27-09-2016, 09:04 PM
(This post was last modified: 27-09-2016, 09:06 PM by Hartley118.)
Once upon a time, a pair of open telephone wires spaced 9 inches apart on ceramic insulators exhibited a characteristic impedance of 600 ohms. In other words, an infinite length of those telephone wires looked like a 600 ohm resistor. That's the net effect of the distributed parallel capacitance, series resistance and series inductance of the pair of wires. Interesting I think that those complexities can be reduced down to a resistance value.
So if a finite length of telephone wires is terminated in a 600 ohm resistor, it becomes indistinguishable from an infinite length. Therefore you get no reflection from the far end because the wires look infinite. This was an immensely important discovery in the early days of the telegraph and telephone.where engineers were plagued with echoes from the far end of lines many many miles long. However, it's a very long time since telephone wires were spaced apart like that in the open air. Nevertheless, that magic 600 ohm impedance figure remains in audio folklore for no particularly good reason.
Fortunately, audio cables, usually twisted pairs or coaxial, just look like capacitors, even up to several hundred metres long. That simplifies things, but of course the impedance of a capacitor falls as the frequency rises. That's why, as Mark makes clear, it's good to drive audio cables from a low output impedance because it avoids attenuation of high frequencies due to cable capacitance. Of course, in the real world, line drive amplifiers usually incorporate negative feedback and the capacitance of a cable slung directly on the output can cause instability, so it's common to buffer the output with a few ohms of resistance.
Interestingly, audio equipment reviewers normally ignore such interesting topics as different output impedances of preamps, CD players etc and their interaction with the capacitance of connecting cables. However, such factors can make audible differences in high frequency performance.
Martin
So if a finite length of telephone wires is terminated in a 600 ohm resistor, it becomes indistinguishable from an infinite length. Therefore you get no reflection from the far end because the wires look infinite. This was an immensely important discovery in the early days of the telegraph and telephone.where engineers were plagued with echoes from the far end of lines many many miles long. However, it's a very long time since telephone wires were spaced apart like that in the open air. Nevertheless, that magic 600 ohm impedance figure remains in audio folklore for no particularly good reason.
Fortunately, audio cables, usually twisted pairs or coaxial, just look like capacitors, even up to several hundred metres long. That simplifies things, but of course the impedance of a capacitor falls as the frequency rises. That's why, as Mark makes clear, it's good to drive audio cables from a low output impedance because it avoids attenuation of high frequencies due to cable capacitance. Of course, in the real world, line drive amplifiers usually incorporate negative feedback and the capacitance of a cable slung directly on the output can cause instability, so it's common to buffer the output with a few ohms of resistance.
Interestingly, audio equipment reviewers normally ignore such interesting topics as different output impedances of preamps, CD players etc and their interaction with the capacitance of connecting cables. However, such factors can make audible differences in high frequency performance.
Martin
BVWS Member
