[Paul[_13_]]

On 2/19/2017 5:55 PM, Phil Allison wrote:
Don Pearce wrote:



This is where simplistic cables models fall over. Four hundred feet of
cable is enough that at 20kHz a real, distributed model will give a
correct answer, but the lumped C/L/C model has failed. Four hundred
feet of typical 300 ohm mike cable ...




** Who sells "300 ohm mic cable" ??.

Or are you saying typical twisted pair mic cables have a characteristic impedance of 300ohms in the audio range ?

IME, 400 feet of common or garden mic cable will significantly attenuate high frequencies from a mic like the SM58 and most others - assuming there is the usual 1500 ohms load at the other end.

The rated impedance of a mic cable is not defined anywhere I can find, but IMO ought to be the value of terminating resistor that minimises or eliminate shunt capacitance in and somewhat beyond the audio range.


Time Domain Reflectometry (TDR) is used in telecommunications to
find impedance discontinuities in cables.

RF engineers use the stand wave ratio (SWR) to find the least
amount of reflected power with a given termination load.

Not sure if audio frequency people use these techniques....








connecting a 150 ohm mike to a 1500
ohm preamp will actually result in about 0.25dB RISE at 20kHz, not a
drop. The reason for this is that at high frequency the cable is
starting to act as a transformer, slightly improving the match between
150 ohms and 1500 ohms.