Re: OK to use TV coax for microphone?

"Dead Paul" <dead_paul@[EMAIL PROTECTED]
> wrote in message
news:fvjvod$mej$1@[EMAIL PROTECTED]
> On Sun, 04 May 2008 08:13:12 +0100, don pearce wrote:
>
>>
>> Phil Allison wrote:
>>> "Richard Crowley"
>>>>> (1) Is it ok to use this sort of coax for a microphone?
>>>> Not enough details to answer your question adequately. First of all,
>>>> the kinds of microphones used for audio applications (reinforcement,
>>>> recording, etc.) are usually balanced and require cable with two
inner
>>>> wires and an outside ****eld/screen.  If you are trying to use a
>>>> balanced mic, then the cable is unsuitable because it is unbalanced.
>>>
>>>
>>> ** Not true at all !!
>>>
>>> There is  NO reason not to use a (suitable) co-axial type cable with a
>>> microphone  -  either low or high impedance.
>>>
>>> Despite all the nonsense you WILL  have read elsewhere,  co-axial
cables
>>> have as good or better rejection of external hum and noise sources as
do
>>> balanced twin wire cables.
>>>
>>> Try it out if you don't believe this.
>>>
>>>
>> Of course a proper microphone cable is SCREENED balanced twisted pair,
so
>> it enjoys the multiple benefits of electric screening by the outer, the
>> common mode nature of any residual interference and magnetic
interference
>> cancellation by the twist in the balanced pair. A poorly screened coax
>> cable such as TV coax has only a part of the first of those so this
claim
>> is clearly nonsensical. If it were true, professional microphone and
mixer
>> companies would not be going to the trouble of designing balanced kit.
>>
>>
>>>
>>>>> The length I need to use is 3 to 4 metres.
>>>> Not really im****tant to the question.  It wouldn't make much
difference
>>>> if it were 1 m or 1Km
>>>
>>>
>>> **  If the mic is high impedance ( ie 50 kohms), then more than 10
>>> metres or so of cable will cause high frequency response peaking and
>>> early roll off as the cable capacitance loads the transformer inside
the
>>> mic.
>>>
>>>
>>
>>> If the mic is low impedance ( ie circa 250 ohms), then hundreds of
>>> metres can be used  -   but not kilometres.
>>>
>>>
>>>
>> This is true.
>>
>> d
>
> Good quality coax will do the job if you don't mind the impedance
mismatch
> and if you want balanced line then you could use a pair of coax feeds in
> parallel (impedance about 100 ohms for rg58). Also there's coax and
> there's coax, I've seen rg58 like TV down-lead and others like shrunk
down
> UR67M.
>
>
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>

Impedances are almost never matched, source to load, or source to
transmission line to load 
in audio.  Telephone lines are the only case that comes to mind.  Mics,
speakers, preamps to 
amps -- just not done.  In short, we don't match source to cable, nor
cable to load.  Nor 
source to load.  Not only that, the impedance of a typical cable, such as
rg-59, though 75 
Ohms above 100KHz, is much much higher in the audio range.

Earl