Low pass Sallen-Key filter with variable cut-off frequency there are often to find - go to first attachment.

In case of filters with high pass character and variable cut-off frequency in commercial active crossover network devices I find always switches (for variation the values of C1 and C2 in second attachment) but never a stereo potentiometer for R1 and R2.

The reason therefore is the fact, that the character of filter isn't constant over the entire angle of 270° rotation as long as the potentiometer is a linear version.

There was sometimes describe an approach (with schematic) in an Elektor (Elektuur) article (80s or 90s) that solved this problem by adding different resistors to the used stereo- resp. tandem-potentiometer for R1 and R2 (both serial resistors and parallel resistors - but in different kind for R1 and R2). These additional resistors made it possible to use a potentiometer for R1 and R2 in the second attachment without affecting the characteristics of the filter over the intended frequency variation range.

This is from my view much more better and more easy to realize than the variation of C1 and C2 (from second attachment) with stepper character switch.

Who can upload this article from elektor ?

Thank you very much.

P.S.: The attachments are from here:

http://www.stefanv.com/calculators/hp41c_sallen_key.html

In case of filters with high pass character and variable cut-off frequency in commercial active crossover network devices I find always switches (for variation the values of C1 and C2 in second attachment) but never a stereo potentiometer for R1 and R2.

The reason therefore is the fact, that the character of filter isn't constant over the entire angle of 270° rotation as long as the potentiometer is a linear version.

There was sometimes describe an approach (with schematic) in an Elektor (Elektuur) article (80s or 90s) that solved this problem by adding different resistors to the used stereo- resp. tandem-potentiometer for R1 and R2 (both serial resistors and parallel resistors - but in different kind for R1 and R2). These additional resistors made it possible to use a potentiometer for R1 and R2 in the second attachment without affecting the characteristics of the filter over the intended frequency variation range.

This is from my view much more better and more easy to realize than the variation of C1 and C2 (from second attachment) with stepper character switch.

Who can upload this article from elektor ?

Thank you very much.

P.S.: The attachments are from here:

http://www.stefanv.com/calculators/hp41c_sallen_key.html