[Scott Dorsey]

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 ?

Actually, typical 24 ga mike cable tends to run around 110 ohms or so, which
is why that was picked as the impedance for AES/EBU, since they wanted to be
able to run AES/EBU over existing cable plant.

Older 18 ga mike cable with thicker rubber comes in around 150 ohms.

If you wanted to get up to 300 ohms you'd have to separate the conductors
more, or make them tiny.

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.

If you're using an SM58, you probably want a load of around 600 ohms in order
to make it happy. If you're using a condenser microphone, the output Z is
likely far lower than the load and so if you're thinking about it as a lumped
sum system with the impedances in parallel the source impedance is dominant.

The loading issues are more of an issue than the cable issues.

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.

The problem is that microphone noise and microphone dynamic behaviour due to
electrical damping (in the case of dynamic mikes) are usually much more
important than any HF behaviour.

Still, the HF behaviour gets more interesting with cables like star quad that
have far higher shunt capacitance without any change in series inductance.

I'd like to see a model of the system. Shouldn't be hard to do, and I would
not be surprised to see an HF peak appear in the top octave if you get the
loading right for the cable.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."