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Thread: Monolith refresh - Panels, woofers and rail stain updates

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    Super User JonFo's Avatar
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    Default Monolith refresh - Panels, woofers and rail stain updates

    Iíve had Monolith IIIís for eight years now, having bought them used in Ď98. They were made in January of Ď93.

    They sat unused for more than a year as I finished up the custom home theater room designed around them and the rest of Martin Logan gear I have.

    Once in full deployment in the new HT, they were marvelous, although running on their factory passive crossover for the first months. After playing with placement, tuning the room acoustics and all other non-invasive approaches, it was clear that a better sound should be possible with these great speakers. So I went with an active external crossover and bi-amped. Big improvement.

    However, in measuring the new x-over setup, it was clear something was not right in the bass, especially the mid-bass. Keep in mind Iíve always used subs (back in those days it was two Velodyne ULD-18ís) so the Monolith woofers were not being taxed by ultra-low frequencies. The x-over then was at 60hz.

    Looking at the stock 12Ē driver, it was clear the distortions and Ďflabbinessí I was hearing were a product of one tired speaker. So I updated the driver with an ACI SV12 unit. Read the specific story and see pics of that one at my site.
    The sound, especially the 100hz region was vastly improved. Been very happy with them as long as the x-over is below 160hz.

    Having experienced dramatic improvements from the active x-over, I then went all-out with a Speaker processor, the DriveRack 260. This allowed true panel to woofer (and to sub) integration on a level that seemed a pipe-dream before. This unit is awesome, and results are likewise.

    Now, here we are in the spring of 2006 and the panels have significant high-end frequency drop offs, the ETF measurements also show some panel resonances that I donít think had been there before. Also, the original panels are now 13 years old. Time for a refresh of the panels.

    The Monoliths I bought (since I got them used) came with Oak rails. They look nice, but donít really go that well with my HT setup, where everything is either black, grey or uses purple for a highlight color. When using front projection, light control and reflections are critical. The light Oak rails pick up the light of the PJ and are somewhat noticeable when playing a movie. Since the replacement of the panels requires some disassembly of the Monolith, Iíll be removing the rails and refinishing them.

    The plan is to use an Aniline dye to stain them a dark purple and then put a satin finish poly coat over that.
    First, lotís of sanding to get down to the bare wood.

    In addition to the panels and rails, Iíll also be doing something with the woofer, but thatís detailed in another post.

    Some of you have PMíd me or posted asking for details on Ďhow-toí perform some or all of the above, so this thread is for you guys. Pics, measurements, descriptions and crying on your shoulders if I screw-up, all to come in the next few days.

    Lotís of work in the next day or two
    Last edited by JonFo; 04-30-2006 at 01:00 PM.
    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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    Super User JonFo's Avatar
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    Default Woofer Update selection

    Since the woofer project a few years ago, Iíve been doing two things, one is testing higher crossover points to move out of the dipole rear-wave cancellation issue area, and the other is researching drivers that can play cleanly at up to 500hz. Turns out there are a couple of suitable drivers, one is a Peerless brand unit, the other is an AudioTechnology Flexunits model.

    The Flexunits seems like a good choice if you do not use a sub, as it has better LF response and a better motor. However, itís more expensive and does not quite go as high as the Peerless, but definitely goes high enough for the factory x-over points or even higher.

    I settled on the Peerless 830669 12Ē driver. This unit will play clean past 1KHz, and bass was reasonable considering Iíll be crossing over at 24db/oct at 80Hz. It is not suitable for a full-range Monolith, as itís low-end extension is limited. But for my purposes, seems like a good choice. Proof will be in the measurements.

    As seen the pic below, the woofers and panels are here.
    The other stuff is for my little center channel project
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    Last edited by JonFo; 04-30-2006 at 01:01 PM.
    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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    Super User JonFo's Avatar
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    Default As-is measurements of ACI SV-12

    Before we go moving and ripping anything up, I captured some Ďas-isí measurements of the current ACI woofer.

    This graph shows both the detailed Bode response (blue line) and the smoothed third octave response (green line) as measured at 1m distance

    As you can see, the smoothed response is reasonable, being +/-3db from 35 to 435Hz. It falls off pretty rapidly after that. In the Bode response, the distortion elements at >250Hz start accumulating. The cone is pretty massive and does not do mid-bass that well.
    The big peak at 64hz is a room mode. I donít have anechoic chamber, so we live with spikes here and there

    I expect the peerless to go higher and smoother.

    More to comeÖ
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    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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    Super User JonFo's Avatar
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    Default Back on track

    Well, after a few weeks of hard work, first on the SL3-XC center channel project , and then some catch-up at work, I finally had time to get back to the Monoliths.

    The plan to deal with the trim rail is a bust. Hereís why:

    The monolith is constructed of essentially three parts:
    1. The Bass cabinet, which houses the 12" driver and the ESL Electronics (but no Crossover, Monolith x-over have always been external, with a choice of passive or active)
    2. The ESL frame. This is what holds the panel and the trim elements.
    3. The Rails that tie the two together.

    The rails are a part of the support structure and require some major bolts and screws to be removed. And I didnít feel like a total disassembly project. So the rails will remain Oak, and their light color nature will be dealt with differently. Iíll post about that when I get around to solving that issue.

    In the meantime, I had the new panels and the Peerlesss drivers to install.

    So, doing one each weekend, Iíve updated the panels and woofers.

    The speakers were repositioned and measurements taken.
    Last edited by JonFo; 05-21-2006 at 08:23 PM.
    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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    Super User JonFo's Avatar
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    Default Woofer Update

    As noted above, the ACI woofer as big improvement over the tired-out factory Eminence unit.
    However, the ACI SV12 is really more of low bass /sub driver, which means it canít go cleanly up into the mid-bass. Not too big a problem if you stick to the 125hz factory crossover point.
    But as many threads have discussed, dipole speakers, no matter how big the panels, have issues with rear wave cancellation at low frequencies. Physics gets in the way it seems.
    Therefore I like to cross-over around 312Hz to the Monolith panels.

    To better match the higher crossover point (and use of 18db/octave slopes), I needed a speaker that would be clean up in the 800Hz range.
    The Peerless also does not go down very far, its FS3 is 40hz. So it needs to be crossed over to the sub above 65Hz. In my application, Iím using 80hz.

    As can be seen in this graph of frequency response, the old ACI and the Peerless are roughly equivalent in the 100 to 450 range. The ACI clearly is more of low-end driver, easily posting 3 or more DB better numbers down low.
    But the Peerless goes substantially higher (and as weíll see in the impulse graphs, cleaner) and is essentially flat at 800Hz. The graph does not show it because of the floor bounce (notice how both curves track it). I really need an anechoic chamber

    Measured from different distances (with different room modes affecting), itís clear the Peerless is better at the mid-bass.
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    Last edited by JonFo; 05-21-2006 at 08:26 PM.
    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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    Super User JonFo's Avatar
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    Default Impulse response

    The real difference is in the distortion products of these two drivers. The larger mass of the ACI will get in the way of clean mid-bass transients. This Impulse response shows us that the ACI can start and stop its initial impulse fairly well, but then the cone oscillates substantially more than the Peerless unit. The Peerless has a lighter mass cone, and less compliant surround, which is why it does better up in the mid-bass.

    The Blue trace is the ACI, the Green trace is the Peerless.
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    Jonathan

    System #45 (Monolith IIIx, Sequell IIb, SL3XC)

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