sebackman, Thanks for the wonderful tips! I will keep them in mind next time I change diaphragm, which I am contemplating swapping in another pair of D8R2450SL into my 2441. Given that the dia that came with the 2441 are very old, new genuine D16R2441 are unavailable, and how well the D842450SL look on the measurement. And that pair definitely need some new foam/pads!

I'd just like to point out about the measurement method. I wasn't using the driver body as an absolute reference. I wiggle the dia/coil to it's limits inside the gap, figure out the range of motion (usually around 0.07mm to 0.1mm), and then fix the dia at the middle of that limit. That I think is the key of the method and give the result some consistency and less guesswork.

As for the matching, I guess I got lucky. I got them from ebay I think a year ago and only managed to have some free time to work on it recently (caught covid and was isolating myself in my room for a week, so that was a blessing in disguise! )

Quote Originally Posted by sebackman View Post
Great info and graphs.

A few observations.

I like your "depth gauge" approach but it may be valuable to consider that the outside of the driver is a "cover" which is not always centered and probably glued to the core. I have seen several cores where the small space between the cover and the core is concentric (with the back cap removed). That may mean that even if it is centered compared to the out rim it will not be so in the gap.

For the quadrant approach I do the same but using a signal generator. I use a good analogue signal generator providing a clean sinus wave from about 300Hz to at least 10kHz.

I found that the 1,5" cores typically are sensitive to resonances in the 500-600Hz sector and 1300-1400 Hz area. If that is fine I rarely see other problems if the gaps are clean. It is almost always audible when sweeping a sine wave. The sound changes to reminding of the sound from a Square wave. It is also visible on a impedance measure with something like a DATS3 or similar.

The drives are sensitive to if the back cap is on or off and also must see free air for at least 0,5-1m in front. If you put it on a table with the back cap facing down it must be on a soft surface as a hard surface may affect the back cap. I use some 1 inch foam on a short pedestal up from table when measuring the driver on its back.

And always change to foam to felt when you have it open. The foam may look fine but if you push with your finger it often falls apart. It is easy as it sits with a large double-sided self adhesive tape. It comes out easy. I typically glue the woolen felt to back back with contact glue. The size of the felt is easy too. The outer diameter of the back cap is perfect as a template it will cover the sides on the inside. You just have to cut a small piece for the terminals on the inside of the back cap. I apply contact adhesive on the inside (including the angled sides) and just put in the felt circle in. Then I put a quarter or a washer in the center to pust down the felt on the resessed square in the middle of the inside of the back cap and the just apply suitable weight for a few hours to. I often use a round flat bottom breakfast bowl that I fill with water.

I typically run about 80-75 db without horn with no capacitor but a series resistor of about 20-50 ohms.

One thing to consider with the newer Mexico diaphragms is that the lacquer on the coil often has blobs and spill. I typically "feel" with the fingers on the coil and on the inside to detect blobs and often find that I have to adjust them with a knife or a file.

It may prove more or less impossible to get a "Mexico diaphragm" with a lacquer blob silent and that can be darn annoying when the cores may be clean and in good shape.

The phase plugs are glued together from sections so they can differ quite a bit from core to core. It sounds like you were lucky finding two good cores with similar paterns. Good news.

I did spend quite some time a few weeks ago to find two well matched drivers from a sample of quite a numer of cores and diaphragms. I did find a few good pairs but it was very time consuming and then several of the cores were NOS and never used.

Have a good weekend
//Rob