Active filters for subwoofers. The art of audio

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Active filters for subwoofers

The art of audio

When using a modern radio with an acoustic sensitivity of 89 dB or more, the volume level is usually quite sufficient. Therefore, the first (budget) amplifier, as a rule, is intended primarily for the 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.

Active filters for subwoofers

The proposed schemes are designed just for such cases. Most of them were developed "at the request of the workers."

During development, two conditions were set:

get by with only a 12-volt unipolar power supply, so as not to mess with the manufacture of converters and not to climb inside the amplifier for increased voltage;
the scheme should be extremely simple and not require special qualifications to repeat

PCB for this device

The first scheme is intended for the simplest installations. Therefore, its characteristics are far from ideal, but the possibilities are quite sufficient. A large range of cutoff frequency tuning allows the subwoofer to be used with almost any acoustic system. If the radio does not have linear outputs - it does not matter. The circuit can also work with the "column" outputs of the radio. To do this, you only need to increase the resistance of the resistors R1, R2 to 33 ... 100 kOhm.

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

Active filters for subwoofers

However, the possibilities of the simplest schemes are limited. The passive adder gives a large attenuation of the signal, which forces the use of the maximum sensitivity of the amplifier. In addition, when working from an unbuffered linear output of the radio (and they are all like that in budget lines), the separation of stereo channels may deteriorate due to the low input impedance of the adder.

Therefore, you need to go to the active mixer of the signals of the left and right channels. It is most convenient to perform it on field-effect transistors - when using transistors with a cutoff voltage of more than 3 volts (KP303G, KP303E), the required operating mode is achieved without gate bias. In this case, a decoupling capacitor at the input is optional. And this is an additional improvement in sound quality. And the field-effect transistors themselves are "nobler".

Active filters for subwoofers

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

Active filters for subwoofers

And, finally, when there is everything that is needed and only a phase shifter is needed.

Active filters for subwoofers

Finally, if the subwoofer is anything more than a closed box, a sub-low cut filter should be included in the gain channel. In the "Filters not according to the rules" cycle, the filter of the OI-OK structure is considered. It is the best suited for this purpose. True, to increase the quality factor, it was necessary to perform it according to the third-order scheme, although the frequency response corresponds to the second.

Active filters for subwoofers

Publication: www.bluesmobil.com/shikhman

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