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# [SOLVED]Passive crossover filter for 20W loudspeakers

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#### T3STY

##### Full Member level 4
This is a project of a friend of mine, and he's having some troubles making a crossover filter for his 20W loudspeakers (8Ohm). The crossover is made using RC filters and the loudspeakers have a medium-low speaker with a range of frequencies from 20Hz up to 3kHz, and a medium-high speaker with a range of frequencies from 3kHz up to 16kHz.
Now, theory says that a LPF and a HPF made as a RC circuit should work, but when actually connecting the crossover to the speakers no sound can be heard. I suppose, RC is not the way to build crossovers, moreover, I've always seen LC filters used for passive crossover, not RC. Can you confirm this is the issue?

What value resistors are you using? The speakers probably have an impedance of 8 ohms. If you are using, say a 100 ohm resistor in your crossover, you'll be dissipating 90% of the power in the resistor, not the speaker.

THAT'S why they use inductors and not resistors.

T3STY

### T3STY

Points: 2
Also LC filters have a sharper rolloff then a simple RC filter.

I'm not sure which values is he using, some fast calculations brought me a 3.9K resistor and a 10nF capacitor for a 4kHz filter.
So I was right that RC filters aren't good for a crossover... I would try making an LC filter, but actually I have no idea how to properly build an inductor of a specified value. Any advice on that?

You can just buy the components, but if you feel the burning desire to wind your own inductor, there's lots of information on the web.

And with that 3.9k resistor that means you are attenuating the signal by a factor of about 500!!!

Here is a link to a handy calculator for a 2-way crossover.

https://www.erseaudio.com/First-Order-2-Way

The above is for a simple 1st-order type. (Other calculators can be used for more complex designs.)

It gives a value of 0.425 mH for the woofer (8 ohms, 3000 Hz).

A typical crossover coil consists of several dozen loops of wire, a few inches across.

Other websites have a calculator which you can use to figure diameter, length, number of turns, etc., in order to obtain a desired Henry value.

T3STY

### T3STY

Points: 2
if you feel the burning desire to wind your own inductor, there's lots of information on the web.
Oh no, I don't want to bother about building it! Just, what if the desired inductor does not exist as a standard commercial inductor? Then I would be forced to build it on my own... which actually I'd like to avoid if possible...

Ok then, thank you very much guys. If there will be any further problem stay sure I'll be back

The coil does not necessarily need a metal core, in case that's what you're thinking. A core complicates matters when your goal is high fidelity audio reproduction.

The wire also needs to be thick enough to handle the current. Probably 1.3A peak (calculating from 20W and 8 ohm). I think 24 gauge should do it. (0.5106 mm)

Wrap a few feet of it around your hand, and you have 0.425 mH. That's all it takes.

Updating the thread for solution:
RC passive filters won't work because the resistor in the RC filter will add more impedance. Eventually we built LC filters which worked just fine.

Regarding non-standard inductor values: An easy way to solve this problem is to purchase an inductor with an inductance that is slightly higher than your desired value. Then remove turns until the desired inductance is obtained. DIY inductors with easily accessible windings are available from many sources. Parts-Express (www.parts-express.com/) is a good source in the USA. Be sure to specify an inductor with wire size suitable for the current that will flow.

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