TCI Cobra interconnects against Chord Chameleon
 

John Phillips wrote:
On 2008-07-15, Don Pearce wrote:
Jim Lesurf wrote:

... If I have two pieces of
cable, one of 10 microhenries and 100 pF per metre, and another of 20
microhenries and 200 pF per metre, they will perform identically.
I think not in a mismatched system.
Indeed.

No, the degree of match doesn't make any difference. In RF, you can take
a piece of PE coax, a microstrip, an edgeline, some Andrew's Heliax -
they all have totally different inductive and capacitive values. But if
they were put in a black box with only connectors visible, there is not
a single measurement you could make that would tell you which is which.
All you need to specify them completely is the characteristic impedance
and electrical length (I'm ignoring resistive differences of course).

Inside the black box:

- 1 metre of 100pf/m (10uH/m - 316-ohm) cable open circuit at the far end;

- 1 metre of 200pf/m (20uH/m - 316-ohm) cable open circuit at the far end.

Are you saying a capacitance meter operating at (for example) 1 kHz will
measure the same capacitance value? What value will it measure?


No I'm talking end-to-end measurements transmitting signal.

d

TCI Cobra interconnects against Chord Chameleon
 

On 2008-07-15, Don Pearce wrote:
... In RF, you can take
a piece of PE coax, a microstrip, an edgeline, some Andrew's Heliax -
they all have totally different inductive and capacitive values. But if
they were put in a black box with only connectors visible, there is not
a single measurement you could make that would tell you which is which.
All you need to specify them completely is the characteristic impedance
and electrical length (I'm ignoring resistive differences of course).

Right. I see a difference.

I think you are comparing transmission lines of the same electrical length
(wavelengths) and saying they are indistinguishable,

But I think a TL's electrical length is proportional to sqrt(L' * C').
If you double both (thus keeping the impedance unchanged) you have to
halve the physical length to keep the electrical length constant.

However I have been comparing cables (transmission lines) of the same
physical length. So when I double the C' and L' in my simulations I have
a TL of twice the electrical length. They ARE of different performance.

Re-doing the simulations and halving the length of the 20uH/200pF line
does indeed show an identical performance. However the cable is then
too short to reach the from the amp to the 'speakers.

--
John Phillips

TCI Cobra interconnects against Chord Chameleon
 

John Phillips wrote:
On 2008-07-15, Don Pearce wrote:
... In RF, you can take
a piece of PE coax, a microstrip, an edgeline, some Andrew's Heliax -
they all have totally different inductive and capacitive values. But if
they were put in a black box with only connectors visible, there is not
a single measurement you could make that would tell you which is which.
All you need to specify them completely is the characteristic impedance
and electrical length (I'm ignoring resistive differences of course).

Right. I see a difference.

I think you are comparing transmission lines of the same electrical length
(wavelengths) and saying they are indistinguishable,

But I think a TL's electrical length is proportional to sqrt(L' * C').
If you double both (thus keeping the impedance unchanged) you have to
halve the physical length to keep the electrical length constant.

However I have been comparing cables (transmission lines) of the same
physical length. So when I double the C' and L' in my simulations I have
a TL of twice the electrical length. They ARE of different performance.

Re-doing the simulations and halving the length of the 20uH/200pF line
does indeed show an identical performance. However the cable is then
too short to reach the from the amp to the 'speakers.


No, that isn't how electrical length is specified - it is measured in
wavelengths. All this means is that if the em wave travels slower in
one, you make it proportionally shorter.

d

TCI Cobra interconnects against Chord Chameleon
 

In article , Don
Pearce
wrote:
Jim Lesurf wrote:
[big snip]

OK - this is all on hold for the moment. Interested though why you are
including EM field velocity - is there some point to be made?

You'll have to read the series of articles to see all the reasons I took an
interest in velocity as well as impedance. :-) However...

Two points.

1) I noticed that there seems a distinct pattern where the wave velocity
varies from cable to cable in a way correlated with the cable impedance.
First noticed it when analysing other people's measured results, but my own
measurements threw up the same pattern. Found this interesting.

2) I think some people make a fuss about the wave velocity in terms of
arguing about 'time smearing'. So I looked at that in transmission line
terms. But I then went on to examine the non-matched more realistic cases
to see if the idea stands up. Results in articles, but you can probably
guess my conclusion. :-)

From basic transmission line theory there is no obvious 1st order reason
why the velocity should vary correlated with the impedance. So I covered
this for the above reasons.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

TCI Cobra interconnects against Chord Chameleon
 

In article , John Phillips
wrote:
On 2008-07-15, Jim Lesurf wrote:
In article , Don
Pearce wrote:
Speakers are particularly interesting in that there are many cables
available, some of which (from Goertz) have inductance and capacitane
which together come down to around 8 ohms as a distributed impedance.
These cables, despite having enormous capacitance, are essentially
ruler-flat in frequency.

The last sentence is meaningless as it has no reference to the
conditions of use. A 'cable' doesn't have a "ruler flat" response.
What you get depends on the system. And as soon as the load doesn't
match the cable perfectly, the source matters as well. Problem here is
that domestic LS uses a voltage assertion approach with loads that
have values that vary all over the shop.

However my simulations suggest Don is quite right in the sense that a LS
cable with a lower nominal impedance interacts less with most
loudspeaker loads and in general results in a flatter frequency response
for the voltage transferred from the amplifier to the LS.


yes. The closer the cable and load impedances are, the smaller any
variations in response tend to be. The snag is that in virtually all cases
neither the load nor the cable have a uniform impedance across the audio
band, nor vary in the same way. So although in an ideal world we are all
healthy and rich... :-)

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

TCI Cobra interconnects against Chord Chameleon
 

In article , Don
Pearce
wrote:
Jim Lesurf wrote:


... If I have two pieces of
cable, one of 10 microhenries and 100 pF per metre, and another of
20 microhenries and 200 pF per metre, they will perform identically.

I think not in a mismatched system.

Indeed.


No, the degree of match doesn't make any difference. In RF, you can take
a piece of PE coax, a microstrip, an edgeline, some Andrew's Heliax -
they all have totally different inductive and capacitive values. But if
they were put in a black box with only connectors visible, there is not
a single measurement you could make that would tell you which is which.

Well, you could easily measure, say, their capacitance values, and
inductance values, Indeed, for the work I have been doing I have routinely
been measuring such quantities and using a VNA to measure cable properties.

If what you said above were true all cables would return the same values.
They clearly don't. They simply don't "perform identically" when not used
matched. Even if they share the same Zc value, they show observable
differences. So i'm afraid I've just spent a few weeks showing that your
initial statement above is incorrect.

They may "perform identically " *when matched*. But that is a special case
upon which the idea of matched operation is based. Nor, even then, might
they be "identical" as the wave velocities may differ, for example.
Doubling both L' and C' alters the wave velocity.

They may also "perform indentically" if you have altered some other
'hidden variable' for the above without saying so. But the main one
here is physical length, and we are discussing LS cables in a context
where any normal comparisons would be between cables of similar
physical length. People who buy/use/compare LS cables don't normally
proceed on the basis that, "I have to first measure the wave velocities
then move the speakers so as I can compare cables with the same matched
propagation time." So far as I know, the normal process assumes cables
of similar physical lengths as they will have a given amp-load distance
to cover regardless of choice of cable. Hence selecting the lengths
and varying them from cable to cable in this way is not appropriate
for the purposes we are considering.

Indeed, working on the basis of trying to get the same propagation time
when matched would generally be futile as you don't have a matched
system, and the impedances vary all over the place. When you come to
measure the unmatched behaviour the results are nothing like what
the matched case shows in general.

All of what I have said presumes the use of similar operational use
conditions. i.e. same physical lengths, loads, etc. Not "twiddle
unspecified extra variables to get a special case". I am interested
in teh real-world situation, not a special case which no-one would
normally encounter for domestic audio LS use.

Perhaps it will be useful for you to read the first article in the HFN
series when it appears. :-)

All you need to specify them completely is the characteristic impedance
and electrical length (I'm ignoring resistive differences of course).


In practice: I'd also be interested to know how you would be doubling both
C' and L' prime whilst keeping all values uniform across the audio range.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

TCI Cobra interconnects against Chord Chameleon
 

In article , Don
Pearce
wrote:
John Phillips wrote:
On 2008-07-15, Don Pearce wrote:


Are you saying a capacitance meter operating at (for example) 1 kHz
will measure the same capacitance value? What value will it measure?


No I'm talking end-to-end measurements transmitting signal.

Ah! So your point is on the basis of comparing different *lengths* of
cable, chosen so as to get the same summed L and C values. Not simply on
using changed values of L' and C'. In effect you have now added an extra
variable designed to counter the others for the purpose of your initial
statements. :-)

In such a case then the cables can, Indeed, perform the same way when you
scale L' and C' together. The snag is that simply this isn't what people
normally do in the real world. There they have a gap to cable across, so
use a cable of much the same length of whatever type they choose. So your
model isn't dealing with the practical reality relevant to this discussion.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

TCI Cobra interconnects against Chord Chameleon
 

John Phillips wrote:

Jim Lesurf wrote:
Don Pearce wrote:

Speakers are particularly interesting in that there are many cables
available, some of which (from Goertz) have inductance and capacitane
which together come down to around 8 ohms as a distributed impedance.
These cables, despite having enormous capacitance, are essentially
ruler-flat in frequency.

The last sentence is meaningless as it has no reference to the conditions
of use. A 'cable' doesn't have a "ruler flat" response. What you get
depends on the system. And as soon as the load doesn't match the cable
perfectly, the source matters as well. Problem here is that domestic LS
uses a voltage assertion approach with loads that have values that vary all
over the shop.

However my simulations suggest Don is quite right in the sense that a LS
cable with a lower nominal impedance interacts less with most loudspeaker
loads and in general results in a flatter frequency response for the
voltage transferred from the amplifier to the LS.

This does assume the amplifier is stable driving the load.

And just how the LS and the room then deal with that input voltage is,
of course, another matter which may make that flatter frequency response
less relevant.

The concept of cable 'impedance' at audio frequencies is INSANE for a few metres
length.

It's the bulk R, L and C that effectively matter.

Graham

TCI Cobra interconnects against Chord Chameleon
 

Eeyore wrote:

John Phillips wrote:

Jim Lesurf wrote:
Don Pearce wrote:

Speakers are particularly interesting in that there are many cables
available, some of which (from Goertz) have inductance and capacitane
which together come down to around 8 ohms as a distributed impedance.
These cables, despite having enormous capacitance, are essentially
ruler-flat in frequency.
The last sentence is meaningless as it has no reference to the conditions
of use. A 'cable' doesn't have a "ruler flat" response. What you get
depends on the system. And as soon as the load doesn't match the cable
perfectly, the source matters as well. Problem here is that domestic LS
uses a voltage assertion approach with loads that have values that vary all
over the shop.
However my simulations suggest Don is quite right in the sense that a LS
cable with a lower nominal impedance interacts less with most loudspeaker
loads and in general results in a flatter frequency response for the
voltage transferred from the amplifier to the LS.

This does assume the amplifier is stable driving the load.

And just how the LS and the room then deal with that input voltage is,
of course, another matter which may make that flatter frequency response
less relevant.

The concept of cable 'impedance' at audio frequencies is INSANE for a few metres
length.

It's the bulk R, L and C that effectively matter.

Graham


Really - did you not see this? Have a look and tell me you think the
bulk parameters are what matter again. The bulk parameters are
represented by the curve with 1 as the first parameter. The one with 30
comes close to a true cable performance.

http://81.174.169.10/odds/sections.gif

d

TCI Cobra interconnects against Chord Chameleon
 

In article , Don
Pearce
wrote:
Jim Lesurf wrote:


The lumped version presumes a 'short' cable in wavelength terms. For
audio frequencies and runs of a few metres this is generally quite a
decent assumption.


Ah but how short - that is the question. Here is another graph, this
time of the loss of 10 feet of Monster cable into an 8 ohm resistive
load. There are three traces, one in which the lumped elements are all
in one piece, another where they have been split into 20 equal sections
and yet another with 30 sections. Now, which one is "right"? The
legends tell you which is which.

http://81.174.169.10/odds/sections.gif

Afraid the meaning of the text on the horizontal scale isn't clear to me.
However the three plots seem to be within about 0.05dB of each other up to
about 3 x 10E5 whatevers. If I assume the horizontal scale is angular
frequency that seems to be over 45 kHz.

I'd call the cable 'short' on that basis for audio use. However in reality
*none* of the models is "right" in an absolute sense as all models are
approximations which may be suitable for purpose or not. As with your
decision to base arguments on changing the cable length to suit, your
approach here may simply be misleading you by causing you to think in terms
of 'ideal cases' which may not reflect the practical reality.

You might also care to reflect that when I did VNA measurements the LF
behaviour was similar to what you'd predict from lumped models. Main
problem seems to be the way values for CLRG vary with frequency, and that
affects transmission line models as well as lumped ones. :-) Results for
that in the third article if HFN are happy to publish it.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html