In article , harrogate3
wrote:

"Jim Lesurf" wrote in message
...


http://www.st-and.demon.co.uk/AudioM.../lscables.html

http://www.st-and.demon.co.uk/AudioM...kracables.html


Having read Jim's files with which I have no arguement, there is another
reason for having a very low cable resistance.

When drive ceases to a loudspeaker cone that is not in its rest position
(we're talking here about LF drivers) its suspension tries to put it
back to the rest position. Inevitably it will overshoot a little and
pass the rest position, so it will oscillate slightly but in reducing
amounts until it stops. During this time the coil is moving in a
magnetic field and so the loudspeaker becomes a generator.

That is actually included and implicit in the standard argument based on
the input impedance as a function of frequency. It follows from linear
superposition and that the pattern in time is directly related to the
complex frequency response.

There are two basic reasons that an audio amp has a low output
impedence: one is to follow the time-honoured engineering principle that
the source is low impedence driving a high impedence load; the second is
that the amp 'sees' the energy generated by the loudspeaker as it
settles and must dissipate it and in this instance the amp output
impedence is the load. The lower the amp output impedence the greater
the load it will place on the loudspeaker generator and so the quicker
that generator will die.

There is generally an internal series resistance in the speaker system.
This will tend to have a resistance value which is much higher than that of
the cable or most amplifiers. Thus the effect on what people call 'damping'
is generally dominated by this in practice - unless the cables or amp have
'large' impedances - e.g. of the order of an Ohm or more.

If you now put cables of significant resistance in series between the
amp and the loudspeaker-generator the load imposed on the generator will
be reduced and it will take longer to come to rest. The effect of this
is quite audible - the bass tends to sound indistinct and a tad unreal -
'flappy' or 'flabby' are the usual words. It is thus fairly safe to say
that the larger the loudspeaker cone and the louder you want to play it,
the thicker the cables should be.

Incidently the ratio of the amp output design load - typically 8R - to
the amp output impedence is known as the damping factor. It is normal to
expect a damping factor of at the very least 40 (indicating an amp
output impedence of 0.2R) and more usually 60-80. It can be argued that
going much above this will cause other undesirable effects as reduction
of the output impedence of the amp is often achieved by use of negative
feedback in the amp design which can itself cause audible artifacts.

"Damping factor" as quoted in amplifier specs is a bit of a misleading term
for the reasons given above. In reality, the level of motion damping will
tend to be dominated by the internal series resistance of the speaker
system itself.

IIRC Briggs and others showed this both by theory and by measurement about
45 years ago.

Slainte,

Jim

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