Re: Characteristic Impedance for John Kennaugh/nTaul Andersen
From: John Kennaugh (JKNG_at_kennaugh2435hex.freeserve.co.uk)
Date: 08/11/04
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Date: Wed, 11 Aug 2004 21:54:39 +0100
Androcles writes
>http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/skincoax/page8.html
>
>My last word on the subject of "Characteristic Impedance".
>Fight it out amongst yourselves. The curve speaks for itself.
Congratulations! Against overwhelming odds you have managed to find
someone who knows less about the subject than yourself. Going through
the article:
"3. Effect of varying the cable impedance."
He is not varying the 'cable impedance' in any accepted meaning of the
word. It is clear he has little idea what that is.
"In practice many types of coaxial cable used as domestic audio
interconnects....."
In general you don't use coax in audio connections. He is talking about
'screened lead' as can be seen by the 'impedance' values in the table.
The whole key is in the phrase:
"..... we would expect the combination of a 600 Ohm source and a 25 kOhm
load....."
He is "terminating" a 75ohm? coax with a 25000 Ohm load. As near an open
circuit length of cable as makes no matter, and whether it is coax or,
as I suspect, audio screened lead (or a length of mains cable come to
that) it is simply a capacitor. The frequency response is that of a 600
ohm with a capacitor across it. It has nothing whatever to do with coax,
characteristic impedance or any of the other things we have been
discussing.
To be more precise a piece of coax, open circuit at its end, will look
like a capacitor at frequencies where the two way delay is much less
than the period. 2.5m is about 14ns delay which is the period of 72MHz.
At audio frequencies it is therefore just a capacitor.
Where he talks of 'delay' he means phase shift.
I suspect he is in the HiFi business.
-- John Kennaugh to email convert the number from hex to decimal
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