AudioBanter.com - View Single Post - Speaker wire

On 19 Sep 2003 16:32:15 GMT, (Stewart Pinkerton)
wrote:

On 18 Sep 2003 16:14:04 GMT, Don Pearce wrote:

On 18 Sep 2003 14:36:22 GMT, (Stewart Pinkerton)
wrote:

On Wed, 17 Sep 2003 18:09:09 GMT, (Bob-Stanton)
wrote:

(Stewart Pinkerton) wrote in message

Goertz
MI has very low impedance, around 6-8 ohms, and very high capacitance,
while the classic spaced construction cable, 'balanced' FM antenna
feeder, has 300 ohm impedance and very high inductance. Agreed that
we're talking a lot less than milliHenries here.

Actually, if the Goertz cable (or any other) is terminated in it's
characteristic impedance, it will not look capacitive or inductive to
the amplifier. It's terminal impedance will be a pure resistance.

Yes, but that isn't ever going to happen with a loudspeaker, except at
a very few specific frequencies.

True, but using a decent 8-ohm cable even with a real speaker will
yield an impedance (forgetting the speaker itself) that is very, very
close to a resistive 8 ohms.

No, it won't. Anyone who has dabbled in radio will tell you that you
need at least tolerably close load matching to get anything like a
proper resistive transmission line. Now, since the amplifier certainly
won't have anywhere close to 8 ohms source impedance, and the speaker
won't be anywhere near to an 8 ohm resistive load over the vast
majority of its working range, you are in fact back to the lumped
capacitance model.

I do mean close. With the line being a minute fraction of a wavelength
long, You would be very hard pressed to measure the speaker alone as
any different to the speaker plus line (once the line delay has been
normalised) even at the frequencies where the speaker is quite some
way from 8 ohms. Normal speaker cables, around a couple of hundred
ohms, can make a measurable difference - a fact attested to by the
instability of some amplifiers using them

What I am really trying to do here is dispel the myth that such cables
(like the Goertz) are in some way capacitive when driving a speaker.
The fact is they aren't.

It certainly will not be the highly
inductive load of a traditional twin-flex speaker cable. Of course
some amplifiers need that additional inductive load to stay stable,
but that is another story.

A reality check will indicate that a few dozen microHenries (as you'd
get for ten feet of 'zipcord') hardly constitutes a 'highly inductive
load', even at 20kHz.

Inductive enough to pull a poor amplifier back from the brink of
oscillation, though.

d

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