This guy has a whole bunch of information on how and why the twisting
was done:
https://the-electric-orphanage.com/wp-open-wire-transpositioning-systems/
Credit to Google University :-)
73, Mike W4EF..............
On 12/31/2023 12:34 PM, Jim Brown wrote:
On 12/31/2023 7:55 AM, Jim Lux wrote:
I suspect the popularity of 600 ohms is because that is the standard Z
of an open wire voice/telegraphy pair from the 19th century. And*that*
is probably determined by the physical size of insulators, with the
wire size chosen for the mechanical properties to accommodate the pole
to pole span.
YES! I did some research on this a couple of decades ago. Twisted pair
is well known to have a powerful rejection of noise/crosstalk. Since
the earliest days of telephony, long distance (cross country) lines
were run on power poles, below the lines carrying power. 60 Hz was the
only noise source, and twisting to reject that very long wavelength
was achieved by crossing over the pair every few poles.
All those 600 ohm transformers in telephones and such: even though
they’re actually fed by twisted pair, which is almost certainly in the
100-150 ohm range.
YES again. The Zo of common audio twisted pair is in the range of 70
ohms. CAT5 and digital audio cables are in the range of 100-110 ohms,
which is about the limit of practical cable pairs. CAT5, by the way,
is excellent transmission line, with the four pairs twisted at
different rates to minimize crosstalk between them.
600 ohm matching has not been used in pro or consumer audio since
solid state took over from hollow state devices in the '70s. Audio
circuits use a very low-Z source impedance and a high-Z load
impedance, so that very little current flows.
73, Jim K9YC
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