I tried to explain that a droopy lowpass Bessel filter type has two independent RC filters that are calculated to produce a -3db response EACH then both produce a total of -6dB.Hi audioguru,
The cutoff frequency is defined as the point where the normal passband gain drops to -3db. Thus you cant have a cutoff frequency 'fc' that is -6db.
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I'm pretty sure you know this already so maybe you worded your post a little incorrect.
Then I explained how a lowpass Butterworth filter provides boost from positive feedback so that the total response from both RC filters results in only -3dB at their calculated frequency.