Fixed-frequency, variable-gain bandpass filter

[MrStab]

Hi guys,

I came up with this circuit for someone ages ago, and i doubt i'll ever use it again, but maybe someone else could find it useful. I have no clue whether there are any advantages over other topologies, but its gain behaviour can be customised quite easily. This is a Multiple-Feeback Loop filter combined with a differential amplifier.

The customer at the time requested a boost at 2KHz, so that's the centre frequency in this example. You can calculate values for frequency and Q here, but as always i recommend you at least try to learn the equations for yourself! 10k and 3k in the diffamp gives you +/-15dB. Increase or decrease the 3K for less or more gain, respectively. In all honesty, i can't be bothered finding my notes for that equation, lol.

By cutting either end of VR1 from the diffamp, and shorting it to Vref ("B"), you can make the circuit boost OR cut, instead of boost AND cut. Use a Linear pot for the "and", but for a better response in the "or" configuration, use a Log pot for boost and Rev-Log for cut (though linear will work for all three).

If your input is low-impedance, you can do this with just a dual op-amp. As the output of IC1a will be low-impedance, you can add several filters in series afterwards. Buffered Vref isn't necessary, ofc. I've made two identical, 2-band filters of this type and there were no gig-volume issues to report. Feel free to mess around with it, poke holes in it or use it for roach.

cheers!

[Kipper4]

Thanks Grant.
Nice one.
Thanks for sharing the mfb page too.
I might have to put some old filter design values in and see f I can learn something.

Rich