1st BFM build: 2 X Omni 15 TallBoy in 2-12 configuration

[billkatz]

I used the same 2 3/4" holesaw trick to make the phase plug on mine. I like the way you did the driver holes. I used a jigsaw at a 45 degree angle, and the hole doesn't look nearly as nice.

[Tom Smit]

Do Not Do This: hammer hurricane nuts into the holes. You'll be sorreeee Abide By The Plan, draw the hurricane nuts tight with the driver in place... unless you enjoy modifying your driver to fit the hurricane nuts that are now in the wrong spots, because you hammered them in place... the durn things move.

--aeolos

Um, the plans don't say that. What they do say, is to use the driver to mark the holes. Then when drawing the nut into the hole, use a bolt (and washer). Using the driver to pull in the nuts may damage the driver flange.

[aeolos]

Tom, apologies for getting it wrong yet again. For some reason this particular step has been a stumbling block over and over again. Should have re-read that passage before posting.

But would you agree that if one is very careful, as I finally was on the fourth and last attempt to get hurricane nuts placed correctly, one can use the driver as a guide? You are doubtless right that if one ham-handedly grinches the things in there you can crack the cast driver frame, or do something else awful.

The reason I finally did it that way was that I could not seem to get them in the right locations any other way. Even a carefully-performed geometric construction, using the specs as published on the Eminence website, with markings made through the driver frame and nuts placed as you prescribe, resulted in 2 out of 8 bolts being off enough to require easing the mounting hole in the driver frame. But with great care, using the actual driver as the guide, the fourth set went in exactly right, a cause of some satisfaction after getting it wrong three times in a row--without damage to the driver.

None of us have $200 drivers to waste I am sure. So Abide By The Actual Plan y'all.

Now this also has a lot to do with using 1/4-20 nuts and bolts rather than 10-32, and I know the plan says 10-32. Another reason to Abide By The Plan. 10-32 bolts look so wimpy somehow, for those drivers. I'm using 10-32 for the midrange driver. In the old days we always used 1/4-20 for cast-frame drivers. That was longer ago than the median age of the population of the USA, and has nothing to do with building BFM Omni 15 Tallboys, other than to prove, if proof were needed, that long-established habits may sometimes trump good sense.

Thanks for a much-needed correction
--aeolos

[aeolos]

Thanks Bill! The reason I went looking for that RotoZip (took me a long time to find it, knew I had it but where?) was that I first tried to cut out the midrange baffles with a handheld jigsaw.

I am not very good with a handheld jigsaw, as I discovered. The reject midrange baffles are still on the workbench downstairs, ugly ugly ugly.

As with the hurricane nuts for the 12" drivers, without a precise guide, my handiwork leaves much to be desired. So I now try to fudge up some sort of guide for any operation requiring precision.

There is always a way
--aeolos

[miked]

Cab looks great, especially the horn. I read that you had to "modify the driver" to get it to fit. What do you mean exactly? I hope just maybe widened the mounting holes in the frame a bit. As long as the driver and it's gasket seal absolutely air-tight, that's really all that matters. T-nuts have as much, if not more of a "Love/Hate" reputation around here than PL does. When they work, they work well and are a pleasure to use. When they mess you up, it's always time consuming and painful. Good luck w/the rest of the build and keep the pics coming!

ps
x2 for getting used to sawdust in your orifices and PL stuck to your arm hair. That's what compressed air and hot showers are for.

[aeolos]

Hi miked, yes, that was the modification: drill out the mounting holes, doubtless voiding the warranty but at least now they will mount, tightly and with a good seal. I had to take them all the way out to 23/64" on all 8 holes for both drivers the first cab, agony. Fortunately, I guess, the frame is cast aluminum, so at least I can't bend it much. Break it, yes, but bend, not so much.

The thing that really bothers me is that, sometime in the future, either I or someone else may blow one of those drivers and need to replace it in a hurry. That is not going to be easy unless whoever it is has a drill and a 23/64" bit handy at the moment of truth. I'm going to glue notes in the cabs documenting the bug and cover the notes with clear polyurethane.

Of course these are little bitty Omni 15s and it is extremely unlikely that anyone will ever use them in a mission-critical application. I tell myself.

As a sometime writer of application software, maintainability is a primary goal for things I build. These cabs are not maintainable in that regard.

Thanks for your good words! Even if I have blown it in some aspects, this build is still tremendous fun and it is teaching me a lot. Someday I hope actually to hear the speakers in operation , man this is taking a long time.

Right now I'm working on the various filters for the one cab I have the drivers for. Low pass is sitting on the desk in front of me, physically built but not yet wired: I think I will take care of that detail right now. It is an oinking pig as you will see, biggest passive crossover (or part of one) I have ever seen. Hardwood (air, sort of) core, 1.1 mH inductor wound from 12 gauge copper foil, rated at 650 watts RMS, and a 68 μF 400V polypropylene cap. No doubt others have seen bigger but nevertheless..

Live and learn
--aeolos

[miked]

More often than not there is a whole lot of "DO IT" in DIY. The end result is more than worth it. I built my own electric bass cab (before I knew about BFM) and 4 OTops and I'm almost done with 4 T48s. You get performance that you'd have to pay at least 5x the cost in parts alone and you get to DIY, so you know it's done the right way.

[aeolos]

miked, I know you're right. This pair of speakers looks to me as though it will more than outperform any pair of single cabinets I've ever used. Long ago I had a commercial rig that used JBL-factory "Perkins bins" (that's what we called them, can't remember the model number) for full range, that each had a single 15" midbass driver, a compression horn, and that supertweeter with the wedge-shaped phase plug (2405?).

These Omni 15 Tallboys are smaller, lighter, and give every indication of blowing those JBLs out of the water.

It's a matter of skill, like most things: if I knew at the beginning what I know now, this build would have been done long ago. That learning is one of the great benefits of the DIY approach of course.

best
--aeolos

[miked]

It's a matter of skill, like most things: if I knew at the beginning what I know now, this build would have been done long ago.

Absoultely! It took me about two months, IIRC, to build my 4 OTops. I could bang out another four in easily half that time now. Good luck w/the rest of the build.

[aeolos]

Thanks miked!

And I took a look at your build threads, and my word, nicely done. Your shop is just fantastic, at least to me who uses a corner of a garage, and your dust filter is a truly beautiful bit of engineering. You inspire me to look at my woodworking environment and find ways to make it better.

Please allow me to add my congratulations on the completion of one career, the beginning of a new one or ones, and profound thanks for your service.

All the best, most truly
--aeolos

[aeolos]

Crude but (one hopes) effective: a low-pass filter for an Omni 15 in the 2 12" (4 Ω) configuration. 12 ga. 1.1 mH foil inductor, 68 μF 400V polypropylene film capacitor. An electrolytic cap could have been used for the purpose. OCD manifests itself again. Will be mounted on rubber washers to provide space under the board.

The return to ground runs under the inductor and has curves in it, since I cut it too long and was in too much of a hurry to trim it. Never enough time. Looking at it this morning, I have to ask, is that going to cause a problem? Can always re-route it.

CD included for scale.

Thanks
--aeolos

[miked]

Thanks miked!

And I took a look at your build threads, and my word, nicely done. Your shop is just fantastic, at least to me who uses a corner of a garage, and your dust filter is a truly beautiful bit of engineering. You inspire me to look at my woodworking environment and find ways to make it better.

Please allow me to add my congratulations on the completion of one career, the beginning of a new one or ones, and profound thanks for your service.

All the best, most truly
--aeolos

Hey, thanks very much! I am proud to have served. I saw and did many things during my career in service to our country and couldn't be prouder to have been a part of something so much bigger than myself.

As for the dust filter pre-separator (AKA "cyclone"), you can thank one J.P. Thein for that. http://www.jpthien.com/

He invented the separator concept. I just built a copy of what's on his webpage...with a little help from the woodworking community, too, of course. I waited 20 years to have a garage shop. I have built MANY car stereo trunk boxes in my kitchen, on a 4x8' balcony, turned a bathroom into a spray booth (did not go well at ALL), etc, etc. I have learned a bunch of new woodworking skills and tricks from the BFM forum community. Bruce Weldy in particular. I learn by doing and repetition, but THAT guy just has the knack, naturally. In 30 seconds he figures out a 2-step process to get something done. For me it's 3-days, 11 steps and a pile of firewood later. LOL!

[miked]

Crude but (one hopes) effective: a low-pass filter for an Omni 15 in the 2 12" (4 Ω) configuration. 12 ga. 1.1 mH foil inductor, 68 μF 400V polypropylene film capacitor. An electrolytic cap could have been used for the purpose. OCD manifests itself again. Will be mounted on rubber washers to provide space under the board.

The return to ground runs under the inductor and has curves in it, since I cut it too long and was in too much of a hurry to trim it. Never enough time. Looking at it this morning, I have to ask, is that going to cause a problem? Can always re-route it.

CD included for scale.

omni15212lowpassfilter1.jpg

omni15212lowpassfilter2.jpg

omni15212lowpassfilter3.jpg

Thanks
--aeolos

Holy overbuilt crossover, Batman! I've seen something like that mounted on transmission towers for capturing lightning strikes! I don't have the Omni plans; I am assuming your component values are correct? If so, no harm at all in overbuilding. I am of the overkill mindset myself. Build it mil-spec/bulletproof one time and never worry about it again.

[aeolos]

Yessir, it's a good big 'ern all right. The 1.1 mH inductor is of the plan-specified value, but the cap is 68 μF where the plans say 66. But the plans also allow for 10% tolerance, so it's close enough I guess.

I like designs with headroom, but this one doesn't really have any: that inductor is specified to handle 650 watts RMS, while the 3012LF low frequency drivers are specified to handle 450 watts each. Just looking at them, 450 watts seems to be a little on the high side, or maybe there is an unstated time limit, or something. They are very nice drivers though, can't wait to hear them in action.

The planned application for these cabs is PA for a single performer on guitars, keyboards, and vocals, in very small venues, or reproduction of recorded music in those same venues, so it's hard to imagine running them at even as much as 300 watts RMS in normal use and service. My own rating for one of these cabs will be 300 watts continuous 20 Hz - 20 KHz pink noise, of course at nominal 4 Ω, for 1 hour, if testing shows they can handle it.

That inductor is so big because it's an air- (well, hardwood-) core design. Weighs quite a bit, almost all of the mass being copper. One hopes it will not saturate or generate distortion at the rated power level. The cap is rated at 400 volts, so perhaps that is why it is so big. Plenty of headroom where the cap is concerned .

You are right: build it to withstand unforeseen use/abuse conditions, and gain peace of mind thereby.

All the best
--aeolos

[aeolos]

Here's one of the cabs with the interior bracing (but not the back flanges) in place--another mistake on my part, the vertical brace should have been offset 1 inch to the right in this view, but fortunately the drivers still fit, barely:

The braces at the top of the midrange horn need a little shaping to allow the use of a Selenium 220Ti driver and HC23-25 horn, here's the view from the front--the driver does not touch the braces, so no buzzing from this source anyway:

omni15tallboymodifyhorntopbracesforseleniumhc2325and220ti1.jpg (33.3 KiB) Viewed 1667 times

And here's the view from the back:

Here are the 20 pieces of the back flanges, soon to be installed:

The HC23-25 horn will be glued&screwed onto the front panel to preserve air-tightness. There will be a small padded block on both the top and bottom of the driver itself to reduce the chance of the throat of the plastic horn cracking off under stress (as when somebody drops the cabinet). The horn is polystyrene (maybe ABS?), looks like, and its threaded entry is reinforced with thicker plastic, but it's still not strong enough for my taste. Chose to use the Selenium after a side-by-side test of the Selenium combination vs. the Eminence APT150, using AudioTool for Android, the little Dayton calibrated microphone, and ears. Your mileage may vary, but the top illustration on the Omni 15 page on this site appears to show a version of the Omni 15 standard (not Tallboy) with an HC23-25 horn in it. Had already decided to use the Selenium combination when I saw that: the 220Ti is a good driver, and the little horn has very good horizontal dispersion. "Let everyone be fully convinced in their own minds" .

more soon
--aelos