How to measure capacitance of cable?

[DiamondDog]

The question is basically the title- what are the ways of measuring the capacitance of cable? I'm redoing my cabling, and the guitar to board and board to amp cables are both 20 foot+ (6.5 metres).

That's long enough for it to be an issue, so I may as well do it properly.   

[GibsonGM]

Offhand I'd say to use a capacitance meter....

There are a few other ways, but it gets complicated. 
Include the cable in a circuit with known resistance and measure a frequency cutoff or time constant, things like that.  But it would be so low as to be very problematic to make practical!  A cable won't give you 10's of uF's, which would make such things do-able.

Ultimately, I'd just accept the extra pF you'll get from a 20+ foot cable, and correct for it accordingly (maybe a 'conditioning box') ahead of your first stompbox??)

[R.G.]

There are a couple of things involved. One is cable capacitance, which is an annoyance to be minimized. The other is buffering, which makes cable not matter.

Coax cable makers specify their cable capacitance. You can just look it up if you are making cables up out of known cable. For example, Canare lists their GS-6 cable at 49pF/Ft or 160pF/m. Their GS-4 is 47pF/ft, 154pF/m. If you know the cable maker and type, you measure the length of the cable and calculate.

If  you don't know the maker or cable type, you measure. The value will be over 20pF/ft, as it is quite difficult to get cable better than that. Multiplying length times 20pF/ft gives you a floor value. Chances are you will measure from two to ten times that number.

When you put a buffer on a cable at the source end, it provides enough current drive to make the cable capacitance essentially not matter. So from your pedalboard to amp, if the last pedal that the signal touches is buffered, the cable to the amp will be driven with negligible loss. There are buffers you can put on your guitar to make that cable capacitance not matter too.

One of the Achilles heels of true bypass switching is that it makes the bypassed signal unbuffered, so it is prone to the problems of cable loss.

[ashcat_lt]

...but HOW to measure an unkown cable?

One probe on the tip of the plug, the other on the sleeve.  Both at the same end.  Other end not connected to anything.  Make sure the cable is laid out as straight as you can get it with no coils or sharp bends.  Meter on appropriate capacitance range.

[R.G.]

...but HOW to measure an unkown cable?

Same way as measuring any unknown capacitance. It's only an interesting problem if you don't have a meter that reads down in the 10pF range and up. 

[Brymus]

This got me curious...
My 8ft cable I always use measured 24.6pf  ,pretty low I guess.
My 12ft cable I also use alot measured .5nf ,not nearly as good.
The 8ft one is a premium Musicians Friend cable (forget the brand)
The 12 is a Realistic POS,I guess the hype is somewhat true in this regard.

[Paul Marossy]

One of the Achilles heels of true bypass switching is that it makes the bypassed signal unbuffered, so it is prone to the problems of cable loss.

Which Pete Cornish likes to point out here: http://www.petecornish.co.uk/case_against_true_bypass.html 

[PRR]

> ways of measuring the capacitance of cable?

Do you need the exact value? Or just a "how bad can it be?" value?

For coaxial ("shielded") cable useful in audio:

You won't go far wrong assuming 30pFd per foot.

It may be as low as 20pFd/foot. Which is better. But not a heap better.


> a circuit with known resistance and measure a frequency cutoff

Not trivial. Any measurement has added capacitance. Some ACVMs have quite a lot.

A tube amp input may also have >100pFd of grid capacitance.

> Make sure the cable is laid out as straight as you can get it with no coils or sharp bends.

In "normal use" a guitar cable is usually not-straight. And that really does not matter unless you are a precision-geek. That's the advantage of coax. External fields (even its own looping back) hardly matter.


What do you really need to know? Whether full-range audio will pass.

As R.G. says, buffering reduces the problem. So much that the source is more important than a few feet of cable.

Take a few common sources:

A) Chip direct output -- this is under 1K source impedance. It can pull nearly 10,000pFd to 20KHz. This is 300 feet of cable. A simple opamp chip with a few hundred ohms in series will easily feed a cable longer than any you are likely to have.

B) Tube output or anything with 100+K pot on the output -- now we have ~~50K source. It can pull only 200pFd to 20KHz. This is 6 feet of cable. Or conversely if you must have 20 foot cable, response will be droopy by 6KHz.


While electric guitar has little above 6KHz that we want to hear, we sometimes want "all" the 3KHz-6KHz output we can get. So a droop at 6KHz is starting to take the string-zing off the signal.


> My 8ft cable I always use measured 24.6pf  ,pretty low I guess.
> My 12ft cable I also use alot measured .5nf ,not nearly as good.

I think your C-meter has the same problem as my new calculator: I can't see the danged decimal point! It is unlikely that 8 feet of cable is 25pFd; highly likely that it is near 240pFd.

The 0.5nFd is of course 500pFd. We would expect 12 feet of cable to be ~~360pFd. Realistic does not use much copper in their cables, and we'd expect C to be lower. Maybe they got some very high E plastic.

[DiamondDog]

Thanks for all the info, folks. Greatly appreciated. 

Really what it boils down to for me is it may not be the most critical thing in the world, but why accept a compromise when I don't have to?

I've already got the buffers in place at effects chain entry and exit. I'll take my DMM with me next time I'm at my endorsee's warehouse for cabling, and come up with a set of numbers- and see if there is an audible difference. That for me is the final test.

Again, thanks!

[petemoore]

  A stretch of wire is a capacitor...cool.
  It can also be a pickup...drag.
  Otherwise if I needed 100pf, just get out my crappiest cable, draw out 100pf worth...adjust as necessary...
  About the only place I ever see coil referred to in effect schematics is for wah and pickup simulator [outside of power supplies.

[brett]

Hi
Because we are DIYers, it seems sensible to point out that building a DIY capacitance meter isn't too hard.

If you know the resistance (R) and inductance (L) of a tuned RLC circuit, you can calculate C if you know the resonant frequency.  Not as hard as it would seem.  Do you have a signal generator?  If you do, you can pick R and L so that with C=500pF (a guess to start) the system resonates at a few kHz.  By varying the frequency until the throughput is a maximum (measure AC volts with a DMM), you can find the exact resonant frequency, and therefore find a value for C.

I've used a similar method for years (a fixed R and C to measure L). Use the search function to find my post about a DIY inductance meter.  I expect that you'll get a similar accuracy - about +/- 20% of the true value.
cheers

[newfish]

Has anyone seen the Zerocap product?

http://www.aqdi.com/zerocap.htm

This is not a commercial 'plug' (no pun intended) for this product - I saw the advert for the ZC the other day.

Whilst there *is* an audible difference in the sound clips (in the advert, naturally), I've been wondering how the thing works.

Given that the ZC allows DC through (one demonstration allows a flashlight bulb to be supplied with battery power through a ZC Cable - so there are clearly no input / output caps in there), my first thought was some sort of inductance 'happening'.

This might be an excellent DIY project to answer all long cable / true bypass arguments.

0.02

[Lurco]

NIC? Gyrator?

[CynicalMan]

ZEROCAP Device Specifications

      Power Requirements:   Type A23 Battery, 12VDC
      Signal Loading:   1 million ohms
      Weight:   1.2lb (20 foot cable)
      Warranty:   One year parts and labor (see User Manual for full warranty)
      Support:   Contact us for free support

So it does have a 1Mohm input impedance. Could be negative capacitance. The little box is closer to the amp, so it must reverse the effect of cable capacitance, not prevent it. I'll bet there's some tone control in there. I would prefer just having a buffer in the guitar.

Looking on their website tech page, I can see some blatant lies. For example, the graph below says that with a 5.6k series resistance with a 352 pF capacitance to ground, the -3dB point is at about 20kHz. It's actually at 81kHz. One of their measurements is wrong. Also the writing below the graph says all other cables sound the same, while up top it says "The higher the capacitance, the more muffled the sound." They claim that a tone control rolled down sounds the same as a high-capacitance cable. Because of pickup loading, a high-capacitance guitar cable can give a high-mids boost, something most tone controls can't do. Finally, a high-capacitance guitar cable does not sound bad. It does not kill all treble, it shifts the guitar's pickup's resonant peak lower, which reduces treble and increases high-mids at the resonant peak. Most amps and guitars can't do this with tone controls, so a cable should be a consideration for tone. Higher capacitance is not bad.

This might be an excellent DIY project to answer all long cable / true bypass arguments.

Way easier to build a buffer. Probably would sound better too.

::done ranting::