Re: OK to use TV coax for microphone?

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 shield/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 important 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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