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View Full Version : 2450SL vs 2452SL



pos
06-13-2011, 04:20 AM
Both share the same 1.5" aperture, same bolt pattern, same diaphragm, same flux in the gap, but the 2452 only weigths 2kg whereas the 2450SL wigths more than 4kg.

The 2452 core looks like a lower price evolution of the 2450SL/2451 core.

2450SL:
51651


2452SL (pictures from Zilch):
51652

For one thing, the back cap looks smaller, and the damping pad is also much smaller.
Any idea on what was given up during this transformation?
Low freq extension? (not really useful anymore with these rapid flare deigns)
Flux "stability" ? (if that makes any sense)

It is interesting to note than JBL came back to the older bigger design as the root of the 476 core:
51653

subwoof
06-13-2011, 09:11 AM
The 2450SL magnet design first came out in 1989 so it's an old one by todays standards. Better mfg methods, magnet design / composition mean smaller mass all around.

Do you still have your 19" computer monitor from then?? a tad bit large?

And the consumers who pay that kind of money for a top-end driver want to look at it too. The rest of us hide them in cabinets and listen..

:cheers:

pos
06-13-2011, 09:20 AM
But then why did they go back to the bigger magnet for the 476 core? Only to include the longer copper sleeved pole piece?

And what about the effect of the smaller back chamber?

grumpy
06-13-2011, 12:17 PM
could be that the industrial design for the 476 cap (sort of matching in theme
with the 1500AL "Legion of Doom headquarters" look) was completed early on...
requiring a 4-hole cap mount pattern (vs the three, in the 2452 assembly) :D

... and one could certainly insert more effective back damping material into
a deeper cap... I'm missing a 476 to do a 3-way comparison.

all pure speculation, of course (no smiley).

subwoof
06-13-2011, 12:26 PM
The 476 was specifically engineered to work with the Be diaphram. There is a recent post describing it somewhere here. My bet is that they needed a little more room inside for the mods and therefore went with the larger size structure.

And the backcap measurement fiasco was also documented here and it was determined that the extra volume did not impart any better LF response.

sub

grumpy
06-13-2011, 02:05 PM
right... acoustic venting under the suspension, and the copper pole piece sleeve.

you could be right about the ability to perform further mods on the older/larger case/motor
assembly (more modular?)

I remember the back-cap discussions, measurements and that the FR's looked
essentially identical in the area of interest. That put it to bed for most, and rightly so for most uses.

I was thinking more about absorption of delayed/reflected HF energy...
more difficult to sort out at home, and more pertinent (maybe) with diaphragms
that have a smooth(er) extended response, such as the Be. I should go back and
refresh my memory re what was measured and how.

pos
06-13-2011, 02:17 PM
Would you have a link to that particular thread?
I remember reading some 435Be/2435 comparisons, but nothing about 2452/2451/2450SL

grumpy
06-13-2011, 02:44 PM
you might be right... I -know- there were 3" dia rear cover comparisons,
I could just be imagining such a 4" version/thread. :dont-know:

I don't have time to tackle this right now (or a larger rear cover).
If someone thinks it sounds better with a bigger cap or certain
pad material, I have no need or reason to argue.

pos
06-13-2011, 03:30 PM
I just noticed you created a similar thread not so long ago:
http://www.audioheritage.org/vbulletin/showthread.php?30668-2450SL-vs-2452H-SL-question

I even remember having read it now...
sorry :banghead:

grumpy
06-13-2011, 04:05 PM
no problem for me. I hardly remember writing it :p
nothing about rear caps though...

I've been using the 2450SL cores with TruExtent dias
and haven't even felt the urge to measure. I have a single
2452H that came with a dead diaphragm, so if I ever get
the urge to start up yet another measurement thread,
I'll be sure to post some comparisons (and try to remember
to include a with/without rear-cap set).

subwoof
06-13-2011, 04:25 PM
The aforementioned back cap discussion was for the 3" not 4" coil but one could interpolate similar results.

And for interest: the air volume behind the sealed 2416 type diaphram is pretty tiny and they were still used to 1000hz in some of the older SR boxes...even though the motor structure is very similar to the 2426. They were rated at lower power (I think) because they just couldn't get rid of the heat. 90% of the ones that came in bad were bubbled.

Haven't had a chance to play with a 476 so all I know is what I read but it looks to be a specific design that wasn't required to "back" fit into existing applications ( like the cone were....)

JeffW
06-13-2011, 04:42 PM
1audiohack has a 2452 and a bucketful of diaphragms including TruExtent that he's going to measure when time allows. Not sure what kind of horn he'll use.

grumpy
06-13-2011, 07:01 PM
Cool :)

cooky1257
06-14-2011, 11:12 AM
I now have 2452's with Truextents and can say they sound lovely.
The pad in the 2452 back cap doesn't feel like it is absorbent at all-it isn't felt or foam to these eyes anyway.

pos
06-15-2011, 01:18 AM
Nice! How much of an improvement is it compared to your 2435? And how is the UHF, also compared to when you used the 045ti?

Concerning the 2452H damping pad, I can see a recurrent dip at 3khz on my measurements on different horns, and I was wondering if this could be the cause... But it sure does not appear on any other measurement by other people I have seen so far (though smoothing blurs it quickly).

Concerning the magnet assembly, looking at the curves from the 2452SL spec sheet (http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=33360&stc=1&d=1216773713) it looks like the impedance is rising fast in the highs (x2.3 between 4khz and 20khz), whereas it is more stable on other modern 4" drivers: look at the second graph (plane wave tube measurement) of the 2451H (http://www.lansingheritage.org/images/jbl/specs/pro-comp/2451/page2.jpg). Is is difficult to compare because the impedance scale is in log, but you can see that it is lower at 20khz than the peak at resonance ~600Hz, contrary to the 2452.
So, lack of copper pole caps?...
Anyway, the 2452SL seems to maintain a good efficiency in the highs, similar to what you would expect from a 2450SL...
Maybe that is due to the stronger flux density in the 2452: 1.9T vs 1.875T for the 2450H, and BL 12.8 vs 12.7.


EDIT:
I just pulled the trigger and bought a pair of 2450SL.
I also have a pair of 2452H (not SL) here, so I will try to measure them on the same horns with the same diaphragms.

cooky1257
06-15-2011, 09:40 AM
The main improvement is in terms of scale and impact, detail/resolution is at best marginally improved but there is an ease to the mid/hf that just sounds effortless.
Elements within the stereo image appear physically larger and more 3 dimensional.
UHF well as far as my ears can tell is good out to 16khz-I think they do sound best rolled off about 15-16khz. They work really well on the SAM horn.
Wether the extra expense over the 2435 is worth it is another matter. The performance from an aquaplased 2435 is hard to argue with given the prices they go for 2nd hand is less than half what a Truextent /2452 cores will cost you..A this 'upgrade' was a 50th birthday present and I wont be going back to the 2435 so I'm a happy bunny-my 2435's are for sale now..

pos
06-24-2011, 12:11 AM
One difference I noted today that I did not expect:
The 2452 has a shorter phase plug than the 2450SL/2451!
Measured from the diaphragm plastic ring to the throat I get 3.3cm for the 2450SL and only 2.2cm for the 2452.
This is a 33% decrease in length!

1audiohack
06-24-2011, 08:48 AM
Good observation. Have you calculated the flare constant of them yet? I just woke up, I'm not to capacity yet. :p

grumpy
06-24-2011, 09:57 AM
... of the phase plug? from slot to exit? I'm happy to show some ignorance here...
and perhaps ask some ignorant questions:

I can see where a rate might be -estimated- (not having access to the exact exit path
used... I believe these are complex curves, and perhaps not equal length or even
expansion rate ... e.g., Geddes patent appl. describes such a plug [minus curves]).

So, is the concern to try to match a continued flare rate with a horn throat
expansion (at least initially) in an attempt to match acoustic impedance? or
to use this for conversation about differences which may be measured (pos or 1audiohack)
between the 2452 and 2450SL cores? ... or just curious? :)

I thought that the 'flare rate' was more a design/limit regarding the built-in throats
of non-OA compression drivers (set up for the horns of the day), vs the internal
phase plug expansion (?)

I suppose the path length (and some effective internal flare rate) could still be the
same if the 2452 plug paths were made a bit "curvier." ... I hadn't even thought to
measure the phase plug thickness ...

am I rambling and/or making any sense?

cooky1257
06-24-2011, 10:45 AM
Is there a cutaway drawing of the 2452(like the 476 one) showing the phase plug?

1audiohack
06-25-2011, 10:23 PM
This interests me because in most of my measurements of the various two inch drivers if you include the fundamental, 2nd and 3rd harmonics and the phase response of all three magnitude traces, they only really behave from about 1500Hz to 7-8.5kHz, diaphragm dependant.


So, is the concern to try to match a continued flare rate with a horn throat
expansion (at least initially) in an attempt to match acoustic impedance? or
to use this for conversation about differences which may be measured (pos or 1audiohack)
between the 2452 and 2450SL cores? ... or just curious? :)

For me its both. If one decides to only use these drivers in this limited range, it certainly narrows the scope of appropriate horns, and, if one of the throatless motors has a higher phase plug flare rate and will better match to a shallow, rapid flare horn with lower passband distortion in both time and magnitude that would be cool to figure out.

I have much to learn as there is much to know. I hope the plane wave tube will help.



I thought that the 'flare rate' was more a design/limit regarding the built-in throats
of non-OA compression drivers (set up for the horns of the day), vs the internal
phase plug expansion (?)

Yes, no, kinda? There is expansion from the front chamber through the phasing plug and some of what I have read considers this the first part of the horn and the transition there not to be ignored.

What's keeps rattling around in my head is these throatless motors, the Charlie Hughes' quadratic throat theory and a horn with simple geometry like a conical, I don't know it just looks right, seems to model well, I just want to know a little more before embarking on the construction venture.