Re: OK to use TV coax for microphone?

On Sun, 04 May 2008 09:37:50 +0000, Don Pearce wrote:

> On Sun, 04 May 2008 10:30:53 +0100, Dead Paul <dead_paul@[EMAIL PROTECTED]
>
wrote:
> 
>>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.
>> 
>> 
> A pair of coaxes in parallel is balanced, but you miss out on the close
> proximity and twist. The first makes the magnetic loop very small,


yeah but there is no "mag loop" outside of the ****eld with coax.

> while
> the second causes the polarity of any residual pickup to swap every inch
> or so, giving a net cancellation.

no pickup inside coax.

> As I said in my first reply, if I was forced to use coax for a
microphone,
> domestic TV grade would be a very poor choice because in general it has
> perhaps no more than 10% screening (just enough in fact to give the
cable
> a stable characteristic impedance, but no more). Microphone signals are
> too small to mess around this way. Decent audio grade cable has
screening
> approaching 100%.

fully agree.

> 
> d

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