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Teeny tiny PA 15" subwoofer
Teeny tiny PA 15" subwoofer
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Old 16th April 2015, 07:13 PM   #11
Brian Steele is offline Brian Steele  Grenada
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Quote:
Originally Posted by chris661 View Post
Brian,
I quite like the idea of going isobaric to remove the need to boost around Fb. Should give the drivers a much easier time thermally. Plus, I suspect it'd reduce distortion a little, too.
If this is for PA use, I'd recommend against using an isobaric arrangement. A little boost around Fb isn't going to be an issue, particularly if Fb is around 40 Hz or below, unless the music you plan to play through this thing happens to consist mainly of 40 Hz sine waves . The increased efficiency in the main part of the passband is going to be of considerably more importance compared to the isobaric approach.
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Old 17th April 2015, 11:18 AM   #12
chris661 is offline chris661  United Kingdom
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Yeah, looks like you're right.
Isobaric (grey line) at 4v, drivers in series vs standard loading, 2.83v. Not sure which design this one is, but they all come out fairly similar, with 1x driver having a downward sloped response, and isobaric having peaks at each end.

Click the image to open in full size.

With the drivers I have, I could even make 4 of these, which would go astonishingly loud.

Sometimes they'll be used for live music, other times house parties. For the latter, I'd set more aggressive limiters so the drivers don't cook if someone decides a 40Hz sine-wave track is what we need.
Wonder if the dynamic EQ on the DCX2496 can help here...

Cheers
Chris
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Old 17th April 2015, 02:33 PM   #13
cyclecamper is offline cyclecamper  United States
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I use sealed Isobaric for PA. For perspective:

On the plus side, it is visually compact, and wide-range with extended lows.

On the negative side, I use 21,600+ watts for the mains at the moment, and I bring my own assortment of circuit breakers and plug directly into the fusebox. Sometimes infrasonic rumble is only detected by the subtle yet disturbing odor of many cooking voice coils. Consider switching-mode amps; mine have switch-mode power supplies but conventional audio amplifiers, and the stack of amps is nearly as big as the stack of speakers but more difficult to move; but they can be hidden. Bass peaks tend to dim the house lights, as the power line voltage from the service pole dips. Buy a driver distribution franchise from the mfgr, so that you can get the drivers in quantity at wholesale prices.

600 watts into tapped horns is not quite as smooth but considerably louder within a narrow bandwidth. And you can plug the amp into a wall outlet, and carry it without a fork lift.
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Old 17th April 2015, 03:02 PM   #14
chris661 is offline chris661  United Kingdom
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I'd've thought sealed boxes would lose too much SPL compared to tuned designs for PA use. I guess you can get away with really tiny boxes, though.

I'm small-fry in the PA world at the moment. Currently I've a pair of 15" Deltamax tops with these Beyma drivers in. Soon I'll be using 12" tops and some subwoofers. Nothing spectacular, but I'd rather a small, nice sounding system than something huge that just sounds okay. Of course, if, with clever engineering, the small system can keep up with something physically bigger, that'd be a plus.

Just took delivery of another NU6000DSP, so now I can have one per side, plus HF amps. One channel will drive sub(s), the other will do midbass.
I did think about using one NU6k for subs and the other for midrange, but having read they're not too happy with 4ohm a side for prolonged durations, I think one channel at 4ohm and the other at 8ohm should keep them from shutting down.

Now all I need is a little 1U amp that doesn't have a switch-on thump. This old Peavey amp isn't playing too nicely any more. I think it unmutes the outputs before its quite ready - suspect some caps are on their way out.


That's all for now. Looks like I need to go away and contemplate patch panels and figure out which Hornresp sim looks easiest to build.

Chris
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Old 18th April 2015, 04:42 PM   #15
tb46 is offline tb46  United States
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Hi Chris,

I took the basic fold from kees52 (Post #9), and my Hornresp simulation from Post #4, and ended up w/ the attached drawing. I tried to get it as small as I could. I added a mouth opening flare, as Brian suggested. Also, it should be easy to convert this into a vented arrangement. (If you actually want to do this, and need additional dimensions, I can provide those.)

Ok, get out some cheap wood for a trial run. :-)

Regards,
Attached Files
File Type: pdf OD_TTL_Beyma_15P1200Nd.pdf (59.4 KB, 168 views)
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Old 19th April 2015, 03:14 PM   #16
chris661 is offline chris661  United Kingdom
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Woah, that looks brilliant.
I suppose, to convert to a ported box, I could cut holes into the panel behind the driver (as discussed in the 12" thread) and see how that comes out.

I've done one OD-TL before, which was an 550 sqcm cross-sectional area, L12=~67, L23=~144cm. It sounds okay, but doesn't work as the sims suggest. IIRC, it shelved down around 6dB from 70Hz to 50Hz, and stayed flat until dropping off around 30Hz.
You can see the impedance plot here. Hornresp suggests the lower impedance peak should turn up at 24Hz, but it looks like it'll peak <20Hz. I didn't add any stuffing, and just ran a brace along the centre of the internal panels.
Adding some filling to the open end of the TL in Hornresp gets me somewhere close to what was measured. It still serves duty when the Deltamax cabinets need some help in bigger spaces, but I'm certain the design above will do better, despite the serious size difference.


Anyone got any thoughts towards the wiggle at the top of the useful frequency range? - there's certainly some kind of "feature" in the group delay plot, though I'll be crossing over in the 80-100Hz range.

I'll draw up a cutsheet for that cabinet of yours, tb46, but it looks like we might be on to a winner. +4dB Q=0.7, F=39Hz parametric EQ gives a wonderfully flat response in free-field. I'm still playing with the DCX2496 dynamic EQ - cutting down the boost at high levels would wring the last bit of SPL out of this little box.
Cheers, I owe you a drink if we're ever in the same country.

Chris

Last edited by chris661; 19th April 2015 at 03:17 PM.
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Old 19th April 2015, 03:59 PM   #17
tb46 is offline tb46  United States
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Hi Chris,

Yes, to convert it to a ported box you'd have to modify the panels behind the driver, and keep the portion of the bottom duct that you want to use as a port.

As to the wiggle at the top of the useful frequency range, it almost goes away w/ very light filling as shown in the attached simulation. Just a little fiberfill around S1, and a little past the driver in the upper cabinet corner should do it, the more you evenly increase these two points the lower the wiggle, but you start loosing low end SPL (Hornresp gives a total of .055kg Polyfill, w/ .017kg @ S1.).

I got to run, but I'll remember the drink the next time we're in the same country. :-) Let me know if you need any more detailed internal dimensions.

Regards,
Attached Files
File Type: txt 15Acad.txt (998 Bytes, 37 views)
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Old 22nd April 2015, 12:51 PM   #18
chris661 is offline chris661  United Kingdom
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Hi Oliver,

Struggling a bit with the cutsheet - figuring out the sizes of the internal 5 panels is proving a PITA.
I think I'll keep the 30 degree expansion continuous at the end by chamfering the doubled-up baffle (nice touch).

Need to have a think about how I'll attach a grille, too - I'd like 26mm clearance in front of the driver so the cone never hits anything. I'll probably just stretch the top, bottom and sides forward and put a big square grille over the driver.

Anyway, if you could check those dimensions for me, that'd be wonderful.

Chris
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Old 22nd April 2015, 01:38 PM   #19
tb46 is offline tb46  United States
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Hi Chris,

This should help, board dimensions for Post #15:

Regards,
Attached Files
File Type: pdf OD_TTL_Beyma_15P1200Nd_board_dimensions.pdf (19.1 KB, 87 views)
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Old 29th April 2015, 11:54 AM   #20
chris661 is offline chris661  United Kingdom
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Quick update: did a gig last night with the single 15" OD-QWP sub I have. Definitely need more oomph. My finals finish early June, so I'll be making sawdust in the sun.

Have a look at my thread under PA Systems for more.

Chris
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