For simulating the audio signal, I have used an AC source. The opamps work on a dual rail power supply of +/- 3V. The simulation setup is AC analysis to check the filter response. I simulated the entire circuit in LT spice to check if it works properly. The values for R and C are decided as R = 500 ohms and C = 0.1 uF. The fc for HPF from the table is fc = 3kHz. This will give the values of fc1 = 3.18 kHz and fc2 = 482 Hz.įor the last block, a high pass filter is designed. Hence, using the same formula of LPF, the R and C values for fc1 and fc2 are found to be R1 = 500 ohms and C1 = 0.1 uF and R2 = 3.3k and C2 = 0.1uF. Here, from the table, fc1 = 480 Hz and fc2 = 3 kHz. A band pass filter filters out a band of frequencies which means it has two cutoff freqs, the lower cutoff (fc1) and the higher cutoff (fc2). For calculating the resistor values for gain, the simple gain formula for non-inverting opamp is used. Now, to set the gain of the filter, a nominal gain of about 2-3 works fine. of 482 Hz which is quite near to the actual cutoff freq. Hence, design values for the active LPF are R = 3.3k and C = 0.1 uF. Since we have a huge range of standard resistors available but the range of capacitors is limited, a standard value of C is chosen which is 0.1 uF.īy doing the calculations, the value of R is found to be 3315.72 ohms, so the closest standard value for R is chosen as 3.3 kohms. Considering the same circuit and as decided from the above table, Band 1 means a cutoff freq. In the last blog, I briefly introduced the active low pass filter circuit. Now that we have decided on the bands, let's move to filter design. Lastly, for band 3, a high pass filter is used with a cutoff freq of 3 kHz meaning that it will attenuate all signals below 3 kHz.

This band mostly contains the voice or 'vocals'. For band 2, a band pass filter is designed such that it can filter out the 480 Hz - 3 kHz band. This low-frequency spectrum of the audio is also known as 'Bass'. Since the audio spectrum is limited to 20 Hz - 20 kHz, the first band can be filtered out using a low pass filter with a cutoff freq of 480 Hz, this LPF will allow all the frequencies from 20Hz to 480 Hz and attenuate the frequencies above 480 Hz. Next, step is to decide the bands for the filters.įirst, the audio signal (20 Hz - 20 kHz) is split into three bands as follows- Band The preamp is a very basic non-inverting opamp amplifier with a voltage gain of about 4-5. This signal needs to be amplified such that it can be fed to the three opamp filters. A pre-amp is needed as I'm planning to use the audio out of a smartphone which is in the range of few millivolts. The first block in the design is the pre-amplifier. I will be using RC filters with Butterworth response. In this blog, I will do the calculations for the values of the components required to build the active filters and run some simulations before the final hardware build of the circuit. I also simulated 3 basic filters (LPF,BPF, and HPF) and an active low pass filter to demonstrate the working of active filters. The idea was to split the audio spectrum from 20 Hz to 20 kHz using three active filters and each filter's output is connected to LEDs thereby making a LED music visualizer. In my last blog (part 1) I introduced the overall idea of the project.

Low pass opamp filter designer