Installing JBL compression driver diaphragm
Just want to share my experience and technique in installing a set of D8R2452SL into my JBL 2451J compression driver.
I got a pair of 2451J from ebay a little while ago and after doing some reading here I got tempted in replacing the original factory diaphragms with D8R2450SL with aquapas coating (which I got from speakerexchange).
Before removing the original diaphragm I measured, EQed and paired the 2451J with a Purifi 8” base cabinet. TBH, they measured and sounded really good already. Usable from 750Hz upwards and H2 at 65dB to 60dB. Some resonance starts showing from 7kHz onwards on H2 plot and breaking down starts to show on FR around 13kHz.
Even then, after reading reports of the SL diaphragm sounding, I wanted to try it.
I have read many threads and there are some details on how to install diaphragm. I have previously done it on a different driver so I am familiar with the general process but only by eyeball and ears.
So I installed it, trying my best to eyeball putting the coil into the gap and playing different note to make sure there is no rubbing. And while the FR looks great, the distortion looks rather bad. Not only is H2 higher than the factory diag by 10dB, H3 is off the chart, way higher than H2. I unscrewed the diag and made a 2nd attempt and it was partly better but H3 is still out of control.
So I was confused, either this is the best out of this SL diag, or eyeballing doesn’t work.
I figured I need to have more precision than eyeball the coil alignment and then I remember I bought a depth gauge from aliexpress recently for another purpose. It can be used to measure the range for alignment and precisely place the voice coil. It has 0.01mm precision which is absolutely necessary it turns out. I had to hack it by ziptie it onto the driver body but it was secure enough.
The range of alignment when trying to rock the diag in the gap is really small. Max is 0.1mm and sometimes just 0.07mm. So I need to center it within that range.
Another difficulty is to make all parts of the coil aligned. This I did by aligning the coil by quadrants. Start with aligning and fixing the alignment by very softly tighten one screw, measure the range for the coil movement, and tap it to align to the middle of the range, and then move to the next quadrant. Usually by the 3rd quadrant the motion is already fixed.
The result is a big improvement, H3 is now well controlled but H2 is still higher than the factory diag at around 55dB to 50dB. Not as good but I guess HD is not the absolute judgement of sound quality. The SL dia is better in upper treble, the FR is flatter/smoother, and also there is no sign of rising distortion.
In real music testing, it does sound smoother, not as harsh/piercing as the original diag, but perhaps very slightly less details/texture as a tradeoff.
L/R matching is also very good. Both FR and HD are almost matching.
So far, I am happy with the swap.
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