This circuit features a special filter that can be tuned to eliminate particular frequencies of sound.
One can change it to suit its needs which makes it helpful.
This filter is also known as a band reject filter or band stop filter.
These filters primarily eliminate or reduce specific noises within a given range.
Both higher and lower sounds are permitted to go through.
A particular kind of filter that can both block out any sound frequency and be tuned to target different frequencies is required in electronic music.
The all pass notch filter with a changeable notch frequency is used for this purpose.
Circuit Working:
Parts List:
| Category | Quantity |
|---|---|
| Resistors ( All resistors are 1/4 watt) | |
| 22k | 4 |
| 10k | 2 |
| 4.7k | 1 |
| Preset 1k | 1 |
| Capacitors | |
| Ceramic 15nF | 2 |
| Semiconductors | |
| IC LM324 | 1 |
| Transistor 2N3904 | 4 |
Throughout the whole range of sound frequencies the rejected frequency is shifted by the stated variable notch filter circuit over a 10kHz range.
The voltage provided to the control input controls where the notch is placed.
Above the acoustic range IC1 and IC2 both operate as all pass filters with a flat frequency response.
However they cause the input and output signals to phase shift by 0.5 CR.
Eventually this phase shift nears 180 degrees assuring that signal cancelation happens when the input and output are combined.
The variable notch filter circuit in other words functions as an all pass notch filter allowing all frequencies with the exception of 0.5 CR.
By acting as voltage controlled resistors the two transistor circuits (Q1, Q2 and Q3, Q4) allow the notch frequency to be changed in response to changes in the control voltage.
Formulas:
The IC LM324 is powered by external components connected to its input and output pins.
The following formulas are used to operate op-amp circuits:
The input voltage Vin and output voltage Vout of an IC LM324 op-amp set up as an inverting amplifier which is connected using the formula below:
Vout = – (Vin * R2) / R1
where,
- The inverting input terminal is connected to ground by the resistor R1.
- Between the output terminal and the inverted input terminal lies the resistor R2.
Non Inverting Amplifier:
When a non-inverting amplifier configuration is used the formula changes to:
Vout = Vin (1 + R2 / R1)
- The inverting input terminal is connected to ground by the resistor R1.
- Between the non-inverting input terminal and the output terminal lies the resistor R2.
Amplifier with different:
For differential amplifier designs the calculation is complex and depends on the specific circuit design.
Important Information:
These calculations assume perfect op-amp features such as unlimited gain and zero input offset voltage.
In actual circuits these components could somewhat affect the output voltage.
Because the IC LM324 has four different op-amps one can use these formulas to build separate amplifier circuits for each op-amp inside the IC.
For particular functions (filtering, oscillation etc.) which goes beyond basic amplification users will need to refer to circuit schematics and design standards.
These will specify which configurations and component values are needed to achieve the desired result.
How to Build:
To build a Simple Variable Notch Filter Circuit follow the below mentioned steps for connections:
Assemble the components:
- Collect all required parts using the schematic design as a reference.
- Check the values of the resistor, capacitor and other parts as indicated in the diagram.
The structure of a diagram:
- Get the variable notch filters circuit schematic.
- Suitable schematics can be found online and in electronics reference books.
- The schematic will describe the connections between the various parts and their respective values.
Configuration of the PCB:
- Prepare a PCB for the building of a temporary circuit.
- As indicated in the circuit diagram above put the components on the breadboard using jumper wires to connect them.
Operational Amp Configurations:
- The operational amplifiers IC1 and IC2 should be arranged according to the diagram.
- Connect the positive and negative pins and ensure that the power supply is grounded correctly.
Circuits for transistors:
- As shown in the schematic diagram arrange transistor circuits Q1, Q2, Q3 and Q4.
- Use voltage controlled resistors for the variable notch frequency and be sure all connections are correct.
Initial Connection:
- To change the notch frequency connect a preset to the control input.
- The voltage used to change the rejection frequency will be managed by this setting.
- Turn ON the circuit and check the output using an oscilloscope or a signal generator that has an audio amplifier.
- Watch how the notch frequency varies to make adjustments to the preset.
Adjusting:
- To get the required notch frequency range fine tune the circuit by modifying the values of the resistor, capacitor or preset settings.
The enclosure:
- After the circuit functions properly on the PCB one might want to move it to a more permanent configuration and cover it with an enclosure.
Records:
- For future reference note any changes and the values of the component adjustments performed.
Note:
- Remember it is important to carefully follow the circuit diagram, check connections twice and take proper safety measures while working with electrical components.
- If somebody is unfamiliar with electronics it is best to get help from someone with more experience.
Conclusion:
Depending on the particular needs of the application the Simple Variable Notch Filter Circuit in audio processing serves as a versatile circuit that can focus on and reduce various frequencies.