After 5 years it finally begins

[ivica]

....

Later this week I will remove the remaining two 2451Be’s from the old system replacing them with 2453SL’s and changing them from 3-way way to 2-way.

The measurements I did on D2’s with and without the passive JBL M2 filter show that the capacitor is mainly a protection cap and replacing it with a 6db filter in the BSS/DSP achieves nearly the same curve. The cap in the M2 filter is not a Butter slope, the D2 extends a little further down with the cap compared to 6db Butter I used in the DSP. Please see picture.

The XO used in my measurement corresponds to HP 6db filter at 957Hz. The frequency is the result of running a backwards calculation from a calculated 20,8 ohms load that the 8mf in series in the original JBL filter would be seeing. The 20,8 ohms load seen from the cap is a result of the average impedance between 800Hz and 20kHz (26,3 ohms) of the D2 driver on the M2 and 13 ohms in series and the 11ohms shunt in the filter. Lowering the XO can probably mitigate some of the difference, but the passive cap is probably not a Butter curve with those resistor values.

If you are building an exact M2 clone (this build are rather ClownsJ ) I would recommend keeping the passive filter to mimic the exact JBL curve. However, I do think it is possible to change the slope and/or the cut off frequency (lower) of the 6db filter in the DSP do achieve an identical curve without the cap’s, for the one willing to invest some time.

.....

Hi sebackman,

Nice work and very interesting.
Talking about the VHF driver response round 1kHz +/- I have to say that I can imagine the driver+M2Horn impedance would be a bit higher and more complex then expected in the calculation, so total response would be influenced, but I believe that You would find quite good solution.

For me D2 vs 2451/Be_Trx responses are much more unexpected. I am not sure, but I can imagine that high-frequency dispersion of D2 vs 2451&M2 would be different, as their responses differs more then 6dB over 10kHz. May be their phase plugs 'produce' different "wave front" at the drivers mouths, I would expect quite wider HF dispersion D2&M2 combo then 2451&M2 combo, but that is only my personal expectation looking at the drivers&horn F/R.
"Better" LF response of 2451/Be_Trx is something quite expectable for me, as for the 'ring-radiator type diaphragm' is not the best solution for the larger diaphragm excursion. I can imagine THD would confirm that.

Regards
ivica

[sebackman]

Hi Ivica,

The impedance of the “naked” D2 does indeed shift quite a lot over the range. The passive filter does really matter as can be seen in the plots below.

All is D2/2430K marked “M2” (brand new and I measured two different samples) on a new M2 wave guide, no baffle.

First plot is swept impedance taken with DATS II (calibrated)

Second plot is with the JBL M2 passive filter

Third plot is frequency with the correct BSS DSP correction and JBL M2 passive filter

Not too shabby! Please bear in mind that there is no smoothing at all applied here.

Best regards

[sebackman]

Beryllium drivers installed. UHF will get protection caps but drivers only an attenuator.

Tomorrow they are going into the cabinets.

[sebackman]

Just got my hands on two "Holy Grale" diaphragms in the form of 2pcs NOS original "Northridge" D16R475Nd. Great. They are not for this build but in my opinion the best dias for the large format 4" drivers. To the left new SL dia’s from Mexico, to the left and Northridge NOS. I hope they are ok. If so they are going into the M2 build.

Filter done and cabinets done. Bolts turned out to be too long for the M2 horns so that will have to wait for tomorrow.

I did check the impedance curves for the left and right 2451Be drivers on their respective M2 horns and it turned out to be a pretty good match. Please attached chart.

[pos]

Good find on the SL diaphragms!

I cannot read the impedance measurement, the image got resized too small by the forum engine.
Are your Be diaphragms 8 ohm versions?
What MC2 models are you using?

The cabinets look really good too!

[sebackman]

Hi Pos.

I have no idea why the 475Nd dia's do measure "better" than other SL diaphragms in the 2451 cores (in my opinion) They are from the older K2 475Nd 2” driver, but the SL diaphragms should really all be the same.

These are similarly smooth to the Be's but will go all the way to 20k. Sound wise the Be’s are slightly more detailed but at the cost of needing a UHF unit. With this new build I will have both, 2451Be + UHF in one system and 2451/475Nd (no UHF) in the other. Both on M2 WG’s.

There is also a measuring difference in 2450SL and 2451 cores even if they look the same. I have no idea why that is, but it is consistent in my measures. Maybe the phasing plug is different or mounted differently. Maybe someone else here do know if and what the physical difference might be.

Impedance chart just shows that the pair (L+R) are very well matched indeed. Just sheer luck I suppose. Cores with changed/upgraded diaphragms are normally not that close over the entire spectrum, regardless of diaphragm. You typically never get them 100% equally mounted and seldom the drivers in the exact same position on a horn/waveguide. There are also different cores and sometimes this is marked by handwriting under the back cap with plus or minus measures, showing the phasing plug position, as I understand.

Maybe JBL matched cores and diaphragms at the factory but there is no way, that I know of, to specify a replacement diaphragm in relations to such markings. If they used shims to adjust the diaphragms height at the factory, I have at least never seen that in a 2451 core. The deviations from core to core may be small to the eye but do mercy less show up on the measures. This took some fiddling.

My Be's are 8ohms. Today I would have gotten 16ohms, but they are what they are.

I'm using a MC2 T2000 and a T3000 for driving subs.

I tried, on a suggestion from Widget regarding shallow cabinets, to use acoustic foam instead of fiberglass and there were neglectable measuring differences but since I had already bought it I might as well use it. Maybe it will be better in s shallow cabinets as Widget indicated, and I hope it will not fare worse than the fibreglass stuff.

Today I will give all the drivers a measure to make certain they are ok and then mount the assembly in the cabinets. As they are shallow it will be a challenge to get all 10 bolts in place on the back of the waveguide.

[pos]

Hi Rob,

There is also a measuring difference in 2450SL and 2451 cores even if the look the same. I have no idea why that is, but it is consistent in my measures. Maybe the phasing plug is different or mounted differently. Maybe someone else here do know if and what the physical difference might be.

That is pretty strange indeed, one would expect an exact same internal design between the two. Might be something related to an evolution of the build process that could also appear between two 2450SL or 2451 from different eras?
Do you have measurements of that difference to share, or maybe a description of what was the most striking difference you consistently saw?

I'm using a MC2 T2000 and a T3000 for driving subs.

Nice! What are you using for HF and UHF?

[sebackman]

Hi Pos,

I can post some comps next week. It is odd indeed.

I'm stuck on the old Crows CTS series class AB amps, fed from my BSS BLU800. They are as good as any with an odd topology used in several Mark Levinson amps as far as understand (I have been told they were designed by tech's on loan from ML).

I have tried many other amps and some are no doubt better and more modern but these old workhorses sound good and are available to reasonable prices on the second hand market. They were expensive new. I talked to a Crown tech a few years ago and the guy claimed that they were going EOL as Crown did not earn enough on them. Expensive to build, expensive components and over engineered for the intended use/price point. "More HiFi than installation".... I have no clue if that is correct but I like them.

He turned out to be right, a year later they were EOL (2018). I have many of them and use the 4 and 8 channel units for all active DSP systems. I have two 8-channels (CTS-8200) in the HT rack driving 3-way fonts and 2-way rears.

It is important to have the attenuators at full when using them. That is a weak link.

[sebackman]

Did some sanity measures today before the drivers goes into the cabinets.

Everything calibrated and checked against known ref.

I measured 3pcs of each and will use average for DSP filter design.

2451Be is measured on my butchered M2 waveguides after a BSS BLU50 DSP XO with LR48 @ 400Hz. MLS window is 5ms no visible reflections in window.

045's is measured in JBL K2 horns inserted into my butchered M2 waveguides after a BSS BLU50 DSP XO with LR48 @ 6kHz. MLS window is 3ms no visible reflections in window.

Not too shabby. In fact pretty amazing how good the M2 waveguide is. Very little correction needed here and that is good news.

Please bear inmind that there is no filtering or smoothing, just raw mic data.

I will do new measures in the cabinets but this is a good start.

[ivica]

Did some sanity measures today before the drivers goes into the cabinets.

Everything calibrated and checked against known ref.

I measured 3pcs of each and will use average for DSP filter design.

2451Be is measured on my butchered M2 waveguides after a BSS BLU50 DSP XO with LR48 @ 400Hz. MLS window is 5ms no visible reflections in window.

045's is measured in JBL K2 horns inserted into my butchered M2 waveguides after a BSS BLU50 DSP XO with LR48 @ 6kHz. MLS window is 3ms no visible reflections in window.

Hi sebackman,

Very nice drivers as can be seen.
I have not get anything like this on MY measurements, if not a kind of smoothing have not been applied.

Regards
Ivica

[engineerjoe]

Very very nice work so far. I wish I could hear them in person. Thank you for sharing all this. I will reread thru all this again and see that I didn't miss anything!

[sebackman]

Thank you for the kind words.

All diaphragms centered and measured.

A bit of a setback today. The two 1400PRO I have had on the shelf for many years waiting for this turned out to rub. They are fresh reconed so I have no idea how that is possible, but rub they do. Both of them...

I have a hard time to beleive that I put them away when I bought them without checking cone travel. Well, they can't be used as is. I have to figure out tomorrow what to do. I cannot go back to the seller after so many years.

At least the center is done. Only cloth on the front frame remains there.

The cabinet is angled 15 degrees as the center is located under the ceiling pointing down towards listening position and the screen has speaker cloth in the upper blackdrop.

[sebackman]

A good friend came to assistance. A brand new set of 1400Pro’s. Nice. Big thank you to Mr J!

[srm51555]

Thats a good friend, everything is looking great.

[sebackman]

Good friends allways help :-)

Today was a good day. After +5 years the fronts are finally done, except for the cloth on the fronts.


Thank you Mr. J for the 1400PRO’s. And big thank you LH members 4313b, Widget and Guido for Be’s & UHF horn help. Also a thank you to Kalle and Johan1 for help with finding a good carpenter.

Not too shabby with the butchered M2 horns. If one did not know the originals they may have passed unnoticed. Even the 045’s (K2 9900 horns) fits reasonable good. The goal was a very compact front.

I will try to measure over the weekend. 4-way active with SUB1500 subs XO @ 80Hz, 1400PRO from 80-800Hz, 2451BE from 800-10k and 045 above. I have no idea if it works or if it will sound disaster. The M2's will have to act reference.

This is fun!