A quality branded subwoofer is expensive. There are many explanations for this: these are special acoustic requirements for the case, and a high-quality speaker, and a large output stage power, and a special, very thorough factory adjustment.
So, the case. At low frequencies, all the flaws of any speaker system are manifested. Resonant frequencies lead to unwanted vibrations felt like rattling. In order to avoid malfunctioning, the subwoofer casing is clogged with sound-absorbing fibrous fillers, but if the shape and dimensions are calculated and chosen incorrectly, then such a measure will give only a partial improvement.
The speaker is a very important structural element. Its power should be at least 100 watts, and the characteristics must provide for the reproduction of infra-frequencies. Now they are made relatively small - from 6 to 15 inches in diameter.
An experienced radio amateur who has the practice of installing and configuring complex equipment can assemble an amplifier for the subwoofer with his own hands . This is not a simple matter, despite the fact that modern semiconductor components have a high degree of integration. It is required not only to be able to read circuits and understand the principle of operation of a low-frequency amplifier, but also to master the technique of etching boards and other useful skills, including such a simple matter as high-quality soldering.
If you are going to make an amplifier for your subwoofer with your own hands, you immediately need to decide on the purpose of the device. If it is designed to work in the car, then the requirements for it will be one, but if for a home theater - then others.
In the first case, the supply voltage is limited by the operating value of the onboard power supply, usually 12 Volts with a minus on the case. The task is simplified by the fact that a powerful power transformer, amplifier and stabilizer is not required. But you have to assemble the circuit yourself. TDA7293 received good reviews, withstanding high output current and providing sufficient power (up to 100 watts) to boost the bass.
If a homemade subwoofer amplifier is designed for home movies, then the requirements are different. You can, in fact, follow the same path and assemble it in a similar way, but you need a sufficiently powerful transformer with a power supply, and all this needs to be accommodated inside the case, where a rather voluminous speaker and phase inverter are already installed. At the same time, one should not forget about heat transfer, radiators will also take their place.
There is another way to solve the problem - not to assemble an amplifier for the subwoofer with your own hands, but to use a ready-made circuit, for example, from Amphiton or Brig-001 amplifiers of still Soviet manufacture. You can buy them at a very affordable price, especially if one of the channels does not work. A convenient modular system, a ready-made power supply and adjustment unit create all the conditions for using this high-quality equipment, and you can not puzzle your mind how to make an amplifier for a subwoofer.
LC filters that limit the frequency range of the middle and high frequencies should be located at the input and output of the device, immediately in front of the speaker. Assembling an amplifier for a subwoofer with your own hands or using a ready-made circuit, you should take care of limiting noise and the long-term and reliable operation of this electronic device that is sensitive to overloads.
And I also want to say that you don’t have to too intrusively demonstrate the results of your work to neighbors, they will not appreciate it.