microphone cable as testleads?

[hirada]

For low signal measurements I would like to make some shielded test-leads. The idea using microphone cable, as compared to f.e. RG316, is just the fact, that they are not only screened, but also twisted. And of course, having two cores, you only have one cable, maybe slightly larger in diameter, as opposed to having to use two when using RG316.

But I am not sure, what generally makes up a good test lead, CAT ratings aside, as those do not matter here. Of course there is mechanics (how thick, durable and flexible) a cable is, but those are rather easy to determine and personal preference.
Electrically however, there does not seem to be too much difference, just a quick shot:

Cicada:
Wire: 148 Ohm/km
Screen 30 Ohm/km
capacitance wire/wire(pf/1m) 120
capacitance wire/scree(pf/1m) 65

RG316
Wire 110 Ohm/km
capacitance wire/scree(pf/1m) 105

Insulationresistance is 1GOHm/km or above for both. Going larger in diameter would improve specs of the mic cable. 

Now the capacitance would limit HF measurements, but that problem is shared by any shielded cable and is not planned as of yet anyway.

So would that be feasible or am I missing something very fundamental? I am aware of specialized cables like the ones made by ab-precision, but 100+ Euros just for the leads is a little over the top for me. Though I would wonder, what preciesly makes them worth the money.

Happy for any education and of course alternative ideas

Thanks

[bob91343]

First there is inductance.  It's only important if you are driving a low impedance load or if the frequency is high.

In general, that's the main thing.  The parameters of microphone cable are probably less well controlled so there may be considerable variation.

Coaxial cable is useful if it's terminated in its characteristic impedance.  If not, it doesn't have to be coaxial.  In general, coaxial cable is stiffer and thus not as handy for test leads.  Microphone cable is intended to be flexed so it has that going for it.  Most of my coaxial test leads break at the end so they are higher maintenance.

[hirada]

If not, it doesn't have to be coaxial.

Then how would we archive shielding, if it wan't coaxial?

The parameters of microphone cable are probably less well controlled

That is a very good point, but my hope is, when using some respected brand cable for studio gear, not "high end" audio, the variations to the specs provided are not that much larger than between different brands of RG316. But who knows. The HF guys shurely are more sensitive towards specs than an audio engineer trying to find the proper mic for a recording.

First there is inductance

As of now, I yet have to find any cable documentation, that actually  does specify the inductance. But is inductance worse with a coax cable than with an ordinary test lead? I would have been under the impression, since the returnpath is wrapped around, it may even be a little lower than with a single cable? While having instead the capacitance penalty?
And for microphon cables, the twisting may add another barrier to mitigate induction?
But obviously, the whole induction thing is still somewhat opaque to me.

[RandallMcRee]

First there is inductance.  It's only important if you are driving a low impedance load or if the frequency is high.

In general, that's the main thing.  The parameters of microphone cable are probably less well controlled so there may be considerable variation.

Coaxial cable is useful if it's terminated in its characteristic impedance.  If not, it doesn't have to be coaxial.  In general, coaxial cable is stiffer and thus not as handy for test leads.  Microphone cable is intended to be flexed so it has that going for it.  Most of my coaxial test leads break at the end so they are higher maintenance.

Hmmm. I disagree with everything here, I guess.
First why is inductance a big deal? OP said low-level not RF.

Shielding is usually for noise. Microphone cables are all-about low noise, so check mark on this one.

Good cables like, e.g. Mogami (my choice) are well-specified.
http://www.mogamicable.com/category/bulk/microphone/quality_balanced/
Edit: Link to cables I use for measurements:
http://www.mogamicable.com/category/bulk/microphone/quad/
(The mogami site seems to have been infected by audiophools recently)

Also audio cables in general often have what is called a star-quad configuration which gives you both shielding and two twisted pairs. Twisted pair is very helpful in certain noise situations (google it).

The OP seems confused as to what is a coaxial cable--this is only one certain type of shielded cable and not the most versatile! 
https://en.wikipedia.org/wiki/Coaxial_cable

For low-level measurements (and you did not specify bandwidth!) thermocouple and triboelectric effects can be problematic. Good mic cable controls for these, as well.

Belden is another manuf which can fit the bill. Just stay away from audiofool sites.

[bob91343]

I will clarify what I mean by coaxial.  I mean where the center conductor is in the center of the shielding.  Ordinary shielded cable isn't coaxial by that definition.  The word coaxial means sharing the same axis, not one inside the other.

[gcewing]

Also the term "coaxial cable" usually refers to a cable designed to have a specifiied characteristic impedance, for RF work. A shielded audio cable might have a similar-looking construction, but it wouldn't be called a coaxial cable.

[Wallace Gasiewicz]

I don't know what you are trying to measure.
I assume it is KHz freguencies.
The coax that we use is NOT good at audio freq. It is not designed for that. There are eddy currents that go deep into the copper wire and change the characteristics of  coax at low freq. There is a reason audio does not usually use coax for any length of wire.
Coax design mostly takes into consideration the "skin effect" This is not the case at low freq when there are other electrical forces in the wire at work.
In other words 50 ohm coax is not 50 ohm at audio freq. Besides that, audio "line" is usually 600 ohm.
Mic cable is probably better at audio freq than our beloved COAX. Shielding in mic cable is to reduce interference, not to create a RF environment to "transmit" along the line
Twisted pairs have been used since early telegraph communications to minimize interference, the received outside interference is cancelled by itself due to the twisting of the wires.

[bob91343]

Nothing is ever simple, especially in electronics.  If you have the parts, try it and see if it works well enough.  But never forget or ignore the details, the theory.  Many of us prefer the seat of the pants approach but in fin, the books have most of it right.

All of us are lazy, and studying is too much work.  Decide for yourself what you want to learn and how you want to learn it, but don't be afraid of the math.

Asking questions on a forum like this has its place, but much of the time we can answer our own questions.

[RandallMcRee]

So would that be feasible or am I missing something very fundamental? I am aware of specialized cables like the ones made by ab-precision, but 100+ Euros just for the leads is a little over the top for me. Though I would wonder, what preciesly makes them worth the money.

So this question was not answered, I don't think. The characteristics that ab-precision are addressing are:

• gold--low corrosion and low thermoelectric potential with copper
• PTFE insulation-- no or very small triboelectric effect
• crimped-- no solder to cause (again) thermoelectric potentials
• construction-- twisted pair, shielded for noise considerations
• 18AWG-- low ohms for maximum DC precision

Did I miss any? Anyway, all those things add up to an expensive cable. Edit: saw that its 18AWG, added that.

[hirada]

Thanks again to all for the education, I have learned quite a lot. Sorry for having forgotten the bandwith, it is indeed DC ~ 50kHz. Probably slightly less.

And thanks for the heads up with coax. As the shielding is surrounding the core(s), for me all cables with an outer shield were considrered to be coax. Now I know better. And indeed I have found vendors that also do list the impedance as well.

Very helpful, thanks again