Jim Lesurf wrote:
In article , Rob
wrote:
Jim Lesurf wrote:
Hi,

I've just put up a new webpage that provides some measurements on the
properties of a variety of loudspeaker cables. The page is at

http://www.audiomisc.co.uk/HFN/Cables3/TakeTheLead.html

It is an expanded version of the article published in 'Hi Fi News' a
few months ago.

Slainte,

Jim


FWIW it means very little to me. You seem to assume a correlation
between frequency, resistance and sound.

Not sure what you mean, I'm afraid.


There's a relationship between three things: frequency and resistance
(the things you plot) and sound. If one of the things change, another
one will change ('correlation'). On further reading you suggest a
relationship between frequency, resistance and risk.


Perhaps a paragraph or two on what you might expect any measured result
in the context of your measurments to mean?

The measurements and results serve two purposes.

1) The size of the peaks and dips in impedance will vary with the choice of
cable and end-load (speaker). Using 'open' and 'short' means loads with
impedances as high and low as you can get compared with the cable
impedance. So you can expect the results to give you a guide to which
cables give the highest or lowest peaks/dips for real-world loads. Hence
the results give a sign of which cables would be more risky with amplifiers
that are not unconditionally stable, or whose behaviour can be upset by RF
resonances, etc. In particular, sharp dips down to very low impedance can
be bad news for a poor amplifier. Hence useful as a warning.


Ah, OK, good. But is it *really* risky for any amplifier that doesn't
carry a cable recommendation tag? By risk I assume possibility of
component failure.

2) You can use the measured impedances as a function of frequency to
determine the electrical properties of the cables. Choice of 'open' and
'short' here makes calculating the cable properties simpler, although in
principle any two choices of loading with significantly different values
would do. In the absence of a the amp having an RF problem these values are
most useful for telling you the cable series resistance and inductance as a
function of frequency in the audio band. (Yes, both values can vary with
frequency, although probably not by much in the audio band.) Combined with
the loudspeaker impedance, these series values change the frequency
response in the audible range. So the values determined from the RF data
tell you something about what changes to expect in the *audio* frequency
response. In particular, you want low series inductance and resistance to
minimise alterations in frequency response in most cases. The snag is that
*very* low inductance, in our universe, means *high* shunt capacitance
which can change the response from amps that have an output series
inductor. (Which I would recommend they *do* have.)


It'd be nice, although I expect quite difficult, if you could explain
how these effects could influence sound.

I've certainly known about all the above for decades. It was taken as
standard knowledge by people I've worked with. Although I guess some
audio-only designers may not know how the cable properties can be measured
using a VNA in this manner, but it isn't unusual in RF/microwave
engineering.

There are a couple of follow-on articles, that do look at this further, and
include simple techniques - like the use of a series inductor and 'zobel'
on the amp to help protect it against (1). That is a method I've always
used as it works neatly. But there are commercial amplifier designs that
*don't* do this, so are exposed to RF loading by the cable and speaker.

Sounds daft. Do you know which amplifiers?

And
the use of a series inductor may mean you'd have to be wary of ultra-low
inductance cables for the perverse reason that they have ultra-high shunt
capacitance.


Excellent! It's clear that the technically literate here know what
you're driving at, and if Maplin's own is good enough for you etc :-)

Rob