I am new to electronics and have recently been trying to get my head around audio mixing. I stumbled across a helpful introduction by Rod Elliot on passive and active mixers, but I am stuck on a small part of one diagram.
The annotation describes it as a "Passive Mixer With Channel Level Controls", in this case using variable resistors (faders) to control the levels of the input channels. It is used to explain the issues with passive mixing, but my question only concerns the following statement:
In Figure 3 you can see that the faders change the impedance seen by the mixing resistors. The effective source impedance is maximum when the fader is (electrically) centred, and will have a value of one quarter of the fader's total resistance.
Only one quarter of the the fader's total resistance? I would have thought that to maximize impedance, one would maximize the resistance on the variable resistor, resulting in 20k ohms of impedance.
I think it would be helpful if the terms "source impedance" and "electrically centered" were explained? I couldn't find any helpful search results for these.
How did he arrive at the conclusion that the "effective source impedance... will have a value of one quarter of the fader's total resistance"?