Help! The more I search about Speech Filter - the more I get confused! pls help

Thread Starter


Joined Oct 1, 2017
Hi please help, I get confused more and more as I search on how to design a Speech Filter. For my Signal Processing class project, we are asked to design a Speech Filter, which will have a low pass and high pass filters between 300Hz to 3KHz. But, as I look around on how to design this filter, I encounter Op-amps, Butterworth filters, fourth-order filters, etc.. which made it more complicated and more confusing. I've been reading schematics and youtube videos online - everything seems not to grasp of how to begin the approach of designing this filter. Please help! Thanks in advance!


Joined Feb 24, 2006
The way to start is to develop the actual requirements of the filters. Here is how you do that:
  1. What is the corner frequency? For example: 300 Hz.
  2. What is the allowable ripple in the passband? For example: None - maximally flat
  3. What is the maximum attenuation in the passband? For example 0.3 dB
  4. What is the bandwidth of the transition band? For example: 900 Hz.
  5. What is the minimum attenuation in the stop band? For example: -40 dB
  6. What is the allowable ripple in the stopband? For example: 20 dB
From these requirements you can derive the type of filter, and the order. Then you can test your design to see if it meets the requirements.
For more information on the process check out:
Van Valkenburg, M.E.,Analog filter Design

Thread Starter


Joined Oct 1, 2017
Thank you for the replies guys. I made the circuit on a breadboard but somehow I couldn't manage to produce a sound from the microphone to speaker..

*attached is the circuit I made. please let me know where I made a mistake..



Joined Aug 21, 2008
Are those ohms in your schematic or kilo ohms?

Your opamp can't drive much voltage into a 15 ohm fredback resistor. Multiply the resistors by 1,000 and devide the capacitors by 1,000 and things should start wotking.

After you fix the resistors and capacitors, You may find that your opamp may not be capable of driving the speaker hard enough for you to hear unlesd you put your ear right up against it.