The acoustic spectrum covers a broad range, extending from very low frequencies at around 20 Hz to high frequencies up to 20,000 Hz. In the lower frequency range, our ability to perceive directionality diminishes. This is why we employ a speaker to handle these very low frequencies. The solution we are presenting here is designed to isolate these frequencies and direct them to the appropriate amplifier.
Acoustic filters find applications at various points within sound systems. Familiar applications include Baxandal filters used for adjusting low and high-frequency tones and crossover filters that divide the acoustic spectrum into subregions, each directed to the appropriate set of speakers. The solution we are presenting here is a simple filter designed to limit the acoustic range from 20-20000 Hz to a narrower range, specifically 20-100 Hz.
With the solution we are presenting, you can create an active filter to direct low-frequency sounds to a dedicated subwoofer. By doing so, you can position a larger speaker alongside your existing Hi-Fi speakers. To achieve a comprehensive sound experience, you will also require an appropriate amplifier. To integrate this into your system, you can connect the circuit’s input to either the two outputs of a preamplifier or the output of a preamplifier line.
The manufacturing circuit provides an output that can be connected to the subwoofer’s power circuit. If, for any reason, you lack space to accommodate a third speaker in your listening area, you have the option to select a smaller speaker. The choice of speaker output depends on the type of music you are listening to. If you do have the space available, after successfully creating this filter, you can recommend it to your friends or even create additional filters for them.
The diagram below illustrates the theoretical filter circuit. At first glance, it comprises three distinct circuits primarily built around two operational amplifiers. These circuits collectively form a mixer, an amplifier with adjustable gain, and a variable filter. The final design requires a biasing circuit with a 12-volt biasing tendency.
The active components used in this circuit are dual operational amplifiers such as the TL082 and NE5532. These operational amplifiers belong to a family equipped with a JFET (Junction Field-Effect Transistor) at their inputs.
Each member of this family includes both bipolar transistors and JFETs in their respective circuits. These circuits can operate at high biasing levels due to the utilization of high-biasing transistors. They also possess a high slew rate, low input bias current, and are minimally affected by temperature variations. These operational amplifiers offer a unity gain bandwidth of 3MHz. Another crucial factor for their selection is their substantial noise rejection capability, which proves valuable in noise-prone power lines.
These operational amplifiers have noise rejection levels exceeding 80dB and exhibit low power consumption, ranging from 11 to 3 mA. They are typically housed in an 8-pin DIP (Dual In-line Package) and contain two operational amplifiers per package. In a 14-pin DIP package, four operational amplifiers are integrated, and they are commercially available under designations like TL074, TL084, and TL064. The 8-pin DIP package variants include operational amplifiers such as TL061, TL071, and TL081. In our particular construction, we have employed the TL082, which features two operational amplifiers.
The first operational amplifier from the TL082 serves as a mixer and amplifier for the two channels. Its negative input is connected to a simple mixer consisting of two resistors. The aid or gain of the circuit is determined by a potentiometer within this network. At this junction, the left and right channels of the preamplifier are combined through two resistors. The operational amplifier subsequently amplifies the signal, with the gain determined by the potentiometer’s setting.
The position of the potentiometer corresponds to the circuit’s gain. The second operational amplifier functions as the filter in our design. This filter is a second-order active low-pass filter, and its components are integrated around the operational amplifier. The filter operates with a variable cutoff frequency, which can be adjusted to span a wide range, from as low as 30Hz to well over 150Hz. The cutoff frequency of the filter depends on the component values within the circuit.
By modifying the component values, we can achieve cutoff frequencies of 150Hz, 130Hz, 100Hz, 70Hz, 60Hz, or even 30Hz. These settings are easily accomplished by simply adjusting a dual potentiometer. The filter circuit is built around a single operational amplifier, specifically the TL082, which is a dual operational amplifier. The filter’s output is connected to the output jack where it interfaces with the amplifier. At the circuit’s output, the signal is presented, with its frequency range restricted according to the filter’s characteristics, based on the input signal’s frequency spectrum.
R1 = 39 Kohm
R2 = 39 Kohm
R3 = 47 Kohm
R4 = 10 Ohm
R5 = 22 Kohm
R6 = 4,7 Kohm
R7 = 22 Kohm
R8 = 4,7 Kohm
R9 = 10 Ohm
R10 = 220 Ohm
C1 = 39 pF
C2 = 0.1 uF
C3 = 0.1 uF
C4 = 0.2 uF
C5 = 0.4 uF
C6 = 0.1 uF
C7 = 0.1 uF
IC1 = TL064
To construct the device, you will require a printed circuit board (PCB) as depicted in the diagram. On this PCB, you will position the components according to the provided layout. While assembling the components, it’s important to exercise caution to prevent potential errors. In the event of any discrepancies or malfunctions, a thorough inspection of the circuit is recommended.
As previously mentioned, this circuit functions as a filter, and therefore, it is essential to use materials of high precision and quality, particularly for the capacitors. The capacitors used in the filter should have a tolerance of 5%. However, it is possible to experiment with lower-quality materials during the construction phase.
To ensure the functionality of the device, you can employ an acoustic signal generator. By connecting the generator to the input of the device and measuring the output voltage using a voltmeter, you can monitor the response of the circuit. Adjusting the potentiometer should result in corresponding changes in the output voltage, indicating that the circuit is functioning correctly.