How does one measure the capacitance of a cable?

J!m

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#1
So, for a phono line, low capacitance is a good thing. I get that, but how do I measure that? Do I short one end and see what the capacitance is across the center and shield at the other end? Is there some other way (that does not involve 1.8 billion dollars in test equipment)?
 

BlazeES

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#2
Dedicated capacitance meters are pretty dang inexpensive. DMM's have the mode too. Do you own a Fluke?

1580491467098.png

And when you say the phono line, I take it you are referring to the leads running down a tone-arm?
 
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mlucitt

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#3
The capacitance of a cable is normally expressed per foot, no? So the total capacitance is divided by the length for comparison to other cables.
 

J!m

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#4
Ok, so one probe at each end... so I probe the shield at one end and the center conductor at the other? Or, will I be able to measure capacitance just prothe center at both ends?

The meter itself is a separate issue. Right now, I’m just trying to understand HOW to apply the probes to take a reading.

And thank you for walking me through it.
 

J!m

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#5
So I’m clear here: set one probe on center (of RCA) at each end and take a reading.

I’ve never done it, so that’s why I’m asking. And, if I do want to get technical, I suppose the capacitance would change on that center conductor if the shield is floating or grounded at both ends...

And on a litz braided cable, measuring one lead with all others floating vs. with signal and/or ground on others in the braid would also alter the reading, no?
 

Skywavebe

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#6
The capacitance of a cable is determined by insulation material and the size of it and then it is distributed per foot. To get an accurate reading the DVM would have to generate a high frequency signal and then measure it's reactance against a known value to determine a readout. I don't know how accurate that will be in these kinds of meters. The makers of cables make QC tests to make sure their cables are within tolerance and they must have some better equipment to measure that. The problem with cables and the length of them is that you don't have just capacitance to deal with but also a inductor reactance from the wire. These could confuse the meter unless some measures are taken to eliminate the Xl reactance.
I would check with a cable company to see how they do it.
 

BlueCrab

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#7
A Network Analyzer would be needed to make an accurate measurement. Also, to be clear, the measurement is from the center conductor to the outer shield and both connections would be made at one end. Generally a DVM's measurement of capacitance is done at one relatively low freq - 10 to 100kHz. Try it on various lengths of the same cable, plot it per foot, and see what you get. Perhaps start with a 10' cable, measure the capacitance, chop a foot off, and repeat down to 1 foot. Plot the results and show us what you get.
 

BlazeES

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#8
Researching application notes is a great way to learn. Here are two, although not relevant to phono cables specifically:

https://www.circuitspecialists.com/blog/using-a-digital-multimeter-to-locate-a-buried-cable-fault/

https://en.customboards.fi/pages/7-testing-cables-with-a-multimeter

I guess the bottom-line is this: If you're worried about capacitance in an audio cable ... then you're an audiophile.

But I'd love to hear subjective commentary on the relative worth of pf values vs. sound quality ... Popcorn standing by !
 

Gepetto

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#9
I have a 4 wire LCR meter that does a decent job of measuring the capacitance of cable. I have a length of Belden 1649a that has a spec of 16.2pF per foot. The length I tested is 28" long which should have a nominal 37.8pF capacitance. My meter reads 39.9pF, pretty accurate.
 

J!m

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#10
Thanks for the details on this.

The reason I’m asking, is one area sensitive to cables is from the cartridge to phono preamp. That capacitive loading is added to the “nominal” capacitance of the preamp. In a nut shell: the cable change in that location should have the largest audible impact on a system. So I bought it’d be fun to categorize some wire and see how audible it is.

And yes, the inductance and resistance are important as well.

I have several ideas and some are non conventional in the audio world. Anyone remember those CAMAC connectors that Mark Levinson was using? Well I’m not thinking about those but other non audio connectors as well. I may contact a cable company and see if they’d terminate with my connector choice.

Anyway, it doesn’t matter yet right now.
 

Gepetto

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#11
To get a decent, repeatable reading, the area you test it in has to be pretty benign from an RF/EMI perspective. If I turn any equipment on near where I am using the LCR to get a reading, the meter is all over the place since the measurement is such a small capacitance.
 

marcok

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#13
Last year I had to replace the cable of my father' s SME III arm ( broken due to age )
reusing the original connectors . Some considerations about the matter .
All TT manufactures use a 125 cm. long . ( approx . )
It' s the right compromise between the possibility of locate TT and preamp ,
capacitance per meter ,noise and cost ( 4€ )
I bought from my dealer 130 cm . of good cable ( Tasker 89 pF / mt )
Checked with test record and VUs at 8 /10 khz : flat response .
There are some exception : Stanton , for instance .
In this case you must add a 180 , 220 pf on phono input .
At page 28 of Dynaco PAT 5 manual you'll find everything .
See also Quad 44 owner's manual, where it's written that an
error of +/- 50 pF is negligible .

Ciao
Marco
 

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