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Preface

The project offers an universal solution that allows you to find and set the best cutoff frequency for an audio system’s subwoofer with no special equipment. Beyond this, in my opinion, practical listening is the best way to compare audio configurations and devices: charts, parameters and calcs are very important but they may not be respected when put into practice.

This crossover gives you the possibility to set six different cutoff frequencies: 63Hz, 80Hz, 100Hz, 125Hz, 160Hz and 200Hz. The right selection is the one that will make your audio system sound better.

The complexity of an universal active crossover

The crossover is formed by six Butterworth’s low-pass and high-pass filters. By the way, thanks to Uwe Beis and his utilities that allowed me to calculate easily the values of the filters’ components saving me lots of time.

On the next two tables (figure 2 and 3) I have collected all the values I calculated: the capacitors are fixed while the resistors change according to the frequency selected. In order to have a good precision, I connected two resistors in parallel to reach each value. Sometimes my choice may seem strange but that is because I used only the resistors I had available at home.

And here there is the resulting schematic. As you can see, lots of components are required. For that reason, in order to contain the board size, I used resistors and capacitors in SMDs technology instead of the usual through-hole one.

Building and testing the PCB

MAKE A PCB WITH SMD COMPONENTS

Double side layers and SMD components require some experience in building PCBs. If you have never built one, or you don’t feel ready for that, you can use through-hole components (⅛W for resistors) even if that means redrawing the whole PCB mask. Feel free to contact me if you need some help

Otherwise, consider that a high level of precision is necessary and making a clear board is essential. Apart from this, the procedure is the usual. Notice that you have to make two boards for a stereo version.

Have you had success on building the board? Nice, now drill it and clean it properly before soldering the components on it.

Place the operational amplifiers, the dip switches and the terminals on the top side of the board and solder them. Then, start soldering the SMD components on the bottom side.

If it is your first time, here are some hints for soldering the SMDs using a traditional solder:

melt a very small amount of tin on the component’s pads; take the SMD with a tweezers, place it on the pads and hold it; now solder both side of the component melting the tin still presents on the pads.

PRACTICAL TESTS

Before powering on the PCB in order to test it, put all the slide selectors of the DIP-Switches to ON. Then, set the cutoff frequency by putting OFF only the selectors that correspond to the number displayed on the table printed on the top layer board (ex: in figure 1 all the selectors n.3 are OFF, 100Hz cutoff frequency is set). Now, you can supply the circuit with a dual voltage between ±5V and ±15V.

Try all different combinations untill you find that one that sounds better for you (in order to prevent annoyng oscillations, power off the board each time you move the selectors). Notice that in some systems with bad output linearity, a different cutoff frequency for the low-pass filters and high-pass ones could be the best solution (ex: 125Hz LPs and 160Hz HPs).

Download the project

Pressing the button below you can download a compressed file with all the necessary material to build this project:

the NE5532 data sheet;

the pictures of the final project, the schematic, the PCB mask and the PCB layout as shown in this post;

the schematic and the board files in Eagle format.

If you need some help please do not hesitate to contact me or leave your comments below. Enjoy it!

SP0006 (2.2 MiB, 513 downloads)

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