Re: mic cables

On Thursday, September 18, 2003 11:03 PM [GMT-7=PDT],Howard Herrnstadt
<herrnstadt@xxxxxxxxxx> wrote:

> Whoa! Why 20 feet? If there is an efficency issue and you need more
> cord to walk the bar or whatever, then upgrade to a thicker guage,

Unfortunately, life in the electrical world isn't that simple.  A thicker
gauge will actually have MORE loss at high impedances, because it has more
cable capacitance.  At high impedance (i.e. a million ohms), wire resistance
(~20 ohms per 1000 feet) is not a factor.  Cable capacitance is the major
loss factor.  This will be especially true of super high impedance mics like
JT-30's - its impedance is rated as "equivalent to a 390 pF capacitor".

I looked up specs for a couple of Belden audio coaxial cables.  9395 is 55
pF per foot.  9778 is 45 pF per foot.  This is pretty close to most cables.
At 20 feet, the 9395 is 1100 pF, and the 9778 is 900 pF.  Your once strong,
full JT-30 signal has just been dropped by roughly 2/3rd, highs have been
bled off, and the cable capacitance is putting an extra load on the element.
And this is just for a regular 20' cable.

If we look at a low impedance mic, the capacitance @ 600 ohms has virtually
no effect.  The resistance is about 20 ohms per thousand feet, again,
negligible for a 600 ohm mic.

There are other factors that have been touched upon elsewhere.

Hope this helps.

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