3rd order low pass filter table

Just to finish the 3rd order theme of the last post here is a Butterworth low pass filter table with 3 equal resistors.

C1	C2	C3	20K	22K	24K	27K
33N	82N	4.7N	340.2	308.9	283.3	252.4
47N	120N	6.8N	236.1	214.4	196.6	175.2
68N	180N	10N	160.8	146	133.9	119.3
100N	270N	15N	107.9	98	89.8	80
150N	390N	22N	73.3	66.5	61	54.3
220N	560N	33N	49.5	45	41.2	36.7

So if you want a filter with 50Hz cutoff look down the 20K column and you see 49.5Hz, the nearest value,  so the capacitors are 220n, 560n and 33n and the associated equal resistors are 20k. The table covers a decade of values. If you want 500Hz as opposed to 50Hz you scale the capacitors by a factor of 10,  so the capacitors would be 22n 56n and 3.3n. Alternatively if you wanted 16Hz the capacitors would be 680n, 1.8u and 100n

Here is a Bessel filter table, the frequencies are the -3dB points.

C1	C2	C3	20k	22k	24k	27k
22n	33n	5.6n	366	330	303	269
33n	47n	8.2n	242	223	204	179
47n	68n	12n	168	150	142	125
68n	100n	18n	115	104	95	85
100n	150n	27n	77	70	65	57
150n	220n	39n	52	47	44	39

PCB’s to produce 3rd order crossover filters available here:

http://www.kk-pcb.net/cross.html

Never mind the old TL074  try the MC33079 quad op-amp for this.

Another way of getting a quad op-amp

https://www.ebay.co.uk/itm/323723632105?hash=item4b5f6e9de9:g:4UkAAOSwL1Rcfx4p