Sallen-Key Bandpass filter question

[AmpFix]

I'm trying to design a Sallen-Key band pass filter with an adjustable center frequency. I'm thinking that I could possibly change the center frequency with a single pot but I'm not sure how the bandwidth would be affected.

I'm using this design as reference:http://upload.wikimedia.org/wikipedi...ss_General.svg

It seems as though rf is the one to modify but most equations simply everything by using rf=r1 and r1=2r1

Any ideas on how to shift the center freq on this filter type without affecting the BW?

[Steve Conner]

To shift the frequency without changing the Q, as far as I know, you need to change all three resistors R1, R2, and Rf in proportion.

You should probably use the state-variable filter instead, it can be swept with a dual-gang pot. You can get 100k dual reverse log pots on Ebay.

If you specifically want a filter that can be swept by a single pot, try the gyrator or wah-wah style, but these aren't constant-Q. Or make a state-variable filter with a LM13700, this allows you to control the centre frequency with a DC voltage, which you can get from a pot. I have one of the old Maplin kit wah-wah pedals with this circuit inside.

[Amplexus]

Check out Bob Pease, his world class design book has something that should do the trick, you may also want a copy of his troubleshooting book. both are classics, I just talked to him about this and I think he may do a column on it. Google Pease Porridge and elecronic design. Be warned you are likely to be sucked into reading decades of back columns. Not only is he a genius when it comes to analog design he is also really funny. Read his camel stuff column, it details his resignation from Philbrick and sets the bar for resigning with style,
Amplexus