Monday, September 10, 2012

Guest Post: Brandon Golwitzer - Capacitance in Cables

Our recent discussions on methods of reducing noise in cables lead us through some talk about quad mic cables. The subject of reduced high end due to capacitance was mentioned in one of the comments and my friend offered to write up a post about it. Here it is and many thanks to Brandon. While I'm on the subject we're open to submissions from any Brethren of the Knob and Fader that would like to contribute. You don't need to be a professor or any kind of big shot, it could be anything that might be educational to the rest of the Brethren. Stories about gigs, tricks and tips, it's all welcome so get in touch!

Serious gearious
A comment to the recent Star Quad Cable post mentioned the possibility of greater high end loss in that type of cable which triggered my thoughts to writing a post about what causes high end frequency degradation in cables. When it comes to audio cables, there are lots of factors affecting sound quality, but capacitance is the culprit that gobbles up high end more than any other. There is a bunch of information out there on it, but let me take a shot at explaining it here.

If you are unfamiliar with capacitance, it is an electrical property that has been harnessed for years to
give us tone controls in our gear. The most basic tone controls in consoles and amplifiers use a simple circuit with a capacitor and resistor to cut frequencies. A simple high pass filter is an example in your console. Guitars have long used a use a simple capacitor attached to the potentiometer on the tone knob to cut high end (low pass filter). The frequency where the cut starts is a function of the component values chosen. These are simple circuits to build and there is a ton of information at the end of your Google search to teach you how to do it. Those are examples where capacitance is used intentionally to a desired end effect, but capacitance exists everywhere. Sometimes it can be our enemy like in the case of audio cables where we want to transmit a signal with the least degradation possible. In short, the cable capacitance creates a low pass filter that begins to roll off high end as cable length increases. You can read all about the math and science aspect of it here if you want to really dig into it: http://audiosystemsgroup.com/CableCapacitance.pdf

Understanding the specs of the cables you use will help you to understand what you can expect for high end loss, and if you want to get crafty you could even choose your cable based on the source to give you some built in EQ. I will admit that is not too practical for a live gig, but could be used in a studio setting. The capacitance of your cable is measured in pico Farads per foot (pF/ft) because the total capacitance is affected by the length of the run. Generally speaking, longer the cable (i.e. the farther you transmit the signal), the more high end you will lose. Most quality cables fall into the “low capacitance” category, and the term has become a bit of a marketing buzz word in recent years. Generally, anything under 70 pF/ft is pretty low capacitance with the lowest coming in around 20 pF/ft. The capacitance per foot will vary among manufactures for a given wire gauge based upon the number of copper strands that make up the wire and the diameter of each strand. Each manufacturer has their formula that they like best. For example, Mogami tends to have the highest capacitance, but is used in some of the best studios in the world. Their cables are slightly “darker” sounding, but it is easy to make adjustment to compensate. So don’t get too wrapped up in the capacitance spec. Let your ears decide.

With respect to the quad cables discussed in the recent post, the capacitance is generally a little higher than an equivalent two conductor wire because there is more copper per foot. All told though, the quad cables are still very low capacitance and have the benefit of much better noise rejection. Here are some specs for comparison.

Two Conductor Cables:
  • RapcoHorizon (p/n MIC1): 21 pF/ft
  • Canare (p/n L-2T2S): 22 pF/ft
  • Mogami (p/n W2549): 76 pF/ft
  • Belden 8412 (p/n W8412): 30 pF/ft

Quad Conductor Cables:
  • RapcoHorizon (p/n MIC4): 46 pF/ft
  • Canare Star Quad (p/n L-4E6S): 46 pF/ft
  • Mogami Neglex (p/n W2534): 65 pF/ft
  • Belden 1192A Quad (p/n W1192A-BK): 40 pF/ft

Understanding your gear is key to making informed decisions on the job, but it is all about what pleases the ear at the end of the day. Remember that there are a lot of ways to get to sweet sounds, and cable capacitance is a small factor in the big picture. So know your gear, Brethren of the Knob and Fader, but always let your ears decide.

Brandon is a long time Brother of the Knob and Fader who supports his guitar and home studio habit by masquerading as a mechanical engineer by day.

1 comment:

You're the Scotty to our Kirk