RG here is more info [schem etc.] on the high pass formula

[donald stringer]

http://smg.photobucket.com/albums/v172/troublerat/    I hope this makes it clearer. I am not saying there wrong. I just dont understand how he got what he got. I am designing a variable state filter around this chip. I want a high pass ,low pass, bandpass similiar to the ca super tone. Any information will be used to develop this circuit. Thanks a bunch for sharing a small portion of your mind.

[R.G.]

OK. that helps.

The turnover frequency for a single RC is 1/2*pi*R*C. If you have more than one R-C network, then the components interact, and the frequency is not the same. A common beginner mistake is to cascade identical R-C networks and then to assume that you get the same rolloff frequency and twice the rolloff rate, which is not true because of the interactions.

In the circuit shown, there is also an opamp giving gain and feedback. I believe the circuit shown is the so-called multiple feedback highpass filter circuit. Notice that there is feedback from the opamp output through both R2 and C3/R1.

Ignoring all except the opamp, C2 and R2, we see that the opamp forms a highpass filter with one rolloff, that of C2/R2. But around that entire circuit, there is the C1/R1/C3 feedback network, and they interact.

The way these circuits are designed is to do the math to analyze the voltages and currents in the circuit loops, and then solve the resulting multiple equations for the output voltage in terms of the components. I have not yet had a chance to do the analysis on the circuit you show, but it is not out of the question that it is correct.

Here are some references that I think you may be interested in, found by googling for "multiple feedback" "active filter"

http://www.du.edu/~etuttle/electron/elect15.htm
http://sound.westhost.com/project63.htm
http://www.gennum.com/audio/hip/pdf/60009DOC.pdf

and the mother lode:
http://www.analog.com/library/analogdialogue/archives/39-05/Web_Ch5_final_PtB_F.pdf

However - the state variable filter is not a multiple bandpass filter. The state variable filter is two integrators with damping added by feedback and losses. It's quite different from the multiple feedback.

[donald stringer]

Thanks a lot. Alot to bite off and chew. So you answered a question I also didnt ask. Its not as simple as cascading several of these together and expecting it to work.

[R.G.]

Sadly, no, it's not as simple as cascading several of those. But you have some interesting things to learn in front of you.

Dig in! It's a feast!

[RedHouse]

There's also a great book available called "The Active Filter Cookbook" by Don Lancaster.