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Old 29th January 2010, 11:49 AM   #1
Drakmin is offline Drakmin  Finland
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Default MEGABLASTER2010 (2x100w TK2050)

Hi.

I'm building my 4th ghettoblaster. What I've learned so far is that stereo setup is no use outdoors. I've also learned lead acid batteries (or whatever they are called) are no go, way too heavy.

I don't know that much about speaker designing so I prefer to buy 2nd hand passive speakers and use them as a basis. Someone with better skills has designed them soundwise. I'm more into designing the looks of the device!

4th ghettoblaster aka Megablaster2010:

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8,75" bass / 1" tweeter from Behringer Truth 2031p speaker built into new box of same volume and same tuning frequency. Should give 89db/w from 55hz to 20000hz.

Sure electronics 2x100w tripath TK2050 amplifier. I'll use only one channel and convert the signal to mono before amplifier.

2x 7,4V (=14,8v) 4000mAh Rc car nimh batteris. They have the best ratio of weight/price/easy to use for me. Amplifier can take voltages up to 36V so if I need more power I'll just throw in more batteries.

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Finished blaster looks like a stereo device. Other channel is only fake speaker grilles. I'll place reflex port behind the fake bass driver.



Here's some previous audio projects I've done so you know what to expect:

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Old 29th January 2010, 02:27 PM   #2
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I think your primary problem is way too ineffective speakers. 89 dB/W/m for outside use is like paddling upstream with a toothpick, no matter how strong you are (the powerful the amp is) you'll get nowhere.

Since you already are aiming at 9" speakers why not go the extra inch, and get high sensitivity 10" PA speakers that can be had for a reasonable money.

I also wholeheartedly recommend a bipolar design as my own has (see sig).
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Old 29th January 2010, 02:36 PM   #3
Drakmin is offline Drakmin  Finland
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Hm my first model had stereo setup with 89db/w speakers. It was loud enough for my use with 2x25w amp but cut off freq was a bit too high. This design is same in size but should go lower. I like I can choose in which direction the sound goes, thats why I don't consider bipolar.

9" is absolute maximum for the size of the bass driver. You can still recommend me 8" drivers which are more efficient than what I have now but would play below 100hz. I prefer lower notes than more spl. Crossover design is still a problem for me.

Thanks for any ideas!
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Old 29th January 2010, 06:19 PM   #4
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Well, your 89 dB/W/m speakers needs about 100 Watts to play as loud as the speakers I use plays on 10 Watts! And those are not even really high sensitivity speakers at 96 dB/W/m (since I only 2 of them totalling 99 dB/W/m).

You looking at the problem the wrong way. You are never going to get useful really low bass output in an outside enviroment in any portable package.

For example, you could have speakers like mine and make a box 4.2 times larger and get a 55 Hz cut-off but it wouldn't help much since you'd still need an amp capable of output 10 times power than the amp6, or about 200 watts, to equal out the bass loss. And so you're back on miniscule battery life times.

Settle for a cut-off at no lower than 80 Hz and you have a practically possible goal.

Last edited by Saturnus; 29th January 2010 at 06:24 PM.
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Old 29th January 2010, 06:29 PM   #5
Drakmin is offline Drakmin  Finland
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Ok, my limits are: 8" driver in 25l enclosure and 14V 4Ah battery. What drivers and crossover do you recommend? I made a quick search but found hundreds of drivers which might work. Too much to choose from

I still try the Behringer in it's original box outdoors, maybe I'll be disappointed.
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Old 29th January 2010, 06:40 PM   #6
Drakmin is offline Drakmin  Finland
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With my low experience this drivers specifications look great: http://www.hifitalo.com/elementit/ei...und/580011.pdf

Eighteen Sound 8NMB420 with average sensitivity of 95db. Specs sheet recommends a 20liter box tuned to 60hz. What would be a matching high frequency driver? What kind of crossover should I use?

And here is Precision Devices PDN.8BM31 http://www.precision-devices.com/asp...s/super/75.pdf

Specs look pretty similar to me, Precision deviced being the cheaper choice under 100 euros.

Last edited by Drakmin; 29th January 2010 at 06:55 PM.
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Old 30th January 2010, 09:00 AM   #7
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I'd probably chose the Precision Device woofer because of price and that you can order them in 4, 8, and 16 Ohms versions. So you can get the 4 Ohms one. You'll not get output lower than about 80 Hz with either of them though.

Second problem is that you'll use a 14.4V battery, so unless you have a dc-dc converter you amp will output maximum 20 Watts RMS per channel into 4 Ohms, regardless of the specs of the amp. You can always throw in more batteries of course but just remember that power consumption rises as well.

Look at it this way, your battery pack has 57.6 Wh battery life. That's about the same as a standard laptop battery but you'll be drawing about 14.4 Watts at maximum volume (divide product of max RMS total for all channels by 4) so you'll effectively only have about 4 hours of battery life time at that voltage.

All these things are explained in my Boominator thread though, either directly or between the lines somewhere.

Last edited by Saturnus; 30th January 2010 at 09:14 AM.
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Old 30th January 2010, 10:18 AM   #8
Drakmin is offline Drakmin  Finland
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Thanks for yout input again. My plan has place for four batteries =28,8V so I can choose between more spl or less weight. I have no need for full day operation, atleast at maximum volume.

This is again said my 4th build so I know pretty good what I need. Extra spl is never a minus so I consider that 8" Precision as a lf driver as it's also light weighted. I quess I can find a matching tweeter from normal tweeter. To match the 95db/w. I take recommendations of tweeter in consideration.

Are ready made crossovers ok? I can get cheap assembled 12db/okt crossovers for atleast 2000, 2500, 3000hz.

25 liter tuned to 80hz sounds okay. (My miniature blaster has a 3" fullrange tuned to around 110hz and it's really nice when listened in couple of meters radius)
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Old 30th January 2010, 10:39 AM   #9
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I wouldn't go with a ready-made filter. Filters must be optimized for the drivers used. For example if you buy a ready-made filter those will most likely be for 8 Ohms speakers and not 4 Ohms speakers as I recommend, you must use needs a completely different filter for 4 Ohms speakers.

As for tweeter, I recommend the P.Audio PHT407, not the N version as metal diaphrams are quite annoying for outside use as they change sound according to temperature. Bang a 3.8 Ohm power resistor across the terminals and you have a 4 Ohm tweeter with 95 dB/W/m sensitivity to match the woofers perfectly.

A 6/12 db filter should work out fine with a little tweaking.
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Old 30th January 2010, 11:56 AM   #10
Drakmin is offline Drakmin  Finland
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Thanks!

Im getting the precision bass from thomann, unfortunately couldn't find that p.audio tweeter there. Would any of the tweeters on the link be acceptable pair for Precision Devices PDN.8BM31? Small size is preferred so I can easily hide it behind a speaker fabric.

Speaker Components for High-Frequency Range - U.K. International Cyberstore
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