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Old 13th Oct 2017, 11:45 am   #125
G8HQP Dave
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Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Quote:
Originally Posted by Craig Sawyers
Speaker cables of such construction are a double edged sword, because some amplifiers object to an almost perfectly capacitive loading. Typically 500pF/m but only 100nH/m inductance..
That looks like a misguided attempt to make a cable with a characteristic impedance which is in the region of typical speaker impedances. 500pF/m and 100nH/m have an RF characteristic impedance of 14 ohms. Misguided because:
1. speaker impedances vary quite a lot with frequency
2. domestic speaker cables are electrically short at audio frequencies so no need to consider transmission line issues
3. at audio frequencies the characteristic impedance is nothing like the RF impedance

For a low loss cable the characteristic impedance is sqrt[(R + jwL)/(G + jwC)]. At RF R and G are negligible (apart from causing some loss) so the characteristic impedance is approximated by sqrt(L/C). Unfortunately people see this and assume it applies at all frequencies. For most audio frequencies the correct approximation is sqrt(R/jwC): this is higher than the RF impedance, not resistive, and varies with frequency. Hence the idea of 'matching' an audio cable to a speaker is misguided. The only benefit of attempting it is that, perhaps by accident, the cable often turns out to have low resistance and that is what is actually needed.
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