[chbot] Measuring capacitance of audio cables

[Trevor Wignall]

I am not so sure of the range of capacitance for audio cable but RF coaxial
cable varies from 100pF/metre for 50 Ohm 0.66 velocity factor cable to
nearer 50pF/metre for 75 Ohm low loss foam dielectric cable. For some
purposes, coax with up to 100 Ohms and even lower capacitance is available.
Audio coax may be higher impedance but not particularly high velocity
factor. I have a digital multimeter somewhere with a capacitance range,
which I could loan (if I can find it :) but decades ago, I built a
capacitance meter based on two 555 timer ICs and using a simple analog
multimeter as a display, which could measure down to 1pF when calibrated
using a bank of reference capacitors. I still have it and (as far as I
know) it still works. (I have in fact used this to determine the impedance
of unmarked coaxial cables by measuring the capacitance per metre.)
A nominal impedance for audio gear is 600 Ohms, but the input ports are
usually much higher impedance than that. The outputs may be less than 600
Ohms at 1kHz, but the impedance may rise with frequency (due to the
required compensation of the amplifiers) so there may be an effect at the
highest audio frequencies - particularly if the cable runs are more than
1m. Of course microphones and turntable pickups may have different
impedances and could be much more sensitive to cable characteristics.
Good luck.
Trevor

On Tue, Sep 8, 2020 at 10:59 AM hamster <hamster at snap.net.nz> wrote:

> Hi Paul,
>
> I doubt you will be able to measure this directly, especially over a meter
> or two of cable. And 1m cables don't act as transmission lines until the
> signals are well in the MHz range.
>
> Assuming a 300 ohm source impedence, and 100pF cable capacitance, the -3db
> frequency is around 5 MHz.
>
> (Rough numbers from
> https://www.diyaudio.com/forums/parts/285128-measuring-capacitance-rca-cables.html
>  )
>
> However, I can either lend or you can come around and use my Analogue
> Discovery 2. It has a Vector Network Analyzer function. that injects a test
> signal and lets you measure gain and phase of the thing under test.
> https://reference.digilentinc.com/learn/instrumentation/tutorials/waveforms-network-analyzer/start
>
> If you want, I can get something like
> https://www.jaycar.co.nz/2-way-phenolic-rca-socket/p/PS0263 to make up a
> test jig ( 300 ohm source resistor, 10kOhm load resistor, test points).
>
> I just worry that getting a "no result" is very uninspiring, and we all
> expect that pretty much all RCA cables are much like each other.
>
> Mike.
>
>
>
>
> On 08.09.2020 09:38, Paul Willmott-Dalton wrote:
>
> Hello all
>
> I am trying to support my 14yo boy with his homework but am a bit out of
> my depth. He is aiming to demonstrate understanding of the scientific
> method. To do this he wants to make a comparison of the electrical
> properties of different brands of audio cables. His hypothesis is that
> there is no difference between 'audiophile' cables vs cheapies.
>
> I under that folks talk about capacitance being a variable. I'm told that
> this will be very small in any cable and potentially difficult to read (in
> the tens of pF range).
>
> Does anyone out there have the equipment necessary to do this? If so,
> could we bring some cables to you for testing?
>
> Kind regards
> Paul Willmott Dalton
>
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