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Thread: Compact monitor

  1. #46
    Member sebackman's Avatar
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    POS, Ivica and other good members,

    Can you please explain to me how the M2 combination of passive electric -6db/oct, DSP electric -6db/oct and acoustic filters -24db/oct (horn/mouth and driver/back cap) interact to protect the D2 driver and get to a 36db/oct LR filter slope.

    The JBL 2430K seem to be 20 ohms impedance and the resistor attenuator in the M2 cabinet reduces sensitivity -9db. The values match exactly (13ohms/11ohms) and also seem to match the system sensitivity taking the DSP settings into account. Also the designation “K” indicates that it should be higher than “J” = 16 ohms following the old JBL nomenclature.

    If that is a correct assumption, the 38mf (33,3+4,7) capacitor in line with 20ohms corresponds to a first order 6db/oct slope with -3db at approximately 209Hz. I addition there is a DSP filter of first order 6db/oct Butterworth with -3db at 782Hz.

    The DSP PEQ settings of -4,3 db @ 760Hz and -5,6db @ 3,3kHz seem to have too narrow Q to help much. And the system input EQ at +1.1db at 500 does not help either…

    Is the horn loading the driver harder below a certain frequency or is it the back cap reducing the diaphragms movement. For the mechanical amplitude of the driver diaphragm to be reduced below a certain frequency, given fixed voltage across the range, the electric impedance in the driver below a certain level would have to increase to reduce current trough it?? -And when the impedance falls again further down in frequency the idea is that the electric filters shall have reduced the voltage level to un-harmful levels??

    From the available JBL impedance curves for the 244X/245X drivers it appears that the impedance peak at about the double on nominal impedance, dependant on horn. Is that alone enough to save a D2 when receiving several hundreds of system watts and only 12db/oct electric protection?? -Or will it indeed give large dome/ring movements but that they are not fatal to the D2 driver??

    The combination of Butterworth and LR is interesting and I assume that the combination of passive XO -6db/oct Butterworth, DSP XO -6db/oct Butterworth and acoustic XO –24 db/oct sums up to a 36db LR slope to give a perfect summation.

    Still learning.

    Kind regards
    //RoB
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  2. #47
    Member sebackman's Avatar
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    Or….

    Is it a game of sensitivity? The 2215ND has a sensitivity of about 95db 1w/1m @ 8 ohms from data in the tech section. System sensitivity of M2 is 92 db. If D2 is 110db @ 20 ohms, as the other JBL large format drivers, that would give a sensitivity difference of about 15db. The resistor attenuator in the cabinet is -9db = voltage gain 0,355

    50V signal applied to each driver would be
    2216Nd = 8ohm = 312W
    2234K= 20ohm = 125W

    Add the – 9db resistor attenuator to the driver would reduce voltage to 50V*0,355 = 17,74V = 15,7W over 20 ohms

    And if impedance increases to 40ohm (double impedance at lower frequency resonance point) the power would be only 7,87W at the resonance max.

    So even if the system get’s 500W RMS the D2 would only see a fraction of that at lower frequencies and maybe that is ok for the drivers and the diaphragm movement when combined with passive crossover @ DSP 6 + passive 6 db/oct ?

    Or am I getting it all wrong here??

    If this is correct, why have many of the other JBL designs had much steeper crossover slopes if it is not really needed?

    Kind regards
    //RoB
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  3. #48
    Senior Member pos's Avatar
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    Hi Robert

    I don't know the ins and outs of JBL's choices here. The S9800 is similar in this regard (6dB/oct HP filter, basically a cap with some resistors...).
    Earl Geddes also uses a similar topology in his passive Summa system (single cap with a knee around 10kHz IIRC).
    The key here is to add a zobel to avoid having an amplitude peak at the impedance peaks down low.

  4. #49
    Senior Member macaroonie's Avatar
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    Changing tack here ( a little )

    Since there is a fair likelyhood of many DiY systems along these lines I was having these very naughty thoughts.
    The 2206 is pretty much the go to driver for this application but it is really a mid bass driver in it's normal use.
    For domestic home brew systems where the power handling available is not required is there any mileage in replacing the pleated surround with an M roll or regular foam ? Perhaps add a little mass ?
    Obviously the T/S parameters will change but given that this driver is good to almost 2kHz and normally will be used with a horn from 800Hz then a little of the top end and power handling could be sacrificed to gain some extension at the bottom.
    Anyone done this ?

    M

  5. #50
    Senior Member ivica's Avatar
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    Quote Originally Posted by sebackman View Post
    Or….

    Is it a game of sensitivity? The 2215ND has a sensitivity of about 95db 1w/1m @ 8 ohms from data in the tech section. System sensitivity of M2 is 92 db. If D2 is 110db @ 20 ohms, as the other JBL large format drivers, that would give a sensitivity difference of about 15db. The resistor attenuator in the cabinet is -9db = voltage gain 0,355

    50V signal applied to each driver would be
    2216Nd = 8ohm = 312W
    2234K= 20ohm = 125W

    Add the – 9db resistor attenuator to the driver would reduce voltage to 50V*0,355 = 17,74V = 15,7W over 20 ohms

    And if impedance increases to 40ohm (double impedance at lower frequency resonance point) the power would be only 7,87W at the resonance max.

    So even if the system get’s 500W RMS the D2 would only see a fraction of that at lower frequencies and maybe that is ok for the drivers and the diaphragm movement when combined with passive crossover @ DSP 6 + passive 6 db/oct ?

    Or am I getting it all wrong here??

    If this is correct, why have many of the other JBL designs had much steeper crossover slopes if it is not really needed?

    Kind regards
    //RoB
    Hi RoB,

    There are three electrical protection of the drivers membrane:

    1. capacitor in-series with the rest of the drivers circuitry
    2. Hi pas filter of the DSP crossover
    3. sensitivity reduction resistors 13 and 11 Ohms

    Depending of the horn loading the driver, maximum diaphragm displacements would occur, but using some simulation programs such as Hornresp,
    it can be seen that around 300Hz~400Hz would be the most critical frequency region (without the DSP influence), with DSP HP would shit such region up say somewhere about 500Hz, (but making driver better protected).
    regards
    ivica

  6. #51
    Member sebackman's Avatar
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    I’m still curious how the passive, active and acoustic crossover interact in the JBL M2, so if anyone can shed some light on that I would be grateful.

    This upcoming weekend we will be playing around with some drivers and horn and I will post measurements next week. We will be testing 2450SL, 2451Ti and 2451Be on the 2332, M2 and the new VTX F12 horns.

    -And here is some additional info on mounting.

    The M2 waveguide has 8 plastic pre drilled fasteners to mount a driver adapter. The plastic material is actually pretty good. To use a recessed plastic or wood screw proved difficult so I actually tapered the holes for a M6 machine bolt. The holes are about 50 mm deep so I had to make a tap that was long enough, so now each hole accepts M6 bolts. It turned out to be very strong and can easily hold a 245x driver. I will later add some LockTite on final assembly just in case....

    The adapter is made out of aircraft quality aluminum and designed to let the snout of the horn attach to the driver with only 0,5mm distance.

    The M6 bolts are completely recessed and hence will let me mount whatever driver I want as I drilled for both bolt patterns. Even a 2430k will fit. Please PM for a drawing.


    I have settled for the 2206 for now and will be drawing some cabinets in the near future. The plan is to get something up running this summer in a 5.2 system using my Crown CTS amps and the BSS BLU800 as DSP.

    More info next week.

    Kind regards

    //RoB

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    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  7. #52
    Senior Member pos's Avatar
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    Hi Robert
    Quote Originally Posted by sebackman View Post
    I’m still curious how the passive, active and acoustic crossover interact in the JBL M2, so if anyone can shed some light on that I would be grateful.
    I don't think there is anything fancy here: the passive network is just a lpad to reduce hiss and a cap to protect the driver from hardware or user error. It is set sufficiently low to only play a minor role in the final acoustical response.

    The M2 waveguide has 8 plastic pre drilled fasteners to mount a driver adapter. The plastic material is actually pretty good. To use a recessed plastic or wood screw proved difficult so I actually tapered the holes for a M6 machine bolt. The holes are about 50 mm deep so I had to make a tap that was long enough, so now each hole accepts M6 bolts. It turned out to be very strong and can easily hold a 245x driver. I will later add some LockTite on final assembly just in case....
    Very nice!

    The adapter is made out of aircraft quality aluminum and designed to let the snout of the horn attach to the driver with only 0,5mm distance.
    So there is a 0.5mm air gap between the driver and horn throat?
    That could potentially cause problems in the UHF. Maybe you could fill that gap with cork or other material?

  8. #53
    Member sebackman's Avatar
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    Hi POS

    It is the same with the JBL original adapter and in reality there is no gap as the gasket on the front of the 245x driver will compress slightly and fill the gap. 0,5 mm is not much...

    I can try to make a detailed picture over the weekend. A gap is at least not visible to the eye on close inspection.

    Also, the initial 2451Be measurements does not show a problem. I'm not shure my spare 2451Ti are up to spec... Need to take them apart and clean/align.

    Kind regards
    //RoB
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  9. #54
    Senior Member ivica's Avatar
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    Horn & Driver Response

    Quote Originally Posted by sebackman View Post
    I’m still curious how the passive, active and acoustic crossover interact in the JBL M2, so if anyone can shed some light on that I would be grateful.

    This upcoming weekend we will be playing around with some drivers and horn and I will post measurements next week. We will be testing 2450SL, 2451Ti and 2451Be on the 2332, M2 and the new VTX F12 horns.

    -And here is some additional info on mounting.

    ............

    More info next week.

    Kind regards

    //RoB
    Hi
    Hi sebackman ,

    If we are talking about the acoustical response of the compression driver with the horn mounted, it seems to me that that the total behavior is almost the same as closed box response of the speaker. Theoretically, that response behaves as high-pass filter of the second order (slope -12dB/oct, or 40dB/dec), introducing a kid of delay depending of the applied frequencies. Such delay diminish as frequency rises. It can be equivalent as distance delay but depending of the applied frequency. A kind of measure of such delay usually called "group delay" .
    Applying any type of the filter (analog, digital, passive or active) would add its group delay to the reproduced wave front.
    Applied horns has "horn length" of several cm, but it can be much more, say 20cm or more. If it is assumed that 1kHz wave length is about 34cm. If a kind of addition of the acoustic wave front from two drivers is the listener perception of the speaker response, say mid-bass driver and the horn driver their mutual positions between the drivers, can make large influences to the total frequency response of such two driver response. If we have two drivers near one the other reproducing the same frequency sound, total level would be twice larger, or +6dB, so if the two drivers using the same type of network (but one high-pass an the other low-pass), and if they have the same efficiency, and the used frequency is in the pass-band of the driver, in order not to produce over or under shoot, in the response at the crossover frequency networks transfer faction have to be designed in the way that at the crossover frequency on each of the driver the leveled of the applied voltage has to be ½=0.5 of the level in the pass band or -6dB. But another important thing has to be satisfied that at that frequency total delay of the wave fronts from either drivers has to be the same.
    Some time is not easy to get such equal delay, as may be the compression driver mounted on the horn, due to the horn length, and even the cross over network have been designed correct, the response is not flat as expected. A kind of "comb filter effect" can be get dye to the differential delay of the reproduced wave fronts from each of the drivers.
    It is known that depending on the horn type, depending on its cross-section surface flare, depending of any kind of "obstacles" that can be find in the horn throat (as can be seen on M2 too) in order to enhance diffraction to "widen" horizontal dispersion, or horn mouth is abruptly finish (to baffle), so more or less reflection would be generated, if passive network is used then there is large influence of the driver&horn impedance influence to the total responses (as in the region of several hundreds Hz up to several kHz, depending of the used horn driver electrical impedance can change its value). All that can make large "bouncing" in the driver&horn response, so the behavior of the predicted the 2-nd order high-pass response can differ much.
    On the following figure it can be seen the different response using the same driver but applying horn with different horn flare surface walls. The calculation has been done using HORNRESP software.
    Horn length (depth) is about 17cm, while horn mouth of 900cm2, while horn throat is 1.5inch diameter driver.
    OSWG- Oblate Spheroid (Hyperbolic)
    EXP -Exponential
    Cos-Hyperbolic
    LeCleach
    From the attached figure it can be seen that if we want to apply crossover at 800Hz the response would be very different, even ideal (mid)-bass driver is used. Not to mention that 17cm length is almost half of the 1000Hz frequency wave length, so the relative position of the horn (relative to the (mod)-bass cone surface would be important too.
    Attached Images Attached Images  

  10. #55
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    Weekend waveguide bonaza

    Dear all,

    Thank you Ivica for the info. Without seeing the impedance curve to verify that a specific driver/horn combo increases the impedance below XO point to protect the diaphragm I’m still a bit nervous to go 790Hz @ 6db/oct on the 2451Be’s.

    The series capacitor JBL added in M2 seem to be tuned much lower XO in being 38mf @ 20 ohms (-3db at about 205Hz). If there is not a significant increase in electric impedance towards the amplifier as a result of the acoustic and electric characteristics of the driver/horn combo, what protect the diaphragm from dangerous excursions?

    Anyway, we have had a completely wild weekend with a 4" driver / waveguide shootout.

    We tested 2450Ti, 2450SL and 2451Be (Truextent) on the 2332, the new VTX F-series waveguide and the famous M2 waveguide. Only 4” drivers so far but a kind Forum member has offered to measure on his 2435’s and that will happen in a few weeks. -By me or him  .

    I thought I would sum up some impressions and observations. All of the combinations do sound really good compared with most other speakers from my horizon. We have used 2332 for years and in a DSP environment and with some correction they sound nice and are well behaved. However when comparing to the new VTX waveguide it is clear that development has not stood still.

    We did MLS measures on all combinations of drivers and horns at 0, 30, 60 and 90 degrees angle. The setup was outdoors on a 2,2m high tripod and I general as described by member POS above. The small “table” on top of the tripod with the driver/horn was rotated and the microphone was kept stationary. The turning circle was not geometrically correct so the phase is off a bit but the SPL should be reasonable correct.

    For interested members please PM for measuring data.

    Listening was conducted in a normal living room in a fully 3-way active setup with 2206 below the driver/horn and SUB1500 subbs.

    As can be expected they all measured good but the SL and the Be diaphragm’s shows less break-up and are more stable over the higher frequency range. Albeit there is a significant difference in the visible curve shape from the measurements, it was not perceived to be that great in reality when listening. No doubt the SL and Be was an improvement but Ti was certainly no slouch.

    We decided to minimize confusion by focusing on one driver type to test all horns in sequence. We chose the standard 2451Ti as that is what most people would have and that was good it is likely that the others diaphragms perform better. The 2332 we know well and we have used them with 2451 (Ti/SL/Be) for many years. Given some DSP “help” they are sharp and detailed. Maybe not the best coverage and a bit narrow at the top but still very enjoyable. -To my ears, even better than some horns perceived by others to be superior. They do what they do and our test setup verified that.

    Then we moved to the VTX waveguide and now the sound opened up and felt more “alive”. It is difficult to point out exactly a particular field of improvement but we all thought that definition and precision was improved. This even if the 2332 had reasonable accurate DSP settings but the VTX did not. Clearly this is a better and more modern horn. I started a thread some time ago asking if the older PT-F95HF would be an improvement over 2332 and I still cannot answer that question, but the VTX waveguide certainly is an improvement. They are not very expensive, so I would recommend users of the older compact horns to get a pair of VTX and try them out. The have the same footprint as the 2332 and PF waveguide and should be a direct drop in. Even if being all plastic they are sturdy enough to hold any of the Neodym drivers but if used with a 244X ferrite driver I would make a support bracket. Worth mentioning is that they are only drilled for the small bolt circle and if used with the 2451 with the larger pattern, an adapter is needed.

    We were all quite happy with the improvement in the perceived sound quality of the VTX when we turned to the M2 waveguide. First of all, the instruments and details in the M2 are not very unlike the VTX, so from that aspect they are both an improvement compared 2332. But the way it delivers the sound is completely different. It is kind of hard to describe but some reviewers has described it as moving into the music instead of being in front of it. The sound is delivered with complete ease, more like a silk dome than a driver/horn combo. Voices come closer but keep the “room” is intact. -A bit weird actually. It is a dramatic difference.

    We all liked the sound of the VTX but the M2 is something different. Given the time constraints we did only rather crude DSP settings so there is still much ground to cover.

    I did see a comparison between the 2451 (I think) and the 2430k on a PT-F95HF a while back and as I remember the curves they were really not that different in that combo. So we dialed in the original M2 DSP settings just to try. And to our surprise it worked significantly better than we expected. It sounded really good. From memory better than most other systems I can remember. -Accurate and clear as a driver/horn combo but delivered smooth as a soft dome unit. We used 790Hz HP XO with B12 slope since there was no capacitor in series.

    Our setup was a three way system so we could add UHF trough the DSP as we wanted but with the original M2 DSP settings none of us felt the need for it. Not even with the Ti diaphragm. The M2 waveguide is so smooth and effortless so a pure two-way with 4” drivers will be just fine for most people. With a 476Mg it would probably be a killer…

    All in all, the VTX shows that almost 15 years of development really imply improvements. The DMS-1, from where the 2332 came from, was designed in the late 90-‘s. The VTX came to market 2014. Not that 2332 is bad, but the VTX horn just does it all better. And it measures better too.

    Now M2 is something different. Even with crude DSP settings a big smile was seen on all of us as we flipped through different CD’s. Clearly the original JBL M2 system is even better since it was designed as a system. However, after this weekend I think that a reasonable portion of the well-deserved M2 praise may very well come from the waveguide performance alone. The 1500Nd is of course a gem in its own right and the combination must be fantastic.

    Not having tried the 2430k and understanding that it may well be even better still, I think my original thought to combine a 2206 with a 2451Be or a 2450SL on the M2 may well be the end of the search for me.

    Having established that the M2 waveguide is a keeper and is by far “good enough” even with the older 4” drivers, the question is should we build 2 Compact monitors or 3 to the HT…

    Sorry for the lengthy post…

    Kind regards
    //RoB
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  11. #56
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    Quote Originally Posted by sebackman View Post
    Dear all,


    Not having tried the 2430k and understanding that it may well be even better still, I think my original thought to combine a 2206 with a 2451Be or a 2450SL on the M2 may well be the end of the search for me.

    Having established that the M2 waveguide is a keeper and is by far “good enough” even with the older 4” drivers, the question is should we build 2 Compact monitors or 3 to the HT…

    Sorry for the lengthy post…

    Kind regards
    //RoB
    Thanks for a great write up.

  12. #57
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    Quote Originally Posted by sebackman View Post
    Having established that the M2 waveguide is a keeper and is by far “good enough” even with the older 4” drivers, the question is should we build 2 Compact monitors or 3 to the HT…
    LCR

    Yeah, the M2 waveguide is pretty sweet. Interesting that it went from a closely guarded device to an over the counter device. JBL was downright militant when it first came out. NO! NO! NO!

    I took mine into work yesterday and one of the machinists countersunk the flange holes for me so I can bolt 476Mg's to them.

    Great write up, thank you for taking the time.

  13. #58
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    Wonderful review !

    Thanks a lot <>

  14. #59
    Member sebackman's Avatar
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    I just got some additional guides. That is good news. For those looking to embark on the M2 DYI journey you could try to get them from reconingspeakers.com and ask for Randall. Part no is # 5025594. They also seem carry the mounting flange (# 5025224) which was not available when I got mine. Please remember that the original flange only accepts the narrower nose drivers like 243x and smaller JBL driver mount bolt circle.

    You can mount the adapter plate using screws for plastic and potentially recess them in the adapter plate like 4313b mentions. But I opted for making my own adapter plate with dual bolt patterns and cutting M6 threads in the waveguide.

    When cutting threads in plastic I recommend doing this in steps not to get the plastic “floating”. I used a pre, mid and end tap and only cut about 1-2 cm at a time before cleaning out the hole and continuing. The 8 holes in the waveguide are deeper than a standard tap so I got a “long” tap and grinded down almost the entire shaft to be able to thread deeper as a final cut. It went very well and I believe the adapter plate easily can hold any 245x driver and even potentially a 2447 (subject to baffle sterngth).

    Kind regards
    //RoB
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  15. #60
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    What studs did you use to mount the c.d.'s?

    Did you use any goop to hold the studs in to the c.d.'s?

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