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