Originally Posted by 1audiohack
My "shim" was far more elegant, doesn't require butchering the cone, and achieved the same results. My beef with the 075 is not the high end roll off so much as the dip observed at ~9KHz. The dip removes both cymbol "splash" and well as intelligability.
The "elastomeric filter" can easily extend the FR of the 2405-diaphragmed 2402 well about 20KHz while further smoothing it out. There is a loss of few dB though.
The answer to using a D16R2405 diaphragm in the 075 / 2402 is a bullet / phase plug with the correct geometry machined into the bottom. The example shown, nicks dings and all is from a brand new 2402-05. The profile machined into the bottom of the phase plug is that of the 077 / 2405, done at the factory. Hardly butchery in my book besides the obvious lack of care.
Don't get me wrong, I am all for experimentation. For clarification, the nonsense I made reference to was all the serious negativity that came out in that thread. Not to be mean honestly, if you want to use an 075 for an ash tray I couldn't care less. There yours, do with them what you will.
If we knew what the hell we were doing, we wouldn't call it research would we.
We also had a mystery with 2 different diameter phase plugs in the 2404. Using this as a guide would it be correct to say the the smaller phase plug would go with a 2404-1 using the 2402 diaphram?
Rob
"I could be arguing in my spare time"
Butchery in my book.
Hi Rob;
As a guide I would say that is right. However,,,
I have been working to get a good handle on the 2404 / 2404-1 thing. There is a fair amount of uncertainty, I will review my notes tonight and post what I know.
Tod, for clarification, machining the correct geometry in the phase plug for the diaphragm to be used is the right way to go about it. The clown that installed the beat up phase plug pictured as is, is a butcher.
If we knew what the hell we were doing, we wouldn't call it research would we.
No one wants to pull all the drawings and parts lists so I doubt there will ever be a definitive answer. I know that there are more variants than anyone cares to know about, especially amongst the 075's. That might be why no one wants to pull all the drawings and parts lists...
"Would you want to compile a list of all the ring radiator variants?"
"Oh HELL no!"
I have done some measurements all with AlNiCo 'motors' and 077/2405 horn ("slot")
First figure 077 with "Blue-Blue" rings diaphragm
Second figure 077 with "Golden-Golden" rings diaphragms (D16R2405)
Third figure with 2405 (AlNiCo) with "Silver-Silver" rings diaphragm (marked as 2405 from year '84)
It seems to me that nowadays D16R2405 dia is somewhere "in between" older sisters 'blue-blue' and 'silver-silver'(from early 2405-AlNiCo) diaphragms.
Did you attempt multiple tests? I imagine you can get that level of change just by taking out and remounting the same diaphragm in the same driver a couple of times.
Widget
Indeed. Simply changing the tightness of the phase plug screw can produce a fair amount of FR variability... That, and ensuring the dia is clean and physically distortion free (especially after snugging the screw).
... not to dismiss the measurements taken, but it would be easy to weigh them too heavily for the reasons stated above, -and- that variability of old, used parts, vs new is likely to be large anyway. It would be a large task to obtain enough samples ...
Yes, I have done several test with same driver after several days. Of course curves ware not absolutely identical, but "trends" remained, even after 'phase-plug' remounting.
I have one 'silver-silver' diaphragm that has to be replaced (inner part of the membrane is crumpled), so I will make some experiments with the influence of the distance of the phase-plug from the diaphragm (as toddalin did -http://www.audioheritage.org/vbulletin/member.php?230-toddalin).
Changing the sound level in to more then 12dB has almost no influence in the response.
I absolutely agree, but concerning that the trends of the responses are quite different, especially between 'silver-silver' on one side and 'blue-blue' or 'gold-gold' on the other side.
Interestingly almost the same results I have got with JBL 2405/077 "look-alike" slot -ring driver from Beyma CP21F (old model under branded name Mulicell) and DCR4541 on one side and newer model CP21F on the other side.
The older driver (2405,DCR4541 and old CP21F) has relatively flat response up to say 16~18Khz, as 2405 'silver-silver', with some (more or less) peak and sharp "cut-off" after, while 'blue-blue' or 'gold-gold' or new CP21F has wider response, not so flat, but without sharp cut-off at high end.
I believe that there are some technical reason for such behaviors, depending on the aluminum alloy stiffness, thickness, or some minor differences in the membrane shape.
Here we have heard the story about the lady working in JBL who had "repaired" rejected diaphragms for 2405/077 with "blue ball-pointing pen"....so it seems that even small change in the membrane shape has great influence on the driver response.
May be only myth
http://www.audioheritage.org/vbullet...l=1#post187592
http://www.audioheritage.org/vbullet...l=1#post188524
Some part of the pen story is true. I have several D16R2405 diaphragms that have the "rolled rings" in them with the ink color still in them. The proof that it was done during manufacture is evidenced by the fact that the radius and ink is visible under the voice coil lead-ins. I will try to get a good picture of some of them.
Did the measure better? I can't remember.
If we knew what the hell we were doing, we wouldn't call it research would we.
Such photos would be very, very interesting to be seen.
From the fact that the ink and radius is "under" voice coil leads-ins, mean that there were some kind of 'test-bead" for checking the diaphragm operation before fixing outer ring.
Here are measured responses what has been done with 2405 (AlNiCo) with 'silver-silver' diaphragm, that has some amount of mechanical
imperfection in inner part of the membrane. Response presented on second figure.
I believe that that is the main reason why this driver does have almost "ruler-flat" response
as the second one with "silver-silver' diaphragm whose FR was presented earlier
( http://www.audioheritage.org/vbullet...l=1#post319031 ).
I have realized that in this special case there is some amount of "hole' around 9kHz, with some frequency "boost" around 11~12kHz, with some kind of hole and peak about 18kHz
I have used "paper shims thick about 0.1mm", that I have put under "central phase plug" only.
Generally my conclusion would be:
-no shims or only one shim has almost no influence (may be flatten 11~12 kHz boost)
-adding any shims more would reduce efficiency by some amount
-2 shims would reduce mentioned HF peak which is about 18kHz
-up to 4 shims would reduce HF peak, but HF response would remain over 16.5kHz
-8 shims would reduce efficiency much more (about 3dB) and would reduce HF response more, but total response would come flatten almost up to 16kHz.
I found that the 9kHz hole is probably a product of the ring radiator design and it shows in the bullets (the worst), butt cheeks (about as bad as the bullets with 2402 phragms but better with 2405 phragms, and slots (the best).
Even JBL recognizes this and the crossovers are designed with this dip in mind to augment this area. That's why they have the 1 mfd/0.18 mH choke/1.5 mfd. The combination of the two caps creates a peak in this area.
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