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Heavily dependent on output impedance of the source

It turns out that if the source impedance is matched to the characteristic impedance of the cable (75 Ohms in this case) you get the flattest possible response (even though the load is not matched at all). For the case of a 30 ft cable and perfect 75 Ohm source, rolloff at 20 kHz would be unmeasurable with most equipment. I'm using a 20k resistive load, which for all practical purposes is an open (ZL >> characteristic impedance of cable).

Just as an example of what you're asking, take a pretty high 1k preamp output impedance. That case is about 0.2 dB down at 20 kHz. For 300 Ohm preamp Zout, it's negligible (thousandths of a dB). Keep in mind that these calculations are somewhat idealized, since the output impedance of the preamp will not be constant over all frequencies (high-value coupling caps becoming inductive, op-amp output impedance becoming inductive, etc.).

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  • Heavily dependent on output impedance of the source - andy_c 19:06:29 05/14/06 (0)
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