When you are working with electronics you are always in need of filters. Sometimes to avoid certain frequencies or to amplify that specific frequency you actually need. In the basics section of this website I showed the most fundamental passive filter (LowPass and HighPass) that only consists of one capacitor and one resistor. These filters are not so powerful. Their behaviour actually also depends of the circuit that is connected to it (its load) and the resonance frequency will change as well.
An explanation about active filters can be found here (https://www.electronics-tutorials.ws)
A solution to make better, more powerful and more frequency specific filters, is to make use of active filters. These filters make use of opamps. If you search the web about active filters, you will find a lot of information. In this section I will show some of the basic active-filters that are applied in a lot of audio-devices. To calculate the right values for C1, C2, R1 and R2 can be quiet complex. There is a lot of documentation about this, but your math skills should be good. You can also use some on-line calculators to determine the values and experiment with this particular design.
Take a look at :http://sim.okawa-denshi.jp/en/OPstool.php
Or this one: https://www.beis.de/Elektronik/Filter/ActiveLPFilter.html
Active Low Pass filter (Sallen-Key):
Active High Pass filter (Sallen Key):
Active bandpass filter:
