How does one work out cable capacitance?

[realysm42]

Considering trying out a passive preamp and don't want to waste time.

Please can you tell me how to do this and is there anything else I need to be aware of, ie connectors, plug type etc?

Thanks bros.

[Stratmangler]

You thinking of trying out the Tisbury?
I've placed an order for a 50 k/ohm three input jobbie, so I'll hopefully get to hear it sometime next week.
The cable capacitance shouldn't badly affect things unless you have really long interconnects.

[realysm42]

Cool, thanks.

I'm currently scoping a number of options, and that's one of them.

I'm actually considering getting a hypex ncore jobby sometime in the future and so will need an preamp for them.

[Firebottle]

A rule of thumb figure for good coax is 100pF per meter.

You may find this interesting http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/skincoax/page8.html

:cool:Alan

[Reffc]

Just use a standard screened 75 Ohm interconnect as the geometry of most result in capacitance of between 65 and 75pF/m. I'd consider 100pF/m quite high for a good songle cored coax interconnect but it depends where it's being used.

Do not use long ICs with passives. Remember, the source voltage has to do all the work and the since output impedance from a passive can't be as low as a buffered active, you can end up causing a low pass ffilter effect when using longish ICs on the output from the passive. You need to check the imput impedance of your power amp to work out if longish ICs will be an issue. Rule of thumb...keep them screened, keep them short where passives are concerned would be my advice.

[Oldpinkman]

At the risk of stating the obvious you can't work it out. I suppose you could pray for devine inspiration. You can either ask the manufacturer or measure it with an mm with a capacitance measure (remember to zero it properly first). Measure a metre. Once you know the value per metre you can work it out for any length.

[Reffc]

You can work out capacitance from cable geometry providing that you know what the dielectric materials used are and the dimensions/materials used for the conductors and shields are.

Here's a link from the Mogami site which provides a useful guide.

http://www.mogami.com/e/cad/electrical.html

You cannot measure cable capacitance using a multimeter!

[Barry]

You can work out capacitance from cable geometry providing that you know what the dielectric materials used are and the dimensions/materials used for the conductors and shields are.

Here's a link from the Mogami site which provides a useful guide.

http://www.mogami.com/e/cad/electrical.html

You cannot measure cable capacitance using a multimeter!

You can if the meter has a capacitance range, though I would trust the figure quoted by the manufacturer.

Calculation of the capacitance from cable dimensions and material properties of the dielectric will usually agree within 10% of the measured figure, because it is difficult to know the exact relative permittivity of the dielectric filling.

Sometimes the manufacturer will quote the velocity factor (expressed as a percentage of the speed of light), from which the relative permittivity (epsilon) can be inferred: epsilon = 1/(velocity factor)2

Agree that cable with the lowest capacitance/unit length should be used.