Cable Long Runs.Experience,Theroy - Page 2

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Re: Cable Long Runs.Experience,Theroy



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skin effect is for AC only.

Bye.
   Jasen

Re: Cable Long Runs.Experience,Theroy



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High frequency AC only.

MrT.



Re: Cable Long Runs.Experience,Theroy


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DC that varies over time has an AC component.

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thats a common misconception; it applies for all AC, e.g. mHz to GHz

the skin depth in 25C copper at 50Hz is d = 9.34mm. go look at some huge
AC busbars.

This means that for all but the largest transformers, the conductor
thickness is a small fraction of d, so the ac-dc resistance ration Fr ~
1 and both skin and proximity effect can be ignored.

Cheers
Terry

Re: Cable Long Runs.Experience,Theroy



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**Wrong. ALL AC is subject to skin effect.


--
Trevor Wilson
www.rageaudio.com.au



Re: Cable Long Runs.Experience,Theroy



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True, but pretty hard to measure at audio frequencies though!
Do YOU really worry about it Trevor?

MrT.



Re: Cable Long Runs.Experience,Theroy



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**Nope. A trivial exercise to measure at 20kHz. More difficult as the
frequency gets lower.

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**Of course not. That, however, is not what you stated.


--
Trevor Wilson
www.rageaudio.com.au



Re: Cable Long Runs.Experience,Theroy



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Just how much signal loss do you usually measure at 20kHz due to skin effect
Trevor?
Trivial would apply there as well!

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True, and I admitted that, but if you weren't being simply pedantic, you
would have clarified what you actually meant for the benefit of others.

MrT.



Re: Cable Long Runs.Experience,Theroy



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**That would depend on the constrution of the wire and the impedance of the
load. That, again, is not what you stated. You stated:

"True, but pretty hard to measure at audio frequencies though!"

I suggest you study up on electrical theory. Sadly, the equations
surrounding skin effect are not trivial ones. Measuring skin effect,
however, is pretty straightforward.


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**Not necessarily. Unless you know all the factors, you can't say.

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**I don't need. I am merely stating fact. Anyone who cares to dispute what I
say, need only refer to the appropriate text, to see that I am correct. I'll
say it again: Skin effect takes place on ALL AC signals. Whether it is
significant or not, depends on a variety of factors. At 50Hz skin effect can
be a significant loss factor. That does bother electricity supply companies.


--
Trevor Wilson
www.rageaudio.com.au



Re: Cable Long Runs.Experience,Theroy



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the

Why not give us some typical examples then! Not hand selected exceptions.
Or in your case, NOTHING at all.

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I suggest you pull your head in or give us some REAL figures to prove what a
pedantic prick you are being.

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Measuring skin effect,
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Yep, sure is if you have the right equipment, and the results at 20kHz are
pretty trivial.

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We do know typical figures for Hi-Fi equipment. Obviously you want to prove
you are right by using outrageous examples.

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And adding NOTHING usefull at all, simply grandstanding it would seem.
I hope you are feeling suitably smug.

MrT.



Re: Cable Long Runs.Experience,Theroy


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yes they are pretty trivial.

delta = 1/sqrt(pi*f*mu*sigma)

mu = permeability of material = 4*pi*10^-7 * mu_r,

mu_r = relative permeability of material = 1 for copper

sigma = conductivity of Copper


bung in the numbers, and for copper at room temp you get:

delta = 66mm/sqrt(f) where f is in Hz. which aint that hard. at 100C it
changes to 75mm/sqrt(f).

at 20kHz, delta = 0.467mm in Cu at 20C, and 0.53mm at 100C.


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easy. a signal generator and an audio amplifier, along with a length of
non-inductively wound cable (eg a single tightly twisted loop), say
4mm^2. Use a CT (or a small resistor) to measure the current, and slowly
wind up the frequency. watch the current drop, proportional to 1/sqrt(f).

4mm^2 = 5mm OD. At 20kHz, it looks like a 5mm OD hollow pipe 0.5mm
thick. the AC/DC resistance ratio at 20kHz will be about

Fr = R20kHz/Rdc = 5mm^2/(5mm^2 - 4mm^2) = 2.8x

At 100kHz, it will be 6.5x the DC value


in a choke, 4 layers (one atop the other) of 4mm^2 Cu winding will have
Fr = 90 at 20kHz. proximity effect makes things get a lot worse, a lot
faster


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If you were making, say, a 2kW tweeter, you had BETTER pay attention to
skin and proximity effect. normally there aint that much energy at the
higher frequencies....


I'd expect to see proximity effect in mid-range and tweeter crossovers.
Again, the severity is reduced by the lower power levels at higher
frequencies.

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HTH

Cheers
Terry

Re: Cable Long Runs.Experience,Theroy


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I wonder why they use stranded wire for loudspeakkers then...

--

Bye.
   Jasen

Re: Cable Long Runs.Experience,Theroy



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To make it flexible.

MrT.



Re: Cable Long Runs.Experience,Theroy



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Why don't you use hollow core tubes or waveguides?

MrT.



Re: Cable Long Runs.Experience,Theroy



"Jasen Betts"

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**  Bye Bye, Jasen,  bye bye......





.........  Phil



Re: Cable Long Runs.Experience,Theroy


If the calculations on this website can be trusted, the effective skin depth in
copper
at 20kHz is 0.5mm. So with very large diameter cables (high powered systems)
this
could easily influence HF response with low impedance loads.

   http://www.microwaves101.com/encyclopedia/calsdepth.cfm

There is some conjecture about stranded cables. Apparently if the strands are
individually insulated (ie Litz wire) they are effective in avoiding skin-depth
problems but if they are making electrical contact the *bundle* acts similarly
to a
single cable, with some differences.

-Andrew M


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Re: Cable Long Runs.Experience,Theroy



"Andrew M
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** Nope.

Simple, linear inductance has a *much* greater effect on cable impedance
than copper resistance at high audio frequencies.

A 1 inch long copper wire of 1 sq mm cross section has as much inductive
reactance as resistance at 3kHz.




.......   Phil







Re: Cable Long Runs.Experience,Theroy


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I guess that's why they use paired conductors...

-

Bye.
   Jasen

Re: Cable Long Runs.Experience,Theroy



"Jasen Betts"
 Phil Allison
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**  Only works when constructed as a transmission line and terminated with
the load equal to the characteristic impedance.

A twin or twisted pair cable does not match a 4 or 8 ohm speaker load not is
a speaker a defined impedance.



........  Phil







Re: Cable Long Runs.Experience,Theroy


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more likely because 2 wires is a pain....

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rubbish. That wont help reduce the inductance or skin effect at all!

What it will do is prevent ringing due to reflections (electrically
"long" cable runs = distributed circuits) or L-C resonance (electrically
"short" cable runs = lumped circuits). and a simple RC damper would do that.

twisting the wires will reduce the inductance, it should be easy to get
a tenfold reduction in inductance. It will increase the DC resistance
(more cable), but make *NO* difference whatsoever to the skin effect.

no reason why you cant RC terminate a long speaker cable. not that you'd
ever hear cable resonance....for my long cables, C = 2nF or so, and L =
6uH, so Fo ~ 2MHz.


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conjugate matching is a bad idea for power delivery, as the source
dissipates as much energy as the load gets. RF power amps dont do it,
why would audio?


I presume you meant "NOR is a speaker a defined impedance"

thats an understatement.

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do you have a schematic of a really, really good crossover? AIUI its
quite tricky designing a good crossover, theres all the pseaker
resonances, as well as those of the crossover itself.

Cheers
Terry

Re: Cable Long Runs.Experience,Theroy



"Terry Given"

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**  Piss off,  Terry  -   you have no idea what I even said.



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**  Of course a matched transmission line eliminates the effect of linear
inductance.

     What second rate NZ anti-ADHD drugs are you on now ?



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 **  Not even faintly related to my remark.

      Given is an extreme ADHD victim.




.........  Phil







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