Amplifier with filter for a subwoofer - a simple circuit. Active low-pass filter (LPF) for a subwoofer Do-it-yourself passive low-pass filter for a subwoofer

When using a modern radio with an acoustic sensitivity of 89 dB or higher, the volume level is usually quite sufficient. Therefore, the first (budget) amplifier, as a rule, is intended primarily for a subwoofer. Usually there is a signal conditioning unit, but its capabilities are limited. Most often, filters have a fixed cutoff frequency. And a specialized amplifier with smoothly tunable filters is no longer a budget thing.

The proposed schemes are designed just for such cases. Most of them were developed “at the request of workers. Therefore, by the way, there are few drawings of printed circuit boards - this is a purely individual matter, it depends on the details and the layout as a whole. But a lot depends on the board, including the number of “rakes” that the radio amateur will step on when repeating, so all additions are welcome. For now I am designing boards only for designs for “personal use”, I don’t have time for everything...

During development, two conditions were set:

• make do with only a unipolar 12 volt power supply, so as not to deal with the manufacture of converters and not to go inside the amplifier for increased voltage
• the scheme should be extremely simple and not require special qualifications to repeat.

The first diagram is intended for the simplest installations. Therefore, its characteristics are far from ideal, but the capabilities are quite sufficient. The wide range of frequency tuning of the cutoff frequency allows the subwoofer to be used with almost any acoustic system. If the radio does not have linear outputs, it doesn’t matter. The circuit can also work from the “speaker” outputs of the radio. To do this, you only need to increase the resistance of resistors R1, R2 to 33...100 kOhm.

With a wide frequency band reproduced by the subwoofer, to “match” the sound with the front speakers, it is necessary to use an adjustable phase shifter. The circuit of the simplest adder with a phase shifter is shown in the following figure. Compared to the previous scheme, the limits for tuning the cutoff frequency are somewhat narrowed; all other recommendations remain in force. The printed circuit board is not provided - let this be "homework".

However, the capabilities of the simplest schemes are limited. A passive adder produces a large signal attenuation, which forces the amplifier to use maximum sensitivity. In addition, when operating from an unbuffered linear output of the radio (and in budget lines they are all like this), the separation of stereo channels may deteriorate due to the low input impedance of the adder.
Therefore, you need to go to an active mixer of the left and right channel signals. It is most convenient to perform it using field-effect transistors - when using transistors with a cutoff voltage of more than 3 volts (KP303G, KP303E), the required operating mode is achieved without bias on the gate. In this case, a decoupling capacitor at the input is optional. And this is an additional improvement in sound quality. And field-effect transistors themselves are “more noble”.

If the amplifier's built-in filter is satisfactory, the circuit can be simplified.

And finally, when you have everything you need and only need a phase shifter.

Finally, if the subwoofer is anything more complex than a closed box, a low-cut filter should be included in the gain channel. True, to increase the quality factor, it was necessary to perform it according to a third-order scheme, although the frequency response corresponds to the second.

In cases where you need to build a subwoofer signal conditioning unit directly into the amplifier, it makes sense to switch to a bipolar op-amp power supply. Below is a version of the circuit, supplemented by a high-level input and a gain control. Resistor R18 determines the minimum output signal level. If you need to reduce it to zero, the resistor should be replaced with a jumper or the resistance should be reduced to 100-200 Ohms. The input stages and filter remained virtually unchanged, but by increasing the supply voltage to 15 V, the overload capacity was slightly increased. A slight change in the filter ratings increased its quality factor, and as a result, the slope of the frequency response directly in the inflection zone increased. With a wide band, it approaches a third-order filter. When setting up, you need to ensure that the constant voltage at the emitter of transistor VT3 is 6-7 volts.
If you need to increase the transmission coefficient of this filter, you can bypass the resistors at the sources of the field-effect transistors with electrolytic capacitors with a capacity of 10 microfarads and higher. The gain will increase by about 3 times, but there is a risk of distortion.

Parts and installation
To smoothly adjust the cutoff frequency, resistors with a nonlinear resistance dependence (type B) are needed. In the middle position of the engine, the resistance of one half of the “horseshoe” is noticeably greater than that of the other. They need to be turned on so that the engine short-circuits the section with higher resistance.
Ceramic capacitors cannot be used in the audio path due to the microphone effect; they can only be installed in the power supply circuit. Of the inexpensive and accessible ones, it is best to use polypropylene, fluoroplastic or lavsan. For example, K73-17 (from 0.01 to 6.8 microfarads, voltage from 50 to 630V, price from 0.5 to 8 rubles per piece depending on size and tolerance). Capacitors need to be selected in pairs with a minimum spread (it is not the exact value of the capacitance that is important, but the mismatch between the channels). Many modern multimeters allow you to measure capacitance directly. If this is not possible, it is better to use capacitors with a 5% tolerance.
Field-effect transistors across channels must be selected in pairs based on the initial drain current and cutoff voltage. If this is not possible, it is better to use transistors from the same batch - within the package the spread of parameters is usually small. Instead of KP303, you can use assemblies of the KPS series, where the identity of the pairs is ensured technologically. Instead of KT3102E, you can use any other n-p-n transistors with a current transfer coefficient of more than 50. In short, wide possibilities for creativity open up...
To avoid interference, for KP303 transistors you need to connect the “ground” leg of the transistor (case terminal) to the common wire. The input dividers should also be as close to the transistor as possible to avoid long wires in the divider-to-gate circuit. This is especially important when the divider resistance is high.

Source http://www.bluesmobil.com/shikhman/ A. I. Shikhatov 1999-2003

Good day, dear readers! Today we will talk about assembling a simple low-pass filter. But despite its simplicity, the quality of the filter is not inferior to store-bought analogues. So let's get started!

Main characteristics of the filter

• Cutoff frequency 300 Hz, higher frequencies are cut off;
• Supply voltage 9-30 Volts;
• The filter consumes 7 mA.

Scheme

The filter circuit is shown in the following figure:

Parts List:

• DD1 - BA4558;
• VD1 - D814B;
• C1, C2 - 10 µF;
• C3 - 0.033 µF;
• C4 - 220 nf;
• C5 - 100 nf;
• C6 - 100 µF;
• C7 - 10 µF;
• C8 - 100 nf;
• R1, R2 - 15 kOhm;
• R3, R4 - 100 kOhm;
• R5 - 47 kOhm;
• R6, R7 - 10 kOhm;
• R8 - 1 kOhm;
• R9 - 100 kOhm - variable;
• R10 - 100 kOhm;
• R11 - 2 kOhm.

Making a Low Pass Filter

A voltage stabilization unit is assembled using resistor R11, capacitor C6 and zener diode VD1.

If the supply voltage is less than 15 Volts, then R11 should be excluded.
The input signal adder is assembled on components R1, R2, C1, C2.

It can be excluded if a mono signal is supplied to the input. In this case, the signal source should be connected directly to the second pin of the microcircuit.
DD1.1 amplifies the input signal, and DD1.2 directly assembles the filter itself.

Capacitor C7 filters the output signal, a sound control is implemented on R9, R10, C8, it can also be excluded and the signal can be removed from the negative leg of C7.
We've figured out the circuit, now let's move on to making the printed circuit board. For this we need fiberglass laminate measuring 2x4 cm.
Low Pass Filter Board File:

(downloads: 420)

Sand the surface to a shine with fine-grained sandpaper and degrease the surface with alcohol. We print this drawing and transfer it to the textolite using the LUT method.

If necessary, paint the paths with varnish.
Now you should prepare a solution for etching: dissolve 1 part of citric acid in three parts of hydrogen peroxide (proportion 1:3, respectively). Add a pinch of salt to the solution; it is a catalyst and is not involved in the etching process.
We immerse the board in the prepared solution. We are waiting for the excess copper to dissolve from its surface. At the end of the etching process, we take out our board, rinse it with running water and remove the toner with acetone.

Solder the components using this photo as a guide:

In the first version of the drawing, I did not make a hole for R4, so I soldered it from below; this defect is eliminated in the download document.
On the back side of the board you need to solder a jumper:

Low and high pass filters are an integral part of any amplifier. They are usually installed next to the electric coil. There are no moving elements in this case. The main parameters of such devices include bandwidth. Additionally, specialists can calculate signal interception. If we talk about low vibration filters, they can most often be found in subwoofers. In this case, the converter is engaged in changing high-frequency waves.

How to make a simple filter?

In order to assemble a low-pass filter with your own hands, it is best to initially select a magnetic grid. The electric coil in this case should be located behind the resistors. To increase the current bandwidth, a special preselector is used. Additionally, it plays the role of a conductor in the device. The signal interception of the filter depends solely on the types of capacitors.

Field models are considered to be the most common today. Their capacitance on average fluctuates around 3 pF. All this will ultimately stabilize short-wave pulses in the circuit. A reverberator is used to create artificial signals. The conversion in this case should occur without changing the limiting frequency.

Filter calculation

The calculation of the low-pass filter is carried out through cutoff oscillations. Additionally, the formula takes into account the constant signal transmission coefficient. If we talk about active types of filters, the capacitance of the capacitors is also taken into account. To take into account the amplitude of oscillations, the transfer function is additionally calculated. If the output frequency eventually exceeds the original parameters, then the DC gain will be positive.

Active filter types

The active low-pass filter is primarily distinguished by its high bandwidth at 5 Hz. Additionally, elements for signal interception are installed in the system. In this case, the capacitors are soldered on a special magnetic grid. Transistors are used to adjust the limiting frequency. Expanding the capabilities of the device can be done by adding capacitors to the circuit. Their capacitance must be at least 40 pF.

An analog modulator is used for positive feedback. It is installed in the circuit only behind the capacitors. Oscillatory circuits in the system can be stabilized using zener diodes. Their bandwidth must be at least 5 Hz. In this case, the negative resistance parameter directly depends on the frequency range overlap.

Passive filter types

A passive low-pass filter works on the principle of vibration distortion. This happens by installing a reverb. All circuit elements in this case are located on a magnetic grid. A wide variety of modulators are used in filters. The most common today are considered to be double-sided analogues.

A periodic change in oscillations can additionally occur by changing the position of the transistors. The filter should have three capacitors in total. In this case, much depends on the bandwidth of the amplifier itself. If this parameter exceeds 10 Hz, then there must be at least four capacitors in the device.

Additionally, before installing them, the maximum voltage is calculated. To do this, you need to take the rated current of the power supply and, taking into account the capacitance of the capacitors, relate it to the transverse beam. To minimize the sensitivity of the filter, special tetrodes are used. These elements are quite expensive, but the quality of signal transmission is significantly improved.

Devices based on PR1 resistors

A first-order low-pass filter with these resistors is capable of handling a maximum resistance of 4 ohms. All elements of the circuit, as a rule, are located on a magnetic grid. A wide variety of capacitors can be installed in the system. In this case, it is important to calculate the bandwidth indicator in advance. If the capacitance of the capacitors exceeds 2 pF, then a zener diode must be used.

Additionally, some specialists install a reverberator, which can significantly reduce the amplitude of vibrations. The intermediate frequency in this case depends quite strongly on the pairing of the circuits. The rated voltage of the power supply must be at least 20 V. In order for the low-pass filter to successfully cope with interference, the diodes in the system are of the silicon type. If the power supply is set above 30V, the transistors may eventually burn out.

How to assemble a model with PR2 resistors?

A simple low-pass filter with resistors of this type can be used quite successfully with a 30 V power supply. In this case, the bandwidth parameter must be at a level of at least 40 Hz. Positive feedback in the system is provided due to the stability of oscillations.

The negative resistance parameter largely depends on the duty cycle of the pulses. The calculation of the low-pass filter in this case must be carried out taking into account the concentration indicator. It is more expedient to install capacitors in the system of the capacitive type. Diode bridges are used quite rarely in devices. This is due precisely to the absence of resonant frequencies.

Models with powerful converters

Filters with powerful converters can significantly increase the transmittance - up to 33 Hz. In this case, the negative resistance in the system will not exceed 4 ohms. The coils in this case are electric. Movable elements, in turn, are not used. The preselector in the filter is usually located immediately behind the coil. To minimize the risks of various failures, special zener diodes are used.

In this case, resistors should be selected of the analog type. To reduce feedback in the device, capacitors are installed in pairs. In some cases, double-acting zener diodes are used. However, they also have disadvantages. First of all, we should note a rather sharp increase in the sensitivity of the device.

Devices with capacitors

Filters with capacitive capacitors are characterized by stable circuit tuning. In this case, the bandwidth parameter directly depends on the type of electric coil. If we consider chromatic analogues, they are distinguished by their high limiting frequency parameter. Additionally, it is important to consider the volume of capacitors in the filter. The duty cycle of the pulse sequence depends only on the type of converter.

In some cases, the low-pass filter does not work due to a sudden increase in temperature. In this case, it is necessary to additionally install a thyristor near the coil. Filters of this type cannot work with inertial amplifiers. Additionally, it should be taken into account that the power supply must withstand a maximum voltage of at least 30 V.

Models with field capacitors

Low-pass filtering using field-effect capacitors is quite common. This is largely due to its cheapness. In this case, the bandwidth parameter will be at 5 Hz. In turn, the negative resistance of the circuit depends on the installed transistors. If you use single-channel elements, they will significantly reduce the reference voltage.

The deviation of the actual inductance of the filter depends on the sensitivity of the device. Zener diodes are used quite rarely in the system. However, if the negative resistance parameter exceeds 5 ohms, then they should be used. Additionally, you can think about using thyristors. In many ways, these elements will help cope with dipole in the system. Thus, the sensitivity of the device will decrease significantly.

How to use a longitudinal resonator?

Longitudinal resonators are installed in filters quite rarely. These devices are intended to improve the interface of circuits. As a result, the bandwidth parameter can increase to 40 Hz. In order for the system to work properly, zener diodes are additionally installed. Preselectors will be useless in this case. Also, before installing a zener diode, you need to think about the negative resistance parameter.

If it exceeds 5 ohms, then capacitive capacitors must be used. Minimizing system failures can be done in several ways. The most popular of them is considered to be the installation of triggers. Additionally, many experts advise placing special limiters near the coils. These devices will ultimately allow the resonator to operate more stably.

The use of dielectric resistors in a circuit

Dielectric resistors in filters are not very uncommon. They are designed to reduce the negative resistance parameter. At the same time, it is possible to use powerful power supplies. Diodes in this case are mainly used as a reference type. The matching of resonant frequencies depends solely on the output of the resistor.

Capacitors for the filter are selected with a capacity of at least 5 pF. This is necessary in order to increase the bandwidth parameter to at least 3 Hz. All this will ultimately normalize the sensitivity of the device. Additionally, the reference voltage indicator is used to calculate the filter. On average it is at 30 V. If thyristors are not used in the system, then the resistors may eventually suffer.

Models with modulators

A low-pass filter with a modulator is necessary so that the user can tune the device. However, the bandwidth parameter of such devices may be different. The modulator is usually installed on a magnetic grid. A preselector paired with the above element can be used. Additionally, it should be noted that the modulator in some cases is capable of creating low-wave interference. This is due to an increase in the reference voltage. To minimize risks, in this case it is better to install a medium-power zener diode next to the modulator.

Broadband Filter Resistors

A low-pass filter amplifier with broadband resistors has both advantages and obvious disadvantages. If we consider the advantages, it is important to note its high throughput. The cathode connection in this case is made through a small plate. The disadvantage of such resistors is considered to be increased sensitivity.

As a result, the operation of capacitors becomes much more difficult. In some cases, there is an additional load on the electric coil. In any case, to minimize risks, it is important to calculate the filter. To do this, not only the transmittance coefficient is taken into account, but also the capacitance of the capacitors installed in the system.

Many movie fans want to have a personal home theater, but not everyone can afford such a whim. Everyone gets out of this situation in their own way: some will buy simple Chinese speakers, others will adapt Soviet-made acoustics for bass, and the most advanced ones, who have knowledge of radio engineering, will design a subwoofer low-frequency channel on their own. Moreover, it is quite easy.

General information

Let's look at what a regular subwoofer is. In essence, this is a simple active filter whose input is supplied with signals from (right and left channels), an amplifier and a woofer. In this article we will look at the most complex element of the device - a circuit that allows you to independently assemble a low-pass filter for a subwoofer. Such devices reproduce frequencies not exceeding 40 Hz. They are used in conjunction with small satellite speakers. Subwoofers are either active or passive. The latter are a low-frequency head connected to a common amplifier. These types of devices are ineffective and unpopular. The first option is a completely different matter. In such devices, an electronic active low-pass filter for the subwoofer and a separate one separate the bass from the signal that is fed to the main speakers directly at the point in the supply path where filtering this signal will introduce the lowest level of nonlinear distortion, compared to filtering the output power amplifier. Adding a separate amplifier to the subwoofer channel will significantly increase the dynamic range, and also free the mid- and high-frequency amplifier from additional load.

Subwoofer filter: diagrams

The reader is offered three variants of circuits for such a device for consideration. The first circuit proposes the simplest filter for a subwoofer, made in the form of an adder on a single transistor. Serious sound quality cannot be achieved with such a device, but due to its simplicity, it is perfect for beginning radio amateurs. But the subwoofer filter, presented in the following two options, has proven itself with great success as a device with excellent characteristics. Such devices are installed directly after the linear output of the source and the input of the power amplifier. The subwoofer filter is characterized by low noise level, low power consumption, and a wide supply voltage range.

Conclusion

To summarize, adding a type significantly reduces the lower limit of the reproduced frequencies, increases the clarity of the sound in the mid frequencies and provides a fairly high volume level without distortion. Eliminating low frequencies from the spectrum of the main reproduced signal supplied to the satellites allows them to sound cleaner and louder. This is because the cone of the low frequency driver does not oscillate with a large amplitude, trying to reproduce bass and thereby introducing distortion into the signal.

Psychoacoustics (the science that studies sound and its effect on humans) has established that the human ear is capable of perceiving sound vibrations in the range from 16 to 20,000 Hz. Despite the fact that the range is 16-20 Hz (low frequencies), it is no longer perceived by the ear itself, but by the organs of touch.

Many music lovers are faced with the fact that most of the supplied speaker systems do not fully satisfy their needs. There are always minor flaws, unpleasant nuances, etc., which encourage you to assemble speakers and amplifiers with your own hands.

There may be other reasons for assembling a subwoofer (professional interest, hobby, etc.).

Subwoofer (from the English “subwoofer”) is a low-frequency speaker that can reproduce sound vibrations in the range of 5-200 Hz (depending on the type of design and model). It can be passive (uses the output signal from a separate amplifier) ​​or active (equipped with a built-in signal amplifier).

Low frequencies (bass), in turn, can be divided into three main subtypes:

• Upper (English: UpperBass) – from 80 to 150-200 Hz.
• Average (eng. MidBass / midbass) - from 40 to 80 Hz.
• Deep or sub-bass (eng. SubBass) – everything below 40 Hz.

Frequency filters are used for both active and passive subwoofers.

The advantages of active woofers are as follows:

• The active subwoofer amplifier does not additionally load the speaker system (as it is powered separately).
• The input signal can be filtered (extraneous noise from the reproduction of high frequencies is excluded, the operation of the device is concentrated only on the range in which the speaker provides the best quality of vibration transmission).
• An amplifier with the right approach to design can be flexibly configured.
• The original frequency spectrum can be divided into several channels, which can be worked with separately - low frequencies (to the subwoofer), medium, high, and sometimes ultra-high frequencies.

Types of filters for low frequencies (LF)

By implementation

• Analog circuits.
• Digital devices.
• Software filters.

Type

• Active filter for subwoofer(the so-called crossover, a mandatory attribute of any active filter - an additional power source)
• Passive filter (such a filter for a passive subwoofer only filters out the necessary low frequencies in a given range without amplifying the signal).

According to the steepness of the decline

• First order (6 dB/octave)
• Second order (12 dB/octave)
• Third order (18 dB/octave)
• Fourth order (24 dB/octave)

Main characteristics of filters:

• Bandwidth (range of frequencies passed).
• Stopband (range of significant signal suppression).
• Cutoff frequency (the transition between pass and stop bands occurs non-linearly. The frequency at which the transmitted signal is attenuated by 3 dB is called the cutoff frequency).

Additional parameters for evaluating acoustic signal filters:

• The slope of the AHF decline (Amplitude-Frequency Characteristics of the signal).
• Unevenness in the passband.
• Resonant frequency.
• Good quality.

Linear filters of electronic signals differ from each other in the type of frequency response curves (dependence of indicators).

Varieties of such filters are most often named after the names of the scientists who identified these patterns:

• Butterworth filter (smooth frequency response in the passband),
• Bessel filter (characterized by a smooth group delay),
• Chebyshev filter (steep decline in frequency response),
• Elliptical filter (frequency response ripples in pass and suppression bands),

And others.

The simplest low-pass filter for a subwoofer the second order looks like this: an inductance (coil) connected in series to the speaker and a capacitance (capacitor) in parallel. This is the so-called LC filter (L is the designation for inductance on electrical circuits, and C is for capacitance).

The operating principle is as follows:

1. The inductive resistance is directly proportional to the frequency and therefore the coil passes low frequencies and blocks high frequencies (the higher the frequency, the higher the inductive resistance).
2. The capacitance resistance is inversely proportional to the signal frequency and therefore high-frequency oscillations are attenuated at the speaker input.

This type of filter is passive. More difficult to implement are active filters.

How to make a simple filter for a subwoofer with your own hands

As mentioned above, the simplest ones in design are passive filters. They contain only a few elements (the number depends on the required filter order).

You can assemble your own low-pass filter using ready-made circuits online or using individual parameters after detailed calculations of the required characteristics (for convenience, you can find special calculators for filters of different orders, with which you can quickly calculate the parameters of the constituent elements - coils, capacitors, etc. ).

For active filters (crossovers), you can use specialized software, for example, “Crossover Elements Calculator”.

In some cases, a filter adder may be needed when designing a circuit.

Here, both sound channels (stereo), for example, after output from an amplifier, etc., must first be filtered (leaving only low frequencies), and then combined into one using a adder (since most often only one subwoofer is installed). Or vice versa, first sum and then filter out low frequencies.

As an example, let's take the simplest second-order passive low-pass filter.

If the speaker impedance is 4 Ohms, the expected cutoff frequency is 150 Hz, then Butterworth filtering will be needed.