# DIY Voice Coil with Cone

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#### argh

Hello forum members,

Are there any online calculators or is there a mathematical formula that can be used to calculate the target impedance of the voice coil based on the gauge of the wiring, the number of winds etc?

For example, if my initial target is an 8 ohm voice coil and I am using 38AWG wiring, what would the optimal method be for determining the number of winds to get to an 8 ohm voice coil? I have a standard LCR meter and a high precision LCR meter as well, I had thought about periodically melting the wire during the winding process to measure the inductance/capacitance/resistance of the coil on the jig to derive a formula for the length of wiring/number of winds needed, but that's probably not the right approach nor do I think it would work based on the wire still being attached to the spool feeding the winding jig.

I am also curious about how the number of winds affects the magnetic field collapse. I am sure these are basic electronics 101 questions but I can't find any direct answers from the research I've done so far.

Thanks in advance for any input!

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#### argh

No, that's the point of the project. I am building a subwoofer from scratch, and by scratch I mean building a winding jig, winding the voice coil, sourcing parts for the speaker cones etc.

#### JMFahey

So simple you just use pen and paper, and work interactively ... which you must do anyway because wire does not come in "any" size but there's a predetermined size table.

So you start by VC diameter, winding length and amount of layers (usually 2) and start with some value, say: 0.20mm .
You go to a wire table which gives you actual diameter (because it's enamelled).
Then you calculate amount of turns with that wire size.
Simple geometry tells you turn length, multiply this by amount of turns and you have the total VC wire length.
Check ohms per meter from the wire table and you get DCR.
For an 8 ohms nominal VC you try to have around 6 to 6.5 ohms.
If you got too much or too little, select a new wire diameter and start again.
A couple minutes per calculation, not worth writing an Excel table for this, although it's not a bad idea if you do it often.

Interesting subject,

I've been playing with the thought of making a subwoofer myself.

I suppose you have some idea of the gap depth you're going to use? How do you plan to get/ make the pole plate and might there be a specific steel that's optimised for this use?
Once you know the gap depth and have a target Xmax, you know the winding depth, which is where JMFahey started.

Or are you going for the winding only and an excisting motor structure?

PA drivers often use inside and outside winding on the voice coil former to optimise heat transfer (say 2 layers on the inside plus 2 layers on the outside)

If you have a target BL (flux times length), it will tell you the ratio of magnet field strength to wire length, which would result in an estimated wire length and thus thickness.
Most PA manufacturers advertise a magnetic flux around 1 Tesla for subwoofers but 0.35 T throughout the gap would be a good start.

You might find this interesting. It's an older project that looks stranded but Vikashy is also an moderator on here. I for one would be interested to know what happened to it.

Best regards Johan

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#### CharlieLaub

Paid Member
If you know the gauge of the wire and the type of wire material you should be able to calculate DC resistance for a given length, or conversely the length of wire needed to obtain Re= N ohms. An 8 ohm "nominal" speaker will have Re around 5-6 Ohms.

How you wind the VC is another topic alltogether.

This is how you wind it: YouTube

I understand that this man works barefoot if he is hot, but I would not give him a quality speaker.....

This other repairman deserves me more confidence ....

#### Jonhny

I used ohms law for that. Circumference of the winding and number of turns gives you length of wire. Getting the TS parameters right is far more difficult.

#### bentoronto

This is how you wind it: YouTube

Looks like you can get pretty good results working that way. I think I'd just make a little lathe-like mechanism - it would free up at least one hand for those of us who have two "left feet" (meaning: feet that aren't athletic).

I suppose you'd always want to make a VC with substantial overhang because that would eliminate the bad effects of irregularities in the winding process.

One trade-off is using thinner wire in order to get your sufficient turns (BL) into the gap (assuming a novice winding) but at the cost of limiting the max watts you can put through that wire.

I'm forever dreaming of a Vilchur/AR3 woofer with a 10 Hz resonance. Since it is hard to source such a thing, you have to DIY.

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