[Dead Paul]

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

On Sun, 04 May 2008 10:30:53 +0100, Dead Paul 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 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.


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 shield 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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