Coaxial Conundrum. 8 ohm woofer coil vs 16 ohm tweeter. RE-CONE.

[19Brendan80]

I currently have a 16 ohm Coaxial Alnico just like an Altec 604 with resin dome tweeter, but it is not an altec. It originally had a treated plastic/vinyl paper surround, but I opted to replace it with a pleated cloth surround like the 604D when they went from paper to cloth because the cone was in extremely rough shape. This company also went from treated paper to a pleated cloth on their later models which improved Fs.

I wanted to use the original voice coil and was able to salvage the original voice coil however the new cone has a slightly different profile...

This turned out being problematic because i glued the voice coil to the spider according to the correct original height for the magnetic gap which made it 3/16" too short for the new cone once the hole was cut to the correct size.

To account for the difference I then shimmed the coil pulled up slightly and weighed the cone down so they would meet and I could glue them up.

Technically.... it works and it surprisingly sounds way better than an original that I have as well (I a/b compared them), but it is currently applying opposing forces upon itself (spider being pulled up slightly and cone being pulled down....

This is slightly visible to the eye by looking at the woven fabric accordion and I don't like the fact that this is how it turned out and can't help but think it would be affecting the sound in a negative manner. I also don't think the cone moves as freely as it should.... though I'm not really sure about this.

I did buy an altec 16 ohm coil (before measuring my current coil) but they are 3" inner diameter (76mm) flatwire underhung.
They won't fit the pole piece.

My woofer uses a 3.03 inner diameter coil (77mm) roundwire underhung.

I could not find a 77mm/3.03" coil in 16ohm but I was able to find them in 8 ohm, but they are roundwire overhung. they're in the mail.

I don't want to affect the sensitivity of the driver too much either.. I'm not too worried about the overhung /underhung situation.

Here are my options:

A) I can attempt to stretch the altec coils out 1mm diameter with an exhaust stretcher and re-epoxy them to the bobbins.

I tested stretching magnet wire today to see how far it would stretch and if the coating and ohms would be affected at all and I am safe to stretch them out well over the 3mm I'll need for circumference. I'm just not too keen on this idea.

B) I can use the 8 ohm roundwire overhung and but there will be an impedance mismatch with the 16 ohm tweeter. I'd imagine I could account for the 8 ohm coil with a resistor in series with the woofer? (I think there would be a gain in sensitivity going from a 16 ohm coil to a 8 ohm coil but then a reduction back when trying to match the impedance with a 8 ohm resistor. Maybe i wouldn't need a resistor and a proper crossover would be fine to level match them?

C) I can use the 8 ohm roundwire overhung coil and change the tweeter out to an 8 ohm as well.... I've bought a comparable resin tweeter with 1.5" voice coil to use but it will need to be glued to a 1/16" phenolic ring to sit correctly in the gap further complicating things.

D) I know there is one place that might make custom coils but not sure if they'd just make two and I don't have a ton of money to spend on this project.

I'm leaning towards "b or c" but can't help but think about "a" because it's a proper underhung coil and is flatwound (bonus) which will improve magnetic Flux and frequency response along with keeping distortion to a minimum and is 16 ohms like the original coil.

Any opinions would be greatly appreciated.
Questions are more than welcome. Just looking for some direction. Thanks.

 

[bowtie427ss]

A) I can attempt to stretch the altec coils out 1mm diameter with an exhaust stretcher and re-epoxy them to the bobbins.

I tested stretching magnet wire today to see how far it would stretch and if the coating and ohms would be affected at all and I am safe to stretch them out well over the 3mm I'll need for circumference. I'm just not too keen on this idea.

Don't give this further thought. It offers nearly guaranteed instability of the electrical parameters over time as oxidation forms in the thousands of microscopic cracks this method will cause in the enamel insulation, and could ultimately end with a damaged amplifier as the impedance of the coil sags lower and lower with time.

B) I can use the 8 ohm roundwire overhung and but there will be an impedance mismatch with the 16 ohm tweeter. I'd imagine I could account for the 8 ohm coil with a resistor in series with the woofer?

Probably the best route, except instead of adding series resistance(something most designers strive to reduce) why not re-configure the LF section of the crossover with the proper value components for the load?

 

[19Brendan80]

I think that does sound like the best idea. You've talked me out of stretching the coils. I'd hate to do all this work just to have them eventually fail. I'll have to buy a dayton dats v3 so i can measure each speakers parameters (tweeter and woofer) so I can then model a crossover with XSIM. I think that's the best way to design the crossover anyhow. Is there a better way? Thanks for the response as well. I definitely needed some persuasion. Just for hypothetical argument and to throw one more idea into the mix: If the bobbin length was long enough on this 16 ohm coil: Flat Wire Voice Coil - MWA1703-16 · Queensland Speaker Repairs (qsr.net.au) would you attempt to fit it or still use the 8 ohm coil with the custom crossover? The pole piece on my speaker is exactly 3" and the original coil was 3.03" This coil is 3.025 (five thousandths of an inch smaller). Thanks.

 

[bowtie427ss]

would you attempt to fit it or still use the 8 ohm coil with the custom crossover?

IMO, yes as it allows the opportunity to fix the suspension issues, getting the surround and spider in their respective neutral/home positions simultaneously.

Input from @GordonW would be beneficial to your situation. Hopefully, he will see this and post. Keep checking back for a few days.

 

[bowtie427ss]

The pole piece on my speaker is exactly 3" and the original coil was 3.03" This coil is 3.025 (five thousandths of an inch smaller). Thanks.

Assuming a typically .020 shim spacing between pole and coil former, that 5 thou difference on the diameter(2-1/2 thou on the shim spacing) is only slightly outside of a 10% tolerance.

I think that's negligible. Maybe, just a bit more care with the final alignment before you commit to adhesive.

 

[19Brendan80]

I think i feel the same way. I just wasnt sure if the tolerance between the coil and pole would be too tight. So, If the 16 ohm one from that qsr has a bobbin length long enough I'll use that. It also has the advantage of being flatwire. If not I'll be using the 8 ohm roundwire overhung ones i just ordered and make a crossover so the drivers play well together. I never measured the bobbin/ coil length when i had it out, so when i take it apart again I'll be able to measure that. Thanks again for your input!

 

[GordonW]

A couple questions:

1) What exactly is this speaker (make and model)?
2) What was the DC resistance of the original voice coil?


Also, pictures of the frame, cone and the area where the spider and cone meet inside the basket, would be helpful.

Regards,
Gordon.

 

[19Brendan80]

Man, i feel pretty silly for not actually measuring the actual dcr first. I was basing the resistance completely off literature.

1) Stromberg Carlson RF-475

2) i have two. Each woofer coil respectively measures 7.5 ohm and 7.8 ohm. My new "8 ohm coils" measure 5.9. So in reality the original 16 ohm coils are much closer to an 8 ohm than a 16. Generally speaking a 16 ohm coil might typically be between 11 and 14.

 

[19Brendan80]

.... And the original tweeter dcr

 

[19Brendan80]

Hmmm. I just measured the dcr of the new altec 16 ohm coils that don't fit and they measure 9 ohms. I stand corrected on a typical 16 ohm coil measurement being between 11 and 14.