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Hero999 said:In practice you would never use such a long cable 3.75km long to transmit audio power and if you did you would use a matched 600hm: telegraph type system.
Hero999 said:Yes, I would use two 8hm: to 600hm: impedance matching transformers and a 600hm: transmission line.
But an audiophile wouldn't like that as the transformers would ruin the frequency response, even if you went to the trouble of
Nigel Goodwin said:Too much loss down the cables!
That's why I would prefer a higher voltage level.
Hero999 said:But that won't solve the problem with standing waves creating distortion or will it?
If we use a high impedance cable then using impedance matching transformers will increase the voltage like like you say we need to.
Let's say we're driving 4hm: speakers from an amplifier with a peak output of 12V.
[latex]\frac{N_S}{N_P} = \sqrt{\frac{Z_P}{Z_S}} = \sqrt{\frac{600}{4}}=12.25[/latex]
The RMS output voltage will be:
[latex] \frac{12 \times 12.25}{\sqrt{2}} = 103.9V[/latex]
Look, we've killed two birds with one stone!
I'm not imagining it, it does exist, it's basic transmission line theory.Nigel Goodwin said:I think you are probably imagining such distortion?.
That was lucky, if the had used an amplifier with a peak output of 24V I would get twice the voltage but it isn't about the voltage at this distance, matching the impedance is far more important. In the old days, before fibre optics came along, telephone lines used to be matched to 600hm: to prevent these sorts of problems.That's basically only normal 100V line, I'd like to see it a lot higher for the distance you were talking about.
Hero999 said:I'm not imagining it, it does exist, it's basic transmission line theory.
Try connecting your scope to a signal generator, with a 50hm: output impedance running at just 5MHz, via a 10m long twisted and the pulses won't look very square, you'll notice a lot of ringing. Now repeat the same experiment using the same length of 50hm: coaxial cable, the ringing will be greatly reduced. This is because it's acting as a transmission line at the harmonic frequencies, before with the twisted pair which has an impedance of 600hm:you were getting standing waves and transmission line resonance but this has all disappeared with the 50hm: cable because it's matched to the source.
Hero999 said:Well at 3.75km I would agree.
How far you you away from me?
About 75 miles?
That's 120km which is one full wavelength at just 2.5kHz. Now given that things get funny when the cable length even approaches 14 wavelength, I think you'd agree that an unmatched audio link between us would sound pretty bad.
Nigel Goodwin said:Run it to my younger brother's instead, he's not far from you!.
Now that would be getting more problematic!.
Still, this is far more interesting than $13,000 leads!
Or you could used a standard balanced signal cable - You know, the XLR cables that use differential signals. Then it doesn't really matter what interference gets onto the line because it will be subtracted on the far end. Or have I got this all wrong ?Hero999 said:I'm not imagining it, it does exist, it's basic transmission line theory.
Try connecting your scope to a signal generator, with a 50hm: output impedance running at just 5MHz, via a 10m long twisted and the pulses won't look very square, you'll notice a lot of ringing. Now repeat the same experiment using the same length of 50hm: coaxial cable, the ringing will be greatly reduced. This is because it's acting as a transmission line at the harmonic frequencies, before with the twisted pair which has an impedance of 600hm:you were getting standing waves and transmission line resonance but this has all disappeared with the 50hm: cable because it's matched to the source.
Now normally this isn't a problem for audio, this is because the length of the cable is short compared to the wavelengh but this isn't the case with the example I used in my previous post as the cable is so long it needs to be considered as a transmission line. If I wanted to run a cable to distribute audio from my home in Bedford to you in Derbyshire, stepping up the voltage wouldn't be good enough, I'd need to match the impedance too, if I wanted to keep the ringing and standing waves at bay. As I have illustrated previously, in practise this would mean increasing the voltage anyway as practical transmission lines have a higher impedance than most speakers.