On Mon, 20 Feb 2017 03:25:42 -0700, Paul wrote:
On 2/19/2017 11:57 PM, Don Pearce wrote:
On Mon, 20 Feb 2017 05:46:21 GMT, (Don Pearce) wrote:
On Sun, 19 Feb 2017 16:55:31 -0800 (PST), 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.
Users would at least then know how to get the best HF response with long runs.
I've measured the impedance of a few cables. They came out in the
range of about 220 to s40 ohms. 300 seemed a reasonable average for
the calculation.
d
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Here are a few calculations on the behaviour of longish cables in a
microphone scenario. It is interesting that at any frequency where the
cable is starting to have an effect, a calculation using just
capacitance is wrong. It is not only the wrong answer, but the answer
is backwards, showing a loss where there is in fact a gain. Anyways,
have a look and see what actually happens.
http://www.soundthoughts.co.uk/read/cable.html
Interesting, but where is the series resistance and series
inductance per unit length in your lumped element model?
And where is the shunt conductance G per unit length in the
admittance? You only have the shunt capacitance.
Even if you assume a lossless cable, and R=0 and G=0, you
would still have the series inductance per unit length.
And what is being used for the distributed model? Some
sort of finite element analysis, using numerical methods?
I used just capacitance and source resistance, because that is what
people go to when they try to calculate the high frequency limit of a
cable. I could use a C-L-C model to make it appear more accurate, but
in fact it makes things worse. What you end up with is a multiple pole
lowpass filter.
The distributed model is the standard Spice transmission line - which
almost all RF simulators use. The parameters it needs are delay and
impedance.
I currently use normal coax cable at up to 60GHz (60,000,000,00 Hz) so
it is clear that no cable has any kind of upper frequency limit -
until it starts moding. That is when the inner diameter of the cable
approximates a half wavelength.
d
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