While making my Custom Speakers (inspired by Lampizator's Endorphine 17 project) I was wondering how to calculate the crossovers.
I read a lot of theory, downloaded a bunch of programs, etc., but finally I came to the conclusion that the simplest crossover works best and I will describe you how you can make it by yourself.
First you need to find out the impedance of your speakers. Mine are 5Ohm Saba pairs, so I will make the example calculations based on that.
1) For tweeter you can put a capacitor in series which will form a first order High Pass filter which have to cut off frequencies below ~8kHz-10kHz (above those most midrange drivers became weak). In my case this is a 3-4uf capacitor. I choose 3,3uf capacitor.
You can use this calculator to find out what capacitance will suit in your case:
2) For midrange you need to make a Band Pass filter, so you can cut the low frequencies where the midrange driver is weak and the very high where it is also becoming weak and the tweeter will support it.
For Saba midrange driver these are below 300-400Hz and above 9-10kHz, so I needed a coil to filter high frequencies and a capacitor to filter very low frequencies - all put in series to the driver.
In my case this is a 0.39mH (mili Henry) coil and a 100uf capacitor. The 0.39mH coil start to cut off frequencies above 2kHz but with decent step so it can match to the tweeter filter correctly at ~9kHz.
And the 100uf capacitor will cut off everything below 318Hz, which is pretty fine.
Here is a band pass calculator which will show you all the curves:
More straight forward calculator for the High Pass part of the Band Pass filter: http://sim.okawa-denshi.jp/en/CRtool.php
And a straight forward calculator for the Low pass part: http://sim.okawa-denshi.jp/en/LRtool.php
3) Now we come to the bass section. The goal is to support the midrange. You can use any 12" or 15" speakers here. I choose Saba 10" 5Ohm because my room is pretty small and I had a bass extension boxes (without back wall of course!).
So let's see how we can cut off everything above 300-400Hz with a Low Pass filter (passes everything below the cut off frequency)
Here we have to be more aggresive that's why we will use a Second order Low Pass filter which consists of a coil in series to the driver and a capacitor connected to the ground.
I went with a 4mH coil (because of the price) and a 10uf capacitor. I could use a 50uf capacitor so I can get a 350Hz cut off frequency, but since I use a 10" bass speaker it is a little big weaker than 12" ot 15", I decided to go to higher cut off frequency so I can compensate the small bass speaker size.
But you should definitely make your calculations in a way that you cut everything above 300-400Hz if you have bigger bass speakers.
This is the calculator you can use: http://sim.okawa-denshi.jp/en/RLClowkeisan.htm
*) remember you have to connect bass speakers with reverse polarity! So they can cancel each other with the midrange driver where they overlap, e.g. minimize unwanted destructive acoustic interference in the frequency region covered by both woofers and main/midrange speakers.
Here is my crossover as a final schema.
Coils have to be with low resistance (2-5Ohm). I use Jantzen Audio coils - C-Coil for bass and Wax Coil for midrange - http://jantzen-audio.com/html/coils.html. Very very good coils from Danmark!
Final advice - do not use electrolytes. Buy PIO or Polypropilene capacitors. They are much better and you will hear the difference. Even if you use old russian PIOs (Paper In Oil).
Chinese Obbligato http://www.partsconnexion.com/capacitor_film_obbligato_oil.html and http://www.partsconnexion.com/capacitor_film_obbligato_psu.html are very good.
If you have a small budget you can go with electrolytes, of course, but do not forget that the signal wire have to be connected to plus (+) side of the electrolyte capacitor.