Filter Types and Circuit Design Freeware

[Single Coil]

What filter types are most commonly use in stompboxes? Butterworth, Bessel, Chebychev, Gaussian, Linear Phase, or other? I'm trying to learn how to use TI's FilterPro progam for designing circuits. Is there a better free program out there, or is this one satisfactory for a beginner? I'll need a tool like this if I want to begin to design circuits. Since this is a TI program, I was hoping that it would have the ability to select a TI specific Op-Amp to work with...but no. That would be too easy.

Thank you.

[davebungo]

I would have to say other, since the named types have very precisely defined characteristics (given by the associated precisely placed poles and zeros) and although they are used on occasion, there are many arbitrary designs out there which don't fall into such narrow definition.

The most common filter you will see is a simple first order low or high pass filter i.e. one R and one C.  If you want to play with specific types then for general use the Butterworth filter is fine.  Other types are also and more commonly used in applications such as anti-alias filters prior to A/D conversion etc.

Linear phase filters (as far as I am aware - someone please correct me if I am wrong) can only be implemented using digital FIR filters.

[Rob Strand]

In general I'd say "other" as well.  You do get obvious attempts at Butterworth in effects.   You might be wondering, if it's "other" then how do people design "other"? There are many reasons and most of these have no technical rationale, some are cost cutting measures,  often stages are repeated just to simplify things, sometimes a filter is tweaked by ear,  others are designed using elaborate design methods to make the filter match something physical (for example a speaker simulator).

Despite the enormous amount of filter theory around a good deal of filters are designed in an ad-hoc manner.

Take a look at the BOSS BF-2 flanger filters for example and try to see what the designer was thinking - not an easy task.

[davebungo]

Take a look at the BOSS BF-2 flanger filters for example and try to see what the designer was thinking - not an easy task.

I remember looking at a commercial pedal circuit recently (it may have been a TS or BOSS I can't remember) and trying to reason in my head why the "filters" were set up in such a complicated way i.e. lots of repeated C's and R's to ground etc. and I just couldn't see how anyone could come up with such a circuit by up front design alone.  I think you have to combine a little theory with experiment, then listen and tweak to get what you want.  Either theory or practice alone is pretty useless.

It's funny how it is always difficult to reverse engineer a circuit and try to second guess why things are done in a particular way.  This is where experience counts.

[JHS]

The tonefilters are built with gyrators, pretty simple, 2 Rs and 2 Cs define corner frequency, bandwidth and amplitude.
The calculation of the gytator is really complicated, but with a good calc. software where you can set the basic parameter it's done in a minute.

JHS

[Single Coil]

So you guys are pretty much saying to use a breadboard and simply experiment ans start with a basic or already known schematic and play with the R and C values. A Butterworth is overkill or may produce satisfactory guitar tones. Or start with a circuit base on a Butterworth filter and use that as a starting point?

[Jay Doyle]

EE knowledge is useful to a certain degree. Complex filters are normally overkill and require too many parts. Plus, most of them are designed with a strong emphasis on phase angles and shifts which are not really all that important with FX.

I'd recommend going with a breadboard before software. It's all about how it sounds.