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I am currently taking a class on filter design, and I am learning by doing.

I want to make a small project by building a microphone circuit.

I found online a tutorial by Great Scott. He made a circuit like this. His results were pretty good so I will probably leave it as it is. His design already has a low and high pass filter implemented in the design building a bandpass filter.

enter image description here

I am thinking also about implementing an active high pass filter on the end of the circuit to cancel out low frequency noise and get better signal quality and also practice the real world application of what we do in the lecture. I used the Texas Instruments filter design tool to create a 3rd order Bessel filter, with 3dB gain because its voltage peaks were around 3V. I think it would be better if they were closer to 5V, I was thinking a little less than 4.5V.

Now my problem is that I have no idea how to choose the cutoff frequency. I was thinking 60Hz would be induced from AC wires around the house etc. So triple that at 180Hz should be good - I guess. I would appreciate any input and help on the matter.

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  • \$\begingroup\$ Reducing hum is a matter of good shielding twisted pairs from the mic. Increasing R1 affects gain as it lowers DC level. \$\endgroup\$ Commented Dec 31, 2020 at 14:20
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 so is there no point to adding an active filter? \$\endgroup\$
    – Emre Mutlu
    Commented Dec 31, 2020 at 14:26
  • \$\begingroup\$ For "telephone quality" audio, a rough rule-of-thumb for frequency response of human speech maintains a pass-band from 300 Hz up to 3000 Hz. \$\endgroup\$
    – glen_geek
    Commented Dec 31, 2020 at 14:30
  • \$\begingroup\$ @glen_geek could I cascade a low pass filter also before the highpass? \$\endgroup\$
    – Emre Mutlu
    Commented Dec 31, 2020 at 14:35
  • \$\begingroup\$ The input is an RC HPF \$\endgroup\$ Commented Dec 31, 2020 at 14:37

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I was thinking 60Hz would be induced from AC wires around the house etc. So triple that at 180Hz should be good - I guess. I would appriciate any input and help on the matter.

Well induction from AC wires is due to current flow in those AC wires inducing voltages in series with the microphone wires but, the current in the AC wires can easily have multiple harmonics of 60 Hz and you'll find that the third harmonic (180 Hz) is still very dominant in household appliances etc..

Fifth harmonic is 300 Hz so, where do you conceivably stop?

Answer: you use good practises and avoid induction by twisting signal cables so that the net magnetic induction is basically very low. We also use shielded (or screened as they say in the UK) to remove/reduce electric field coupling. In other words, we filter as a last resort but if we are at that last-chance saloon, then surely we have made a wrong design decision a few stages ago.

Sometimes when I mix guitar or vocal tracks from one of my pals in the US, I use a notch filter to remove as much 60 Hz as I can. Reason: his setup is carelessly put together and I don't think his mancave/shed has a decent earth. Mind you, it's usually some trashy rock song we are writing so it doesn't really matter.

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  • \$\begingroup\$ okay thanks. I was looking to use the techniques we learned in the lectures in a practical exxample. Building a Microphone sounded like a good idea. Do you have any recommendations on what else I could use such a filter, that would still be "cool" to build? \$\endgroup\$
    – Emre Mutlu
    Commented Dec 31, 2020 at 16:26
  • \$\begingroup\$ Absolutely - tone controls; bas boost, bass-cut, treble boost and cut. Maybe even add a bandpass filter as something to control mid-range frequencies. \$\endgroup\$
    – Andy aka
    Commented Dec 31, 2020 at 16:32

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