CAMERON KOO

Aspiring Audio Engineer

Notch Filter & Bandpass Filter: Creating Interchangeable Filters Through Electronics

What's this project about?

This project is to create two adjustable filters that can be switched smoothly with two dual gang potentiometers. The filters of the project are notch and bandpass filter, it is controlled by a low pass and high pass filters. The main goal is to create filters that can cut certain frequency. To make it user friendly, it is also designed with sliders, so it is easier to control.

This is a picture of me perfecting the filter.
This circuit diagram shows every step to the creation of the product.

What does each part of the circuit do?

My circuit is essentially composed of five main sections (figure below).
The first section (A) includes the input and a buffer. The buffer can solve a lot of impedance issues within the circuit and it an turn the impedance to a lower value as its output.

A color-coded diagram that can help explaining the circuit.

The second section (B) is a bandpass filter with two dual gang potentiometers. The dual gang potentiometers allow me to simplify my circuit and adjust the notch and bandpass filter with the same sliders. The bandpass filter is composed of a low pass filter and high pass filter. first, the signal goes through the low pass filter. Then, it goes through the high pass filter. with the signal being overlapped by two filters, it generates a bandpass filter. The adjustable sliders allow it to change the cutoff frequency.

The cutoff frequency can be calculated. With a 10k potentiometer, the lowest and the highest cutoff frequencies are 176.83Hz and 17683.4Hz.

The third section (C) is a simple notch filter with the same potentiometeres from B. The notch filter is built in parallel to allow the signals to be attenuated. In a way, it works like a band-stop filter depending on the range of the sliders. With my adjustable sliders, the notch filter gives the signals that were removed by the bandpass filter.

This diagram shows how it works. The Y axis represents the frequency(Hz). With the high pass filter and low pass filter in action, the blue highlighted section is the boosted area from bandpass filter. At the same time, the red highlighted section is the cut from notch filter.

The fourth section (D) is an inverting summing amplifier to achieve a unity gain. In a simpler way, the summing amplifier can add two filtered signals together with equal gains before sending them to the output.
The fifth section is an output that ensures my earphones/speakers are protected with the capacitors and resistors.