What's the capacitance of a wire going from a jack to switch to jack?

[Mark Hammer]

There is much discussion about true-bypass vs buffered FET-switching, and such.  And there is much discussion about cable capacitance and this brand vs that one.

Of course, when we calculate the potential for tone-sucking across a pedalboard of 10-12 pedals, we tend to think of the total cable distance travelled in terms of the properties of the cable we can see: i.e., the patch cables.

So what I'm wondering is this: what does the cable going from the input and output jacks to the stompswitch contribute to the equation?  Typically, it won't be the same stuff used for patch cables, so you can't just add up how many inches of it there are and multiply by the spec'd cable capacitance per line foot.

[earthtonesaudio]

Shielded is easy, just look up the cable spec and multiply by the total distance the "whole" cable travels (before pigtails).

Unshielded/standard single conductor is more difficult, because the first conductor "seen" by the cable is probably the wall of the enclosure, which is at random and unequal distances from any given part (unless you do your wire runs in perfectly straight lines).  In this case, you'd multiply the length by the permittivity of free space, about 9pF per meter. (in other words, almost nothing)

[George Giblet]

I would have given it about 50pF/m for single conductors close to the chassis, which is about 5pF in a typical box, not much.

If you use coax wiring that would bump it up to say 150pF/m, or about 15pF/box.

As a rough rule it adds the same amount of capacitance as a length of cable equal to the total length of your pedal board, when string out in line with taut cables.

[R.G.]

The reason capacitive sensors work is that the capacitance from any conductor to the rest of the universe is a sensitive detector of the exact physical arrangement between the conductor and everything else. Unfortunately, it's only simply calculable in a minority of cases where the geometry is simple, as in coax or a wire very far from everything else (i.e. isolated in free space) or a constant distance from a conductive plane (enclosure wall) or another wire (twin lead TV cable being an example). If it isn't one of those, the math gets hairy and the only thing to do is put a bound on it or measure it.

Fortunately, the capacitance per meter or foot for isolated hookup wire is smallish, as both earlier responders mentioned. It's likely to be on the order of a factor of ten less per length than coax, depending on the coax geometry. I could go look up the equations and will if you'd like more info.

[Mark Hammer]

I guess the question underneath the question is: How much does the cumulative capacitance of patch cables matter, when compared to the cumulative capacitance of connecting wires inside the pedals themselves?

If it is likely to be negligible, then forget about it.  On the other hand, if there is something about layout that people are forgetting, and what looks innocent enough is actually sucking crispness away from multi-pedal true-bypass setups with seemingly short (and presumably risk-free) cables, then maybe we oughta talk about it. 

This is NOT a diatribe against true bypass.  It's really more of an inquiry regarding whether there is something people may not be doing that could conceivably improve the performance of TB.  It's just that we talk about layout and such as if the only things to be concerned with beyond neatness were HF clock bleedthrough, or noise from some lines passing too close to each other.  I'm just curious as to whether there are other things to be concerned with that could be easily attended to.

[puretube]

Those dirty little wiring capacitances are more hurtful inside the circuit, especially in high impedance circumstances,
in dense PCB layouts, when they try to "Miller" you... 

[R.G.]

I guess the question underneath the question is: How much does the cumulative capacitance of patch cables matter, when compared to the cumulative capacitance of connecting wires inside the pedals themselves?

Hmmm... Lemme go look some stuff up. Where the heck is my 35-year-old copy of Millman and Taub? 

The old radio amateur gimmick cap is a short length of twisted pair, about 24 gauge hookup wire, polyvinyl insulated. That's about 1pf per inch, and that's for two wires parallel and separated only by their insulation. Wire roaming around in 3-space not parallel with another conductor and only coming close at odd points will be less, perhaps much less as there's a square law involved. So calling internal signal wire 0.3 to 0.5pF per inch would most likely be conservative.

I found numbers for RF coax: RG178 is 39pF/foot, 105pF/meter. Simple single conductor shielded audio cable like you'd find in stereo cables is as much as 200pF/meter, 75-80pF/foot.

It looks like shielded cable, as used for interconnects, is about 10X as bad as the single cable inside the box. So a 4" shielded shortie between boxes is the capacitive equal of 40 inches of wire inside the box, perhaps a little less. I would say that internal cable is negligible.

If it is likely to be negligible, then forget about it.  On the other hand, if there is something about layout that people are forgetting, and what looks innocent enough is actually sucking crispness away from multi-pedal true-bypass setups with seemingly short (and presumably risk-free) cables, then maybe we oughta talk about it. 

Actually, we probably ought to talk about it. The few inches of coax spliced in a number of times on the way do add up, especially if your pedalboard is laid out for easiest foot access, not in signal order, because the inter-pedal cables easily get to 1-2 feet if that happens. Of course, putting even one buffer in the chain overwhelms the rest of the capacitances, so maybe the right thing is to only consider cable capacitance until you hit a buffer. Don Tillman's active cable looks pretty good in that respect.

I'm just curious as to whether there are other things to be concerned with that could be easily attended to.

And right you should! 

[Ed G.]

This is going on a big tangent here, but this post made me think of my job.
I install dynamic positioning systems on oilfield supply vessels. Big 280' boats.
We were getting 110VAC on a cable that went from the pilot house all the way to the rudders. It was coming from the Rolls-Royce steering control box, the prints were inaccurate and we tied in to one leg of 220V. Lucky nobody got hurt.
The cable should have had only 10v DC on it, it was a rudder feedback signal for the DP. Basically an I/O card sends the feedback voltage to a pot on the rudders, the voltage tells you what angle the rudder is at.
When we figured out what was going on and powered down the box, there was still 110v on it! We couldn't figure it out...then it started to slowly go down.
The 400 foot cable (two conductor, 16 gauge, with a shield) was acting as a huge capacitor!

[George Giblet]

> Unfortunately, it's only simply calculable in a minority of cases where the geometry is simple, as in coax or a wire very far from everything else (i.e. isolated in free space) or a constant distance from a conductive plane (enclosure wall) or another wi

The capacitance tends to vary quite slowly as the physical arrangement changes.  It's effectively being logarithmically compressed so physical changes of a factor of 10 compress down to a factor of 2.  With a wire in an enclosure, as you move closer to one surface you move further from another.  I you think about this as two capacitors one capacitor gets smaller and the other gets larger, so the variation in the sum are further smashed.   The big variation comes about when the cables are pushed very close to a surface (less than two conductor diameters), this makes the capacitance go up quickly, with wires have dielectric sheath which further amplifies the effect.

[R.G.]

> Unfortunately, it's only simply calculable in a minority of cases where the geometry is simple, as in coax or a wire very far from everything else (i.e. isolated in free space) or a constant distance from a conductive plane (enclosure wall) or another wi

The capacitance tends to vary quite slowly as the physical arrangement changes.  It's effectively being logarithmically compressed so physical changes of a factor of 10 compress down to a factor of 2.  With a wire in an enclosure, as you move closer to one surface you move further from another.  I you think about this as two capacitors one capacitor gets smaller and the other gets larger, so the variation in the sum are further smashed.   The big variation comes about when the cables are pushed very close to a surface (less than two conductor diameters), this makes the capacitance go up quickly, with wires have dielectric sheath which further amplifies the effect.

I was speaking in general, not just for a wire in a box. In addition, there is the capacitance to the other wires it passes along the way in the normal spaghetti style of wire routing in pedals. Those other wires also act as variable shields depending on the impedances connected to them at one or the other end, adding to or shielding from the enclosure or one another.

I believe the statement is correct as written. It's only for a quite simple geometry where the calculation is feasible. 

[George Giblet]

> I was speaking in general, not just for a wire in a box.

Yeah, in general there not many closed form solutions for anything in electromagnetics.

[R.G.]

Yeah, in general there not many closed form solutions for anything in electromagnetics.

Dead correct on that!