Electret microphone RC filter design contradiction among information sources I'm building a circuit for an electret microphone and I want to build a bandpass filter around the op-amp. 
I was using this source until I have found that most sources ( e.g. here and here) indicate that RC filter needs to be grounded.
I can't think of a way check this (FFT doesn't seem to indicate whether the first source is correct or not). Any ideas on how to check or information which source is correct?
First, the youtube source shows an 1st order active filter, maxim source shows 2nd order active filter and last source shows first order passive filter. That's three different designs.

A 1st order active filter can be implemented either in the negative feedback branch of the op amp (as in youtube source), this does not need to be grounded.

Or it can be done as a classical RC circuit followed by impedance converter or amplifier. Here, either the R goes to ground (highpass) or the C goes to ground (lowpass)

If you combine these two approaches you get a 2nd order filter. So, wether "the filter needs to be grounded", as you ask, depends strongly on where its implemented.