I am building a chorus and want to make pre and post
LP filter with just one transistor per 18dB filter. I know that
24dB is better, but want to simplify the schematic and save
some space on PC board.
I know how to calculate 12db filter and 24dB are just two of
12dB in series, but I am dont know how to calculate cutt-off
freq of one 18dB.
Can you help me please?
Except in a few special cases you can't write formulas for the -3dB cut-off of 18dB and 24dB filters (and beyond). Generally you have to simulate the filter or solve complicated equations.
For a Butterworth response you can compute the 'f0' or 'w0' for the filter and this is the same as the -3dB regardless of the filter order/slope. Butterworth works out easy.