Impedances and transmission line effects...

[Gryphon]

I would point out that in the past I've seen a short (<50m) run of twisted pair driven from a 'balanced' source that was anything but - and of about 1000 Ohms output impedance - suffer from notable treble loss.

The driver was an op-amp with 1000 Ohms in series with its output in the hot side of the feed: the cold side was a 1000 Ohm resistor in series with a 100uF capacitor to earth.

With RG58 at 58pF/m - let's call it 60pF as we're engineers, not scientists - then 50m would be 3nF. With the total Zout being 2k, the -3dB point is ~26kHz. There would be some attenuation at lower frequencies, of course, but that doesn't seem like much to me. Measurably, certainly, and should be attended to in an installation, but would it leap out as being a big problem unless you were doing an AB comparison?

Twisted pair is likely to have a rather greater capacitance per unit length than RG58, I would suggest.  You only have to double the figure you quote (and I seem to recall rather greater numbers being quoted upthread) to get the 3dB point down to 13kHz.  I suggest this would give noticeable treble loss, even if the clean original wasn't available for an AB comparison.

If it was the only line available, I'd give it to a journo for a speech piece.  I think I'd want something a bit better for Carols from King's.

However, in neither case do we really need to treat the line to transmission line theory, although of course that's what it is. Now let's look at the instance of trying to send 2MHz down a screened line 15m long, where the only known quantities are that (a) it's aircraft grade, and (b) the capacitance of the cable.  We can ignore (a), on the basis that the electrons don't know that they're at ground level.

2MHz corresponds to a wavelength of 150m in free space, so rather less than that in screened cable.  Someone upthread said that you need to start thinking transmission line theory if the line length approaches one-tenth of a wavelength (not the figure I would use myself, but never mind).  Mmm, 150/10 = 15.