75 and 50 Ohm cable ?

[jim]

Hi all,

At work I had a questions about these cables.
How did they come to these values, why 75ohm and 50ohm and not for example 35 ? or 86 ?

Thanks,
Jim

[Claude Abraham]

It's been a while, but I remember 75 ohms as being very close to the radiation resistance of a half-wave dipole antenna, which is around 73 ohms. A 75 ohm cable provides a good match with such an antenna. The 50 ohm cable provides lower loss, or less attenuation per unit length. I seem to remember reading that 52 or maybe 53 ohms provides the lowest signal loss. The equations are quite involved. Now, I'm probably going to spend time searching for the details since you've got me wondering. Anyway, off the top of my head, that's what I remember. If I find more details, I'll post them. Best regards.

[Christer]

Claude,

that is interesting. As I understand you say there is actually an
optimal characteristic impedance higher than zero ohms that
gives minimum attenaution. I am not good at transmission line
theory, so forgive some possibly stupod questions. Would this
optimum impedance be the same irrespective of the signal
frequency? Would it depend on the relative ratios between
R, L and C in the cable? I suppose it might be that L /C ratio
can't be choosen arbitrarily, but I also suppose we could get
the same impedance by for instance lowering L and C and
increasing R. Would that make any change to the optimal
Z value?

[Sch3mat1c]

The reasons range from attenuation (power loss), to um... I forget. Well there are many good reasons and it's mathematical. I know I have a paper around here somewhere but can't find it amid all my other images. John Stewart of rec.audio.tubes sent the scans FWIW.

Tim

[Claude Abraham]

Actually, I was kinda close, but not quite there. The 75 ohm cable offers the minimum attenuation (some references say it's 70 ohms, others say 77 ohms). A 30 ohm cable offers the greatest current density. The 50 ohm value is a compromise between the two. Also, in WW2, there were antennae having around 50 ohm radiation resistance, making 50 ohm cable a desirable match. Now, if I can only find that book of mine with the equations showing the math. I don't even remember which book has it. Best regards.

[Christer]

Maybe slightly off-topic, but since it is an audio forum, what is
the (supposed) characteristic impedance of phono connectors?
Since some people seem to think impedance matchin matters
also for audio, it would seem sensible to use cables that match
the connectors.

[Jax]

The phono connector impedance is undefined and not a controlled parameter in production.

The characteristic impedance is of little importance at audio frequencies anyway, you have mismatches at the transmitting and receiving end so choosing a correct phono is rather meaningless.

That said, it may be useful to terminate cables at high (>100kHz) frequencies to avoid eventual problems with reflections. Having a zobel network at both the amplifier output and speaker end makes sense.

[Frank Berry]

The following link answers the 50 ohm/75 ohm question.

http://www.rwonline.com/reference-ro..._1_nov_5.shtml

Happy reading.

[Christer]

Originally posted by Jax
The phono connector impedance is undefined and not a controlled parameter in production.

The characteristic impedance is of little importance at audio frequencies anyway, you have mismatches at the transmitting and receiving end so choosing a correct phono is rather meaningless.

That said, it may be useful to terminate cables at high (>100kHz) frequencies to avoid eventual problems with reflections. Having a zobel network at both the amplifier output and speaker end makes sense.

According to normal engineering, yes it shouldn't matter. Since
many people claim different phono connectors sound different
and cables sound different and some even use 50 Ohm termination
it could perhaps matter, assuming this is not all just psychology.

[Jax]

Useful reading:

http://bwcecom.belden.com/college/te.../ciocahalf.htm

A 50 ohm coaxial cable has a characteristic impedance of thousands of ohms at 20 Hz

If there are differencies between different phonos, I would say it's contact resistance, capacitance and dielectric properties that differs.