How to build an inductor

[albertoZurini]

I want to build an inductor to make a first-order low-pass filter.

res	cutoff	inductor	available	actual	power	current
4	120	0.00530516477	5.00E-03	1.27E+02	600	12.24744871
2	120	0.002652582385	2.70E-03	1.18E+02	600	17.32050808

I basically have to build a 5mH and two 2.7mH inductors based on which amplifier I'm going to use, but I don't know how to determine:

1. The wire length
2. The wire size (because it depends on the RMS current and the wire length)
3. Diameter and number of windings
4. Which material should I use for the core

For the high-pass filter, it was easier, from the power and resistance of the speaker I determined the maximum voltage (~35V) hence I bought 50V capacitors, but the inductor is a bit more tricky because the current is now involved and I don't know which equations I should use. Even if I did study electronic engineering, I'm able to prove the formula to determine the auto inductance, unfortunately, I have zero ideas on how to build a real inductor🤣. I would also guess that those theoretical formulas wouldn't apply here and that for sure there's some sort of simulator that can do a better job than me.

 

[rsavas]

Winding an air core inductor can be done but it is a pita to do by hand, trying to get tight evenly spaced windings is difficult. It’s best done using a winding machine. You could look at what is available at speaker suppliers such as parts express or Madisound as examples. As for actual construction there are web searches and speaker design books to reference

 

[trobbins]

AlbertoZurini, it's recommended for an engineer to include 'units' with values, otherwise the table is ambiguous for readers.

There is also some doubt as to the reason why an inductor is required in the 'amplifier'. You seem to indicate the inductor is in the lead connecting to the speaker - and it forms a low-pass filter - but there is no clarity as to what is before and after the inductor, or why those inductance values were chosen, or whether the inductor needs to remain inductive to quite a high frequency, or whether the inductor will be located near other parts that could interact with the inductor.

 

[Andersonix]

And why do you have to make them yourself? Can't you buy them, or buy slightly bigger and unwind to your spec?
A long time ago Nelson Pass said he takes a big spool of 'X' gauge magnet wire, and it makes a 'Y' value air core inductor, and more if you stick some nails up the middle...