Cable myths, or truths?

[Don Tillman]

   From: "Arturo Mieussens" <amieuss at telcel.net.ve>
   Date: Thu, 2 Apr 1998 01:06:34 -0400

   As far as I remember from my transmission lines class, for maximum power
   transfer in a line the three impedances: output, line and input, have to be
   the same, otherwise, reflections due to reactance (storal of electric
   energy in circuit elements due to AC) occur and make part of the power 
   travel back to the output stage. 

Close, but you're mixing up two different concepts...

For a power source with a given internal resistance (and include the
resistance of the connecting cable with that) the maximum power
dissipated by the load happens (for that power source) when that load
resistance is equal to the power source (plus cable) resistance.  This
is provable with simple math, but basically a higher load resistance
means that less than optimal current flows and a lower load resistance
means that there's less than optimal voltage across the load.

But this is only applicable for the situation where you say "I have a
power source with this characteristic and I want to choose a load
resistance such that the load dissipates the most power", and that's
never the case with audio.

For ac signals on a cable that's longer than (roughly) one wavelength
you have the opportunity for reflections.  This is never the case in
your living room since the wavelength of 20KHz is about 8.5 miles, but
if it were the signal will reflect off the ends unless the load
impedance equals the impedance of the cable.

----

For a real laugh check out www.monstercable.com and tally up the
ridiculous claims...  "Amplitude balanced", "skin effect", "different
sized conductors optimized for different frequencies", "cable phase
characteristics"....  it goes on and on.  Pretty funny.

"Directional cables" are obviously a hoax, however there are cases
where you'd want the cable shield connected on one end only.  I don't
know if that's what Monster is talking about or not.

  -- Don