Simple Variable Notch Filter Circuit

This circuit has a special filter that can be adjusted to cancel out specific sound frequencies.

This is useful because you can change it to fit what you need.

Another name for this filter is a band stop filter or band reject filter.

These filters mostly get rid of or weaken certain sounds within a particular range.

Sounds higher and lower than that range are allowed to pass through.

In electronic music, there is a need for a type of filter that can block out any sound frequency but can also be adjusted to target different frequencies.

This is what the all pass notch filter with a movable notch frequency is for.

Circuit Working:

Parts List:

The described variable notch filter circuit is engineered to shift the rejection frequency across a 10kHz range throughout the entire spectrum of sound frequencies.

The positioning of the notch is dictated by the voltage applied to the control input.

Both IC1 and IC2 function as all pass filters exhibiting a flat frequency response effectively above the acoustic range.

However, they introduce a phase shift of 0.5 CR between input and output signals.

This phase shift eventually reaches 180 degrees ensuring that when the output and input are combined signal cancellation occurs.

In other words, the variable notch filter circuit operates as an all pass notch filter allowing all frequencies except at 0.5 CR.

The two transistor circuits, Q1, 2, and Q3, 4 serve as voltage controlled resistors enabling the notch frequency to be adjusted when the control voltage is modified.

Formulas:

The external parts linked to the LM324s input and output pins are what make it operate.

A few formula are used to op amp circuits which are mentioned below:

1. Inverting Amplifier:

The following formula may be used to link the input voltage Vin and output voltage Vout of an LM324 op amp configured as an inverting amplifier:

V_out = – (V_in * R2) / R1

where,

• The resistor R1 is the one that connects the inverting input terminal to ground.
• The resistor R2 is positioned between the inverted input terminal and the output terminal.

2. Non Inverting Amplifier:

The formula becomes in the case of a non inverting amplifier setup.

V_out = V_in (1 + R2 / R1)

• The resistor R1 is the one that connects the inverting input terminal to ground.
• The resistor R2 is placed between the output terminal and the non inverting input terminal.

3. Differential Amplifier:

The calculation becomes more complicated and is dependent on the particular circuit design for differential amplifier designs.

Important Note:

The op amp characteristics assumed in these calculations are perfect, with infinite gain and zero input offset voltage.

These elements may have a little impact on the output voltage in actual circuits.

Since the LM324 has four distinct op amps, you may create individual amplifier circuits for each op amp inside the IC using these formula.

You will need to consult circuit schematics and design guidelines for specialized functionality (filtering, oscillation, etc.) that go beyond simple amplification.

These will define the component values and configurations required to get the desired result.

How to Build:

Building a variable notch filter circuit involves assembling the necessary components and following a schematic.

Collect Components:

• Gather all the required components based on the schematic design.
• Refer to the values specified in the schematic for resistors, capacitors and other components.

Schematic Diagram:

• Obtain a schematic diagram for the variable notch filter circuit.
• You can find suitable schematics online or in electronics reference books.
• The schematic will specify the connections between components and their values.

PCB Setup:

• Set up a PCB to build the circuit temporarily.
• Place the components on the breadboard according to the schematic making sure to connect them with jumper wires.

Op Amp Configurations:

• Configure the operational amplifiers IC1 and IC2 based on the schematic.
• Connect the power supply pins positive and negative and ensure proper grounding.

Transistor Circuits:

• Set up the transistor circuits Q1, Q2, Q3, Q4 as per the schematic.
• Ensure correct connections and use voltage controlled resistors for the variable notch frequency.

Preset Connection:

• Connect a preset to the control input for adjusting the notch frequency.
• This preset will control the voltage applied to shift the rejection frequency.

Testing:

• Power up the circuit and use an oscilloscope or a signal generator with an audio amplifier to test the output.
• Adjust the preset and observe the changes in the notch frequency.

Fine-Tuning:

• Fine tune the circuit by adjusting resistor and capacitor values or preset settings to achieve the desired notch frequency range.

Enclosure:

• Once the circuit is working correctly on the PCB, you can consider transferring it to a more permanent setup on a PCB and housing it in an enclosure for protection.

Documentation:

• Document the values of components adjustments made and any modifications for future reference.

Note:

Remember, it is crucial to follow the schematic precisely double check connections and take necessary precautions while working with electronic components.

If you are not experienced in electronics seeking assistance from someone with more expertise is advisable.

Conclusion:

Overall to conclude, a variable notch filter is a versatile tool in audio processing providing the flexibility to target and attenuate different frequencies based, on the specific requirements of the application.

References:

High-speed notch filters

Datasheet 2N3904