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I suspect his friend had a passive pre

If one were to make a passive pre out of a 100k pot (bad idea), and operate it at 6dB down from full output, its output impedance would be very large - 25 kOhm. If you were to load that with a cable that had 392 pF of capacitance, you'd have a circuit that is 4 dB down at 20 kHz. This is not a competent design though. It's certainly not the fault of the cable.

Regarding how RF and video cables are specified for loss, they are done with a source and load impedance equal to the characteristic impedance of the cable (usually 75 Ohms). This matching gives optimum performance for both video/RF and digital. Kimber doesn't say how they measure this, but there's no way on this earth you could get that performance with any passive pre, because its output impedance is way too high. Cables such as Belden are measured with impedance matching at both ends. The Kimber spec looks impressive, but if you download the Belden catalog, you can look up the 89259 cable - a popular item with audio DIYers. At 10 MHz, its attenuation per 100 m is 3.0 dB. This gives .03 dB per meter (at 10 MHz rather than 8 MHz). The Kimber doesn't look very impressive in that comparison.

You can compute the characteristic impedance Z0 from Z0 = sqrt(L/C) where L is the inductance per unit length and C is the capacitance per unit length. So for the Kimber Hero, this gives Z0 = sqrt(0.401e-6 / 76.5e-12) = 72.4 Ohms. This could just be garden variety 75 Ohm video cable :-). I suspect they measured the loss with a 75 Ohm source and load.

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  • I suspect his friend had a passive pre - andy_c 20:16:09 05/15/06 (0)
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