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Re: extension cables

Subject: Re: extension cables
From: "John Lundsten" lundsten_john
Date: Fri Jan 22, 2010 3:15 pm ((PST))
Robb Nichols wrote

| I'm not sure you're comparing apples to apples here. Your test included
| mics with a low-impedance balanced source, where as the original
| question came up regarding PIP mics.

a Very valid point from tests i have done mic source impedance is all 
important. And far far more important than whether an unbalanced or balance 
connection method is used.

I did some kind of quick and dirty tests ....... a 1000' spool
| of a low-capacitance coaxial extension cable in a PIP powered
| application, I saw almost a 20dB loss above 1KHz and a 10dB resonant
| peak at about 225Hz.

Yes i can believe that. The lowest source Z , PIP mic I have come across is 
the DPA 4060 {about 300Ohms} more typically an electret mic with it's FET 
will be 1.5 -4k Ohms. Which means IMO, more than 10ft of cable the HF 
response will for sure audibly suffer.

Then, using the same mic, I set up our UPA-1 as a
| phantom-powered line driver at the source and measured losses with a
| 1500' balanced cable (I didn't have a 1000' spool on hand to make a
| similar comparison)
Sorry I have no knowledge of what a "UPA-12" is.

and found a pretty smooth response curve that began
| to roll off at about 1K and extended to about -20dB @20KHz. The
| low-impedance source and balanced line didn't maintain a particularly
| compelling response with such a long cable, but it was substantially
| better and quite useable in our application.

As i say I don't know what your UPA-1 is (or the out Z of the mic you used) 
but that seems a pretty dam poor performance to me.

But a while back when i was designing an "active" mic splitter I did a 
number of tests.
I used 100m (say 330ft) of cable & found  when i got a source Z < 5ohms the 
frequency response and crosstalk loss was almost un-measurable. The great 
discovery considering multichan audio was my aim is that if you just add a 
transformer that
a) drops a (notionally 0dBu) "line level" output from a mic amp near the mic 
by 30dB the source Z becomes an ultra low Z out of less than an Ohm.
b) a whole load of such signals can travel down 300ft+ of cheap, thin, non 
balanced wires with almost unmesurable loss of frequency response & X talk 
better than -120dB.

But, trying to get back to the orig question. as soon as the source Z rises 
the performance is going to suffer -- Badly.
IMO don't bother with lo capacitance cable, the advantage will be minimal 
compared to the cost involved.
If you have PIP mics & want to use long cables, put a mic amp near the mic, 
then use any cheap cable.

| Rob Danielson wrote:
| >
| > At 9:54 AM -0800 1/22/10, Dan Dugan wrote:
| >
| > Next time I install a long cable run, I'm going to record the test I
| > did where I connected two Rode NT2000's (7dB[A] self noise)-- one to
| > a 20' mic and the other to a 1000' role of cable and placed the mics
| > side by side. Prior to this, I looked at all the charts and did the
| > HF loss computations and then Richard Lee (miclist) suggested the
| > obvious perception test. Result: to my aging ears (significant loss
| > above 9-10Khz), I could not detect any loss of signal or HF. I use
| > Symetrix SX202 Mic preamps. RFI-wise on my two 700'+ runs, I
| > sometimes get very faint radio on one mic from a station in Western
| > Texas when the conditions are just right-- its very,very rare. The
| > other 3, same model mics get none. I'm running 5 conductor
| > studio-grade wire ~$180 for 1000'. Rob D.
| >
| > -- 
| >
| >
| ------------------------------------
| "While a picture is worth a thousand words, a
| sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause
| Yahoo! Groups Links


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