choosing wire for voice coil

[Alnico2]

Hi. So I've got these speakers with blown out voice coils and figured it be fun to repair. They're eminence 10446 and available reconing kit details the voice coils as being: 1.005″ inside diameter,
1.125″ form length,
0.437″ windings width, (read coil length)
8 ohms

I've been unsure of what gauge wire to use, the recone kit doesn't mention gauge, but measuring what's left of the original I count 30 turns per 1/4" of coil length, which would be 120 per inch. This gives me 1/120=0.00833 wire at 32AWG or 0.2mm

This is as accurate as I can get with a rule... can't find my vernier :S

Online calculator gives me: For 2 layers, the length of wire needed is 8950.425mm.

Am I on the right track here because once I coil it I'll have no way of measuring impedance, I only have DC resistance on my cheap, and probably inaccurate, meter. Would 8.95m of 0.2mm copper wire make 8 ohm impedance? I also have no clue about reactance to work it all out. It's done my head in getting this far lol... so any help or advice would be appreciated.

 

[JonSnell Electronic]

Measure the gauge of wire with a micrometer then the length. Purchase the enamelled copper wire of the correct diameter. Prepare your enamel to paint the empty former and whilst the enamel is still wet/tacky, wind the coil.
Impedances vary with the material the coils are in so a reading out of the magnet will be meaningless.

 

[Alnico2]

If I had a mic and complete coil I wouldn't need to ask.

 

[Kay Pirinha]

You can calculate the DC resistance of your magnet wire, using a common formula. The impedance usually is some 10 to 20% bigger in value.

Best regards!

Edit: I've done that for you, inserting your values. It calculates to 5 ohms, which sounds reasonable for a 8 ohms voice coil.

 

[Alnico2]

Thanks Kay, although 5 ohms resistance sounds a bit low to make 8 ohms impedance. 5+20%=6 which is considerably lower than 8.

I had read on this site that a good ballpark figure was to multiply by 1.3 which would be +30%. So 8 ohms impedance divided by 1.3 gives 6.15 ohms resistance.

I've just found a video on youtube where a guy goes into impedance, he states in his description, here: https://www.youtube.com/watch?v=Sf0rx_-MipM , that an 8 ohm impedance speaker would typically be about 6.5 ohms resistance.

So if I'm aiming for about 6.2 ohms resistance, something must be off with my initial assessment of what little coil I have left on this former.

 

[ICG]

You could use the datasheet, take the winding height. You know how many layers are used, the diameter and what's the Re so the only problem left is to wind it as tight as the original.

 

[Cal Weldon]

You are planning to use a winder, yes? Can't imagine doing it by hand.

 

[Kay Pirinha]

Thanks Kay, although 5 ohms resistance sounds a bit low to make 8 ohms impedance. 5+20%=6 which is considerably lower than 8

I did the math, using your data. As the enamel has some thickness, the real copper cross section is somewhat lower. So the real DC resistance is somewhat higher. To get it exactly, you'd have to scrape off the enamel - and just the enamel, nothing else, and measure the thickness of the copper itself. I still think that you have an 8 ohms voice coil, though.

Best regards!

 

[Alnico2]

Thanks Kay. Think I'll just buy some 0.2mm and wind it to the dimensions offered in the recone kit. I can't argue with what I'm measuring here at 30 turns per 1/4" working out to 32awg.

The length and resistance/impedance will just have to take care of themselves lol.
The magnets are pretty small so I imagine maybe with their strength a low resistance might equate to an impedance which doesn't quite follow the 1.3 or 10 to 20% rule which maybe applies better to drivers with bigger ring magnets. idk. Fix them or blow them, it'll be fun either way. Thanks again.

 

[Kay Pirinha]

Fix them or blow them, it'll be fun either way. Thanks again.

Yes, that's the point. You can't do false, your speaker is damaged yet. It's all about learning - and DIY, btw. 🙂.

Best regards!