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?

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    $\begingroup$ You better check this document for electret microphone preamplifier design: link $\endgroup$
    – Fat32
    Commented Mar 28, 2019 at 13:45

1 Answer 1


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.

  • $\begingroup$ Thank You for the reply. The intention is to construct a circuit to output a maximally clear signal from an electret microphone for active noise cancellation headset. It's the first time I'm hearing that some filters have to be grounded and some don't, could You direct me to where I could read up on this? The circuit from Youtube source works for me, but I would like to understand why and how I could manipulate it. $\endgroup$ Commented Mar 28, 2019 at 12:52
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    $\begingroup$ You should probably do some reading on basic electronic circuits (RC circuits, transistor, op amp). No offense, but you seem to be lacking some basic knowledge, i.e. the principal and meaning of "ground" does not seem to be clear to you. $\endgroup$
    – Max
    Commented Mar 28, 2019 at 13:54

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