4 channel 2nd order active filter based on LM324. Circuit from https://www.electronics-tutorials.ws/filter/second-order-filters.html The cutoff frequency is set by (for the first channel) r1,r2, c1 and c2. r3 and r4 control gain and filter Q. For my use I've not been populating r3 and using the default Q. r3 and the equivalents for the other channels are the resistors on the back of the PCB. For r1 = r2 and c1 = c2 the cutoff is given by fc = 1 / (2 pi R C) I was mostly interested in fc between 100 and 1000Hz which can be readily achieved with C of 33 or 47 nF |R ohms|C nF|fc Hz| |-|-|-| |5100 |33 |945.66| |5600 |33 |861.23| |6200 |33 |777.88| |6800 |33 |709.25| |7500 |33 |643.05| |8200 |33 |588.16| |9100 |33 |529.99| |10000 |33 |482.29| |12000 |33 |401.91| |15000 |33 |321.53| |20000 |33 |241.14| |30000 |33 |160.76| |5100 |47 |663.98| |5600 |47 |604.69| |6200 |47 |546.17| |6800 |47 |497.98| |7500 |47 |451.50| |8200 |47 |412.96| |9100 |47 |372.12| |10000 |47 |338.63| |12000 |47 |282.19| |15000 |47 |225.75| |20000 |47 |169.31| |30000 |47 |112.88|