Low Pass Filter for Subwoofer, schematic diagram


Everyone wants to have their own personal very good home theater, which at the current prices of going to the public is quite justified, but not everyone can do it. Someone is satisfied with buying cheap Chinese 2.1 speakers, someone adapts Soviet acoustics for bass. And the most advanced radio amateurs make the subwoofer woofer channel themselves. Especially since the manufacturing procedure is not at all complicated. A standard subwoofer is an active LF filter, to which the signals of the right and left channels of the line output are fed, a power amplifier for many, many watts and a large wooden box with a low-frequency speaker. Calculation and manufacturing of the case is purely a joiner’s job, you can read about it on other resources, the power amplifier is also not a problem – with a wide range of all sorts of STKs and LAs. And here we will dwell on the input LF filter for a subwoofer channel amplifier.

As you know, a subwoofer reproduces frequencies up to 40 Hz, and is used in conjunction with small satellite speakers. Subwoofers can be passive or active. A passive subwoofer is a cabinet-mounted woofer head that is connected to a common amplifier. With this type of connection the broadband output signal of the power amplifier is fed to the subwoofer input, and its crossover filter removes the low frequency signal and feeds the filtered signal to the loudspeakers.

the most powerful sub

A much more efficient and common way to connect a subwoofer is with an electronic crossover filter and a separate power amplifier, which allows the bass to be separated from the signal fed to the main speakers at a point in the path where signal filtering introduces much less nonlinear distortion than filtering the power amplifier output signal. Besides, adding a separate power amplifier for the subwoofer channel significantly increases the dynamic range and relieves the amplifier of the main MF and HF channels from additional load. Below I offer the first, simplest version of the low pass filter for subwoofer. It is made as a single-transistor adder filter and we can’t rely on its serious sound quality. We will leave its assembly to the beginners.

Simple amplifier for old boomboxes

subwoofer filter diagram

But these three options proved themselves as great subwoofer filters with equal success, and some of them are installed in my amplifiers.

subwoofer filter diagram

woofer filter diagram

subwoofer filter circuit diagram

These filters are installed between the line source output and the input of the subwoofer power amplifier. They all have low noise and power consumption and a wide range of supply voltages. Microchips used any dual Op-Amp, such as TL062, TL072, TL082 or LM358. The passive elements have the usual requirements as to the parts of high-quality audio circuits. To my ear, the sound of the lower circuit was especially elastic and dynamic, the subwoofer with this option you listen not even with your ears, but with your stomach

Technical characteristics of the filter for the subwoofer:

  • supply voltage, V 12 … 35 V;
  • current consumption, mA 5;
  • Cut-off frequency, Hz 100;
  • gain in passband, dB 6;
  • out-of-band attenuation, dB/oct 12.

Photos of subwoofer filter boards provided by comrade Dimanslm:

subwoofer filter circuit board details

Subwoofer filter PCB

The addition of an active subwoofer significantly increases the dynamic range, lowers the bass frequency, improves the clarity of the midrange and provides high volume levels without distortion. Removing low frequencies from the main signal spectrum to the satellites allows them to sound louder and cleaner, since the cone of the subwoofer does not oscillate with great amplitude introducing serious distortion, trying to reproduce the bass.

Subwoofer Bass Filter with your own hands

When we say “Subwoofer Filter”, we mean an active low-pass filter. It is especially useful when extending a stereo sound system with an additional speaker reproducing only the lowest frequencies. This design consists of a second-order active filter with adjustable 50 – 250 Hz boundary frequency, an input amplifier with gain control (0.5 – 1.5) and output stages.

The design provides a direct connection to the bridge amplifier as the signals are 180 degrees phase shifted relative to each other. Thanks to the built-in power supply, a stabilizer on the board, it is possible to supply the filter with symmetric voltage from the power amplifier – usually it is bipolar 20 – 70V. The LF filter is perfect to work with industrial and home-built amplifiers and preamplifiers.

Schematic Diagram of LPF

The schematic diagram of the subwoofer filter is shown in the figure. It is based on two operational amplifiers U1-U2 (NE5532). The first is responsible for summing and filtering the signal, while the second provides its caching.

12W class AB audio amplifier

LF filter for subwoofer with his hands

The stereo input signal is fed to GP1, and then through capacitors C1 (470nF) and C2 (470nF), resistors R3 (100k) and R4 (100k) it goes to the inverting input of the amplifier U1A. This element is used to realize a signal combiner with adjustable gain, assembled according to a classical circuit. Resistor R6 (27k) together with P1 (50k) allow you to adjust the gain in the range from 0.5 to 1.5, which will pick up the gain of the subwoofer as a whole.

Resistor R9 (100k) improves the stability of the U1A amplifier and ensures its good polarization in case there is no input signal.

The signal from the amplifier output goes to the active second-order low-pass filter built by U1B. This is a typical Sallen-Key architecture, which produces filters with different steepness and amplitude. The shape of this response is directly affected by capacitors C8 (22nF), C9 (22nF) and resistors R10 (22k), R13 (22k) and potentiometer P2 (100k). The logarithmic scale of the potentiometer allows a linear change in the limiting frequency as the knob is turned. The wide frequency range (up to 260 Hz) is obtained with the potentiometer P2 in its leftmost position and by turning it to the right it is possible to narrow the frequency range up to 50 Hz. The figure below shows the measured amplitude response of the entire circuit for the two extreme and middle positions of potentiometer P2. In each case, potentiometer P1 was set in the middle position, providing a gain of 1 (0 dB).

LF filter for subwoofer with his hands

The signal from the output of the filter is processed with the amplifier U2. The elements C16 (10pF) and R17 (56k) ensure the stable operation of the m/s U2A. Resistors R15-R16 (56k) determine the gain of U2B, and C15 (10pF) increases its stability. Both outputs of the circuit use filters consisting of elements R18-R19 (100 ohms), C17-C18 (10uF/50V) and R20-R21 (100k), through which the signals are fed to the GP3 output connector. Thanks to this design, we get two signals shifted by 180 degrees on the output, which allows direct connection of two amplifiers and a bridge circuit amplifier.

Stereo amplifier on MC of LM4702 series

LF filter for subwoofer with his hands

The filter uses a simple bi-polar power supply based on D1 (BZX55-C16V), D2 (BZX55-C16V) and two transistors T1 (BD140) and T2 (BD139). Resistors R2 (4.7k) and R8 (4.7k) are current limiters for the stabilizing diodes, and were chosen so that at the minimum supply voltage the current is about 1 mA, and at the maximum it is safe for D1 and D2.

The elements R5 (510 ohms), C4 (47uF/25V), R7 (510 ohms), C6 (47uF/25V) are simple voltage smoothing filters on bases T1 and T2. Resistors R1 (10 Ohm), R11 (10 Ohm) and capacitors C3 (100uF/25V), C7 (100uF/25V) are also supply voltage filters. The power connector is GP2.

Subwoofer filter connection

LF filter for subwoofer with his hands

It’s worth noting that the subwoofer filter module should be connected to the preamplifier output after the volume control, which will improve the volume control of the whole system. With the gain potentiometer you can adjust the ratio of subwoofer volume to the volume of the whole signal path. Any power amplifier operating in a classic configuration, such as this, should be connected to the output of the module. If necessary, use only one of the output signals, phase shifted by 180 degrees in relation to each other. Both output signals can be used if you want to build an amplifier in a bridge configuration.

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