Cable: Does Better Shielding = >Capacitance?

It does, which is why better shielding also requires better insulation between the conductor and shield to compensate for that fact. But high quality insulation isn't necessarily expensive, otherwise TV cable would cost a lot more than it does.

 

+1, too bad Leo Fender went with high impedance pickups instead of low impedance that wouldn't be bothered by the capacitance, but then he'd have had to have added another gain stage, and tubes cost money. Cable capacitance and noise pickup is why unbalanced high-impedance mics went away in pro-sound in the early '70s.

 

Just curious! But can you fashion a inst. cable using mic cable?

"I dont know so I ask questions."

 

Bill,
Why does better shileding require better insulation?

 

+1. Poor shielding might wrap the core with 10% shield, the rest air. Perfect shielding would have 100% wrap, and that means a major difference in capacitance with insulation of the same dielectric value.

 

They came along too late. By 1953 the high-impedance pickup die was cast. The changeover to low impedance mics was easy, as it pretty much accompanied the introduction of the mixing console for pro-sound.

 

Great thread and thanks for the education.

Would it be fair to say that the best criteria for us would be:
Lowest capacitance is the first goal with enough shielding to suppress RF but not so much as to add capacitance, and, force the need for excessive insulation (thus creating a stiff cable).

I'm assuming 12' to 18' cable lengths, 99% oxygen free copper inner cable and that its' diameter is not that critical.

Am I on the right track?

 

Personally, I wouldn't think twice about the oxygen content of the inner cable.

What you are asking is which is more important: RF shielding or lower capacitance. There is no right answer - it depends on the environment you play in. If RF is a huge problem, shielding is more important.

One thing is cable durability. As long as the cable doesn't completely suck tone and has decent shielding, I go for the most flexible, durable cable.

 

As an aside, another TB'er and engineer posted not too long ago that they did capacitance testing of a very wide range of cables, and found that 12 ga. solid-core Romex had the lowest capacitance of anything they tried. Unfortunately it's only suitable for speaker cabling, but apparently it does a killer job of that.

 

At the voltages presented by a bass pickup, the quality of the dielectric (insulation) isn't an issue as long at the dielectric doesn't fail mechanically. In other words, as long as the inner conductor doesn't end up touching the outer due to flexing, it's all fine.

The factors that affect capacitance are inner to outer conductor distance, the ratio of conductor sizes, the completeness of the outer conductor and the velocity factor of the dielectric.

If anyone wants to dive deep into this, I'll go dust off one of my old text books.

I don't fault Leo for his approach. But, I've also more than once considered building a balanced, phantom powered preamp for one of my basses.

 

The lowest capacitance results from the widest separation of the conductors, which unfortunately is difficult to accomplish with shielded wire. But if you go active the low impedance output effectively removes cable capacitance as a consideration.

 

I've been thinking about this since my last post.

Bill is right. It is time to move on from unbalanced high source impeadance. The whole cable issue goes away, and we'd sure make the soundman's job easier.

Looks I've got my first winter project. Stay tuned.

 

Go to a balanced output low impedance pickup and you'll never get even a whiff of noise again. It would require a balanced input on the amp, but that's hardly a major technical obstacle, and you could use a transformer with existing amps. Leo was halfway there with the split PBass pickup, too bad he didn't make it to the finish line.