Hi,
Here is more from the articles Jim wrote for Audio Ideas Guide.
"When you’re dealing with interconnect cables the three major performance issues are Resistance (R), Inductance (L) and Capacitance (C). Because the interface, in the case of an analog audio interconnect is a ‘high impedance’ connection, any issue with resistance and inductance is a moot point with high quality cables (especially at the distances typically used in audio systems). What is very important is the parallel capacitance.
Capacitance, unlike resistance and inductance, can cause serious negative consequences in an audio system. The real culprit in this case is the sources’ output impedance. The cable capacitance is charged and discharged by the signal through the output impedance of the source – usually a preamplifier. Any waveform smearing occurs at a rate that is determined by the mathematical product of output impedance (Z) and parallel capacitance (C) and is independent of signal frequency and amplitude.
This characteristic makes the cable behave like a low-pass filter so the output impedance and input load impedance is very important when connecting your equipment.
This is why it is critical when using tube equipment (which generally has high output impedance) to keep interconnects as short as possible. It is also why using passive volume controls can be a serious problem because the passive control will actually change the frequency response of your system as a function of its position of rotation. So at the 9 o’clock position you have one frequency response and at the 12 o’clock position you have another frequency response. The reason this is happening is because the passive control is changing its output impedance with position and therefore its frequency response. No wonder people sometimes report hearing major differences in cables when using these types of systems.
So the major point here is that cables always have to be viewed as an integral part of a circuit and recognize that what you are hearing from cable to cable is in actuality a frequency response shift as a result of the interaction of the cable with the specific source and the specific load. You are never listening to just the cable.
It is generally agreed, with analog interconnects, that your source impedance should be at least 10 times to 50 times smaller than your load impedance if flat frequency response is the goal. The output impedance of the Bryston BP 25 preamplifier is less than 100 ohms. The input impedance of Bryston amplifiers is typically between 20K and 50K Ohms.
If you want a much more detailed explanation of all the issues discussed above have a look at Jim Hayward’s superb article in Andrew Marshall’s Audio Ideas Guide Magazine (Summer/Fall 1994)."
