The first is a situation where there is a wire from + input to + output. This is shunted to ground with a capacitor and a pot in series. The wiper of the pot it connected to ground. This arrangement is used in tone control on electric guitars.

Kevin O'Connor in his book, "The Ultimate Tone, Volume 2" pg. 6-17, states:

"The simplest tone control network is a variable resistance in series with a fixed capacitance hung from a signal line to ground... If C is replaced by an inductance, then a low-frequency roll-off could be imposed. A series L-C could impose a mid-range dip depending on the values selected."

He does not give the math to help you calculate values. Can anyone explain? I am having trouble visualizing how this works.

He goes on to write:

"The problem with the simple tone network is that its performance is not predictable, as it depends on the source impedance of E-in and the load impedance across E-out. In a closed system, these values are known and C can be selected for the lowest frequency to be rolled off, as it interacts with the net resistance. The variable resistance then decouples C, swamping out X-c and reducing the amount of high-frequency attenuation."

Could someone give an example of what he is talking about. The "decoupling/swamping out" part has me completely confused.

Also, I see situations, like with tube amps and op amps where the + input line will be split into a parallel cap and resistor and then joined again at the +output. Could someone explain how this filter work's. I cannot find any explanation of this one either.

Thanks.