Wire length

[JPGraphX]

Hi,

Is there a real difference when using longer wire in a pedal ?

Thanks
JP

[mremic01]

Wire has capacitance, and the more you use, the more it adds up. Although usually you have to have something like 20 feet or more for it to make any audible difference. A more serious consideration is noise. They can kind of act like an antenna. You want to keep your wire runs as short as possible, but also routed away from things that might make noise that they can pick up. In most pedals, it's not an issue. In some high gain distortions you might want to use shielded wire on the inputs to keep any noise that that wire might pick up from getting amplified. The only real issue I've ever had was LFO ticking getting picked up in a Boss CE-2 build. I took a piece of copper clad PCB material and connected to ground, then wedged it between the PCB and output jack so it acted as a shield between the jack and the CE-2 PCB. It didn't cut out the ticking entirely, but it made it much quieter.

[psychedelicfish]

Wire has capacitance, and the more you use, the more it adds up.

Not quite, two wires (or any conductors) close together will have a capacitance between them. Long wires will have  inductance (generally a small amount, unless your wire happens to be wrapped around a ferrous core), as well as a DC resistance, the latter affecting your circuit far more than the inductance ever will.

In some high gain distortions you might want to use shielded wire on the inputs to keep any noise that that wire might pick up from getting amplified.

You do want to use shielded wire for the inputs and outputs of any high gain amplifier, but only because the capacitance between the input and output makes a positive feedback, which can cause your circuit to oscillate. Shielding the input and output makes the capacitance from the input/output to ground, rather than the output to the input, which stops the oscillation.

The only real issue I've ever had was LFO ticking getting picked up in a Boss CE-2 build. I took a piece of copper clad PCB material and connected to ground, then wedged it between the PCB and output jack so it acted as a shield between the jack and the CE-2 PCB. It didn't cut out the ticking entirely, but it made it much quieter.

This is probably because your LFO has sharp corners on the waveform. When passed through an inductor or a capacitor, the latter being far more likely in a pedal, these sharp corners are transmitted as RF, which is picked up by your output jack. The grounded copper soaked up lots of this RF, so it didn't reach the input or output. Another fix would be to put a small capacitor from the wiper of the depth control to ground, which would round off the sharp corners of the waveform.

To answer the OP's question, you should keep wires as neat and tidy as possible, partially because of capacitance etc. and partially because it makes things easier to follow when you're trying to debug your pedals.

[mremic01]

Not quite, two wires (or any conductors) close together will have a capacitance between them. Long wires will have inductance (generally a small amount, unless your wire happens to be wrapped around a ferrous core), as well as a DC resistance, the latter affecting your circuit far more than the inductance ever will.

That actually makes sense. I always wondered why we worry about cable capacitance and not cable resistance. Does this mean that you could have larger cables, with more space between the shield wire and the lead, and get less cable capacitance with longer runs?

[Seljer]

Not quite, two wires (or any conductors) close together will have a capacitance between them. Long wires will have inductance (generally a small amount, unless your wire happens to be wrapped around a ferrous core), as well as a DC resistance, the latter affecting your circuit far more than the inductance ever will.

That actually makes sense. I always wondered why we worry about cable capacitance and not cable resistance. Does this mean that you could have larger cables, with more space between the shield wire and the lead, and get less cable capacitance with longer runs?

If you approach the shielded cable as a cylindrical capacitor, the formula for capacitance depends or the ratio of the inner and outer radius. So yes, if you increased the shield and left the center conductor as it was, yes you'd get less capacitance. But if you for example, doubled the diameter of bother the center condutor and the shield the cable would have actually have the same capacitance per unit of length. The formula predicts the precise opposite for the inductance of the cable (so the capacitance goes down, the inductance goes up).

We worry about capacitance because at audio frequencies it has a much higher influence than the resistance (which at most a couple of hundred milliohms per meter for the wire you'd use in a pedal or your typical audio cable).

[PRR]

> not cable resistance

A foot of #28 wire is 0.08 ohms.

Loads are usually over 50K.

A foot of wire driving 50K, we get-out 50000/(50000+0.08) or 99.9998% of what we put-in.

(Also note that the wire inside the pedal is usually MUCH shorter than the external cord; maybe near-equal if only driving those 6-inch jumper cords.)

Wire length is still sorta important. There is capacitance everywhere. To the cases, and to other parts of the circuit. In particular, if output signal sneaks-back to input via capacitance, it will scream (perhaps supersonically).

OTOH, if wire is too short, you can't get the board out/in for debugging, repairs, mods.

> more space between the shield wire and the lead

Yes but the improvement is very slow. A LOT fatter is a little less C.

Guitar cord is an extreme in the coax cable industry. We are in a far corner of very low frequency and high impedance. Most of the demand is for higher frequency. You'd think capacitance would suck worse, but there's C everywhere so cable C may not dominate. Meanwhile inductance, negligible for lo-F high-Z work, gets in the way, as does resistance. So most coax is made with fairly fat centers to reduce R and L (as Simon says).

There's another factor. C is area times the stuff in the space between. We define vacuum as "1.0". Air is so close we don't care. But all the plastics, glass, ceramics, are 3 to 10 (or more). If you pulled the plastic out, the C would go down about 4X. But of course with the least flex the center would touch the shield and short-out. We need some stuffing.

In high-budget work, sometimes they use a pipe and a rod and ceramic disks. If you have one 1" thick disk every 10", the C is about 1.2, where vacuum/air would be 1.0 and solid stuffing would be 3. However you still can't flex it more than a wee bit; it's for radio transmitters where everything is bolted-down solid.

You can "foam" the plastic. The more gas bubbles, the lower the C. Some cables do this. Most notably the old-old TV twin-lead: the "best" grades boasted foam-filled tube spacer between the two conductors. But in coax, the foam crushes easily which leads to shorts.

[JPGraphX]

Thanks all for your nice answers!

[Pojo]

In particular, if output signal sneaks-back to input via capacitance, it will scream (perhaps supersonically).

So maybe twisting the input/output wires together from the pcb to switch in my last build was a bad idea....hmmm. Good info!

[Tony Forestiere]

In particular, if output signal sneaks-back to input via capacitance, it will scream (perhaps supersonically).

So maybe twisting the input/output wires together from the pcb to switch in my last build was a bad idea....hmmm. Good info!

I feel so ashamed about chuckling over this, but I did it too. IC preamp about 30+ years ago. At least I wrapped the ground wire around both of them from the input jack to board ground.  Squealed like a stuck pig.

[amptramp]

I have used 30-gauge wire-wrap wire for a number of places in my builds.  The advantage is that several wires can fit in one PCB hole and the capacitance to everything else is reduced.  Being single strand, it is easy to ensure you don't have stray wire fragments that can get everywhere.