Filter Value Calculator

[gez]

Came across this excellent site when I was googling for the analysis of a 3-Pole Lowpass Filter.  Not only does it give all the info I needed (via the link), but also a calculator to do the maths for you (scroll down)!  If you click on the first two links you get cool little value calculators for simple Sallen-Key and Mult-Feedback filters.

http://www.beis.de/Elektronik/Filter/Act3PoleLP.html

[bioroids]

Hi! Thanks Gez, great link!

Happy 2005!

Miguel

[gez]

Happy 2005!

Likewise!  

[Peter Snowberg]

Awesome link Gez!

Thanks!

[puretube]

TNX & HNY

[gez]

Is there anyone with some simulation software who’d be willing to do some analysis of the three pole Sallen-Key filter used in the korg/boss octave pedals?

Although I’ve finally figured out how to use the table in the following link to design my own filters, the scaling of the caps is different to that used in the Korg/Boss filter, which makes analysis a tad difficult:

http://www.vzavenue.net/~jpbedinger/projects/an/3polelpf/ds1-3polelp.htm

Although there is an analysis of this type of filter using different value caps in the above link, there’s no mention of frequency so, for me at any rate, it’s not possible to work out what the cut-off frequency is.

I just want to know what the cut-off is in the filter shown in this schematic (Korg octave), the one using R28 & C18, R39 & C14, R38 & C15 and IC6b:

http://www.diystompboxes.com/pedals/schems/korgoct1.gif

Also, what is the cut-off when all resistors are changed to 47k and C18 = 33n, C14 = 47n and C15 = 3n3?  These cap values are from a Penfold schematic and no formulas are given to calculate values, he advises to tweak the resistors empirically (keeping them all the same) until you get the desired effect, which works but wastes time and I need to replace this filter with a different one so need to know cut-off.

Any help would be appreciated.  Thanks.

[spongebob]

You might find this one here interesting, I found it on the TI website when I was searching for some PDFs on active filters:

http://www-s.ti.com/sc/techzip/slvc003.zip

FilterProâ,,¢ MFB and Sallen-Key Design Program is a Windows application that designs Multiple-Feedback and Sallen-Key low-pass and high-pass filters using opamps, resistors and capacitors. It also supports a fully-differential version of the MFB circuit. This program supports Bessell, Butterworth, and Chebychev filter types and can be used to design filters from 1 to 10 poles. The capacitor values in each stage can be either selected by the computer or entered by the designer. An "always on" prompt window provides context-sensitive help information to the user. The response of the filter is displayed on a graph, showing gain, phase and group delay over frequency.

[gez]

Thanks for the link Spongebob but it's not much help I'm afraid.  

Their 3-pole design buffers the first pole so calculations are going to be different.  That aside, it doesn't allow me to input cap and resistor values to get an analysis, it's more for designing filters and designing them the Texas way, ie use the max number of op-amps to clock up sales!