> Thanks for that information. 600 ohms is too low an impedance for the mic=
preamp, tho. For example, the LS-10 mic input is specified at 2000 ohms.
The theory of cable impedance only applies to infinite lengths or, in
practice, long lines terminated at the stated impedance like old fashioned=
telephone pole lines.
What we are talking about is unterminated cables or cables terminated at a=
different impedance. This causes a reflection back from the far end at abou=
t
the speed of light which in turn causes a phase shift with frequency. This=
is not useful theory for audio.
The cable characteristic impedance is not a useful parameter. What you need=
is the combined impedance of the input plus output devices
(1/(1/z1+1/z2))
plus the resistance of the cable (tiny) all compared with the distributed
capacitance of the cable to screen and/or across the balanced line.
for instance 100metres (300 feet) of cable at 100pf per metre which comes t=
o
10nF across, say, 2KOhm, this checks out as a 6 dB drop at 8KHz. (Please
check this - it's late at night and my computer lost my previous draft)
Now if you have a low actual impedance mic like a MKH series which is aroun=
d
10 Ohms running into a long cable at another 10 Ohms resistance and 10 nF
capacitance, you run into a 6 dB drop about 800 KHz in the AM Medium
Waveband.
The important mic match is the optimum noise impedance which is nothing to=
do with getting the maximum power into the preamp, but getting the best
signal to noise ratio. Running a 10 Ohm actual mic impedance into a 10 Ohm=
input would wreck the published parameters.
The reason pro sound recordists buy expensive cable is because it is easy t=
o
wrap and unwrap in cold weather.
David Brinicombe
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