Bob-Stanton wrote:

chung wrote in message news:e5d5b$4012a5ca$c247604

.............. Even in that case, a lumped model
is good enough, considering all the approximations one uses in that type
of anslysis.


FET output devices can oscillate at a very high frequency. One would
need a speaker cable model that was accurate at very high frequencies

The lumped element speakercable models have a limited bandwidth. I
doubt that you could write a lumped element model, for a 100 ft of
cable, that would be accurate above 200 KHz.


So consider the approximations. How accurately can you model the power
amp? How about the connection between the output transistors to the
terminals at the back of the amp? How accurately do you model the skin
effect and resistive losses? How about the loss tangents of the
dielectric? How close to an ideal transmission line is a pair of wires
side by side? How about the discontinuity between the cable and the
speaker terminals?

In the end, you are making many approximations in such calculations.
Whether to approximate the cable with a lumped model or a transmission
line model is a small difference, compared to the large uncertainties in
the rest of the system.

It will be like trying to find an answer to 5 significant digits, when
all your inputs only have 2 significant digits. You are chasing 4th
order effects when you don't even have second order effects under control.

Besides, looking at transmission line loading is the wrong way to
analyze such amps. Much better to vary the output loading and make sure
that the amp is stable for all possible loads. And you would model that
with resistive, capacitve and inductive loads.

Trust me, no one analyzes the stability of an audio amp by using
transmission line models of cables.


One can use Maxwell equations and quantum mechanics to analyze speaker
cables, too. Have you thought about doing that?


I think that what you are saying is: one can go too far in analyzing
speaker cables.

Suppose you were writing a circuit analysis program that was to
include a model of typical 12 gage speaker cable (any length).

Why bother? Suppose you were to write a circuit analysis program based
on quantum mechanics?

This
model could be useful for educational perposes, and also would have
some pratical applications.

Not for audio cables or audio amps.

Would you write a speaker cable model that was "dead on" accurate, or
write a (close approximation) component type model?

See all the approximations above.

Bob Stanton