# Filter questions

Thread Starter

#### dfro

Joined Feb 6, 2006
37
I have some questions about filters. There are some filter arrangements I see all of the time, but I cannot find any simple explanations that make sense to me. I am hoping someone can fill me in with how these filter networks are working and how I can calculate different cutoff frequencies.

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.

#### eeboy

Joined Sep 27, 2007
90
The first filter you describe is a first order low pass filter. Do a search on that and you will find a ton of information and the math to back it up.

The amplifier circuit you describe uses feedback (looks like positive feedback given your description). An op amp is essentially useless as an amplifier without feed back. This is the reason the output feeds back to the non-inverting terminal. Search for basic op amp amplifier configurations and you will find several fundemental cases.

Thread Starter

#### dfro

Joined Feb 6, 2006
37
I understand a filter where the input goes to a capacitor and a resistor in series tied to ground and the output comes off of the top of the resistor. That is covered very well in the All About Circuits e-book. The filter I am confused about is a shorted wire from input to output with a cap and resistor shunted to ground off of that wire.

The output is not taken from between the cap and resistor, like in a first order low pass filter, the input and output are shorted together with the cap and resistor shunting to ground.

I don't know if I am describing the circuit clearly.

Thanks.

#### thingmaker3

Joined May 16, 2005
5,084
It is important to remember the load has impedance as well. For high enough frequencies, the capacitor and resistor will present a much lower impedance to ground than will the load.

Similar threads